@Tables and Formats used by DOS & BIOS
:backup headers
^DOS BACKUP Control Information
% BACKUPID.@@@ Format
% Offset Size Description
00 byte Disk sequence flag:
00 indicates disk is not the last backup diskette
FF indicates disk is the last backup diskette
01 word Floppy disk sequence number (Intel format)
03 word Backup year, four digits (Intel format)
05 byte Day of month (1-31)
06 byte Month of year (1-12)
07 dword System time if /T was specified (see ~FILE ATTRIBUTES~)
0B 117bytes Unused
% Backup File Header
% Offset Size Description
00 byte Disk sequence flag:
00 indicates disk is not the last backup diskette
FF indicates disk is the last backup diskette
01 byte Floppy disk sequence number
02 3bytes Unused
05 64bytes Full pathname (without drive designator)
45 14bytes Unused
53 byte Length of file path name at offset 05 plus 1
54 44Bytes Unused
:BIOS Parameter Block:BPB
^BPB - BIOS Parameter Block
% Offset Size Description
00 word sector size in bytes
02 byte sectors per cluster (allocation unit size)
03 word number of reserved sectors
05 byte number of FATs on disk
06 word number of root directory entries (directory size)
08 word number of total sectors; if partition > 32Mb then set
to zero and dword at 15h contains the actual count
0A byte media descriptor byte (see ~MEDIA DESCRIPTOR~)
0B word sectors per ~FAT~
% Additional/different fields for DOS 3.0+
0D word sectors per track
0F word number of heads
11 word number of hidden sectors
15 11bytes reserved
% Additional/different fields for DOS 4.0+
15 dword number of total sectors if offset 8 is zero
19 6bytes reserved
1F word number of cylinders
21 byte device type
22 word device attributes
- located in the boot sector at offset 0Bh
- see ~BOOT SECTOR~
:Batch Control Block:BCB
^BCB - Batch Control Block (undocumented)
^DOS 2.x thru DOS 3.2 BCB Format
% Offset Size Description
00 byte unknown
01 word if non-zero; segment of control block for active FOR
03 byte type of batch command
0 - normal batch command
1 - FOR-loop active
04 dword offset of next command to execute in batch file
07 word offset of variable %0 (batch file name)
09 9 words offset of %N batch file parameters, 0FFFFh indicates
parameter is null
1C nbytes null terminated path and filename of the current
batch file immediately followed by command line
parameters. Each parameter %0-%9 plus a CR is
appended and resulting string is null terminated.
^DOS 3.3 BCB Format
% Offset Size Description
00 byte unknown
01 byte global echo switch, if exec'd by batch CALL-command
1 - turn ECHO ON on return to calling batch file
0 - turn ECHO OFF on return to calling batch file
02 word batch file BCD segment if executed via CALL
if zero; batch file was called from command line
if non-zero; batch file executed via CALL-command
04 word if non-zero; segment of control block for active FOR
06 byte type of batch command
0 - normal batch command
1 - FOR-loop active
07 dword offset of next command to execute in batch file
0B word offset of variable %0 (batch file name)
0D 9words offset of %N batch file parameters, 0FFFFh indicates
parameter is null
1F nbytes null terminated path and filename of the current
batch file immediately followed by command line
parameters. Each parameter %0-%9 plus a CR is
appended and resulting string is null terminated.
- BCB length is variable and depends on the size and count of the
parameters and fully qualified batch file name
- the MCB for a BCB has a process Id of the transient portion of
the latest COMMAND.COM
- offsets displayed are relative to the BCB segment
- SHIFT command changes the offsets of the parameters in the table
at the offsets 0B0h thru 1Ch
- BCB of DOS 3.3 is the same as earlier versions except 3 bytes
were added after offset 0
- to find a BCB, locate the first block in the MCB chain belonging
to COMMAND.COM (the second allocated block always belongs to
COMMAND.COM). Then scan the ~MCB~ chain for a 64 byte block with
the same owner ID as COMMAND.COM). This will be the BCB.
:BIOS Data Area:BDA:BIOS memory:memory map
^BDA - BIOS Data Area - PC Memory Map
% Address Size Description
00:00 256dwords Interrupt vector table
30:00 256bytes Stack area used during post and bootstrap
40:00 word COM1 port address
40:02 word COM2 port address
40:04 word COM3 port address
40:06 word COM4 port address
40:08 word LPT1 port address
40:0A word LPT2 port address
40:0C word LPT3 port address
40:0E word LPT4 port address (except PS/2)
Extended BIOS Data Area segment (PS/2, see ~EBDA~)
40:10 2 bytes Equipment list flags (see ~INT 11~)
ł7ł6ł5ł4ł3ł2ł1ł0ł 40:10 (value in INT 11 register AL)
ł ł ł ł ł ł ł ŔÄ IPL diskette installed
ł ł ł ł ł ł ŔÄÄ math coprocessor
ł ł ł ł ĂÄĹÄÄÄ old PC system board RAM < 256K
ł ł ł ł ł ŔÄÄ pointing device installed (PS/2)
ł ł ł ł ŔÄÄÄ not used on PS/2
ł ł ŔÄÁÄÄÄÄ initial video mode
ŔÄÁÄÄÄÄÄÄÄ # of diskette drives, less 1
ł7ł6ł5ł4ł3ł2ł1ł0ł 40:11 (value in INT 11 register AH)
ł ł ł ł ł ł ł ŔÄ 0 if DMA installed
ł ł ł ł ŔÄÁÄÁÄÄ number of serial ports
ł ł ł ŔÄÄÄÄÄÄÄ game adapter
ł ł ŔÄÄÄÄÄÄÄÄ not used, internal modem (PS/2)
ŔÄÁÄÄÄÄÄÄÄÄÄ number of printer ports
40:12 byte PCjr: infrared keyboard link error count
40:13 word Memory size in Kbytes (see ~INT 12~)
40:15 byte Reserved
40:16 byte PS/2 BIOS control flags
40:17 byte Keyboard flag byte 0 (see ~KB FLAGS~)
ł7ł6ł5ł4ł3ł2ł1ł0ł keyboard flag byte 0
ł ł ł ł ł ł ł ŔÄÄÄ right shift key depressed
ł ł ł ł ł ł ŔÄÄÄÄ left shift key depressed
ł ł ł ł ł ŔÄÄÄÄÄ CTRL key depressed
ł ł ł ł ŔÄÄÄÄÄÄ ALT key depressed
ł ł ł ŔÄÄÄÄÄÄÄ scroll-lock is active
ł ł ŔÄÄÄÄÄÄÄÄ num-lock is active
ł ŔÄÄÄÄÄÄÄÄÄ caps-lock is active
ŔÄÄÄÄÄÄÄÄÄÄ insert is active
40:18 byte Keyboard flag byte 1 (see ~KB FLAGS~)
ł7ł6ł5ł4ł3ł2ł1ł0ł keyboard flag byte
ł ł ł ł ł ł ł ŔÄÄÄ left CTRL key depressed
ł ł ł ł ł ł ŔÄÄÄÄ left ALT key depressed
ł ł ł ł ł ŔÄÄÄÄÄ system key depressed and held
ł ł ł ł ŔÄÄÄÄÄÄ suspend key has been toggled
ł ł ł ŔÄÄÄÄÄÄÄ scroll lock key is depressed
ł ł ŔÄÄÄÄÄÄÄÄ num-lock key is depressed
ł ŔÄÄÄÄÄÄÄÄÄ caps-lock key is depressed
ŔÄÄÄÄÄÄÄÄÄÄ insert key is depressed
40:19 byte Storage for alternate keypad entry
40:1A word Offset from 40:00 to keyboard buffer head
40:1C word Offset from 40:00 to keyboard buffer tail
40:1E 32bytes Keyboard buffer (circular queue buffer)
40:3E byte Drive recalibration status
ł7ł6ł5ł4ł3ł2ł1ł0ł drive recalibration status
ł ł ł ł ł ł ł ŔÄÄ 1=recalibrate drive 0
ł ł ł ł ł ł ŔÄÄÄ 1=recalibrate drive 1
ł ł ł ł ł ŔÄÄÄÄ 1=recalibrate drive 2
ł ł ł ł ŔÄÄÄÄÄ 1=recalibrate drive 3
ł ŔÄÁÄÁÄÄÄÄÄÄ unused
ŔÄÄÄÄÄÄÄÄÄÄÄ 1=working interrupt flag
40:3F byte Diskette motor status
ł7ł6ł5ł4ł3ł2ł1ł0ł diskette motor status
ł ł ł ł ł ł ł ŔÄÄ 1=drive 0 motor on
ł ł ł ł ł ł ŔÄÄÄ 1=drive 1 motor on
ł ł ł ł ł ŔÄÄÄÄ 1=drive 2 motor on
ł ł ł ł ŔÄÄÄÄÄ 1=drive 3 motor on
ł ŔÄÁÄÁÄÄÄÄÄÄ unused
ŔÄÄÄÄÄÄÄÄÄÄÄ 1=write operation
40:40 byte Motor shutoff counter (decremented by ~INT 8~)
40:41 byte Status of last diskette operation (see ~INT 13,1~)
ł7ł6ł5ł4ł3ł2ł1ł0ł status of last diskette operation
ł ł ł ł ł ł ł ŔÄÄÄ invalid diskette command
ł ł ł ł ł ł ŔÄÄÄÄ diskette address mark not found
ł ł ł ł ł ŔÄÄÄÄÄ sector not found
ł ł ł ł ŔÄÄÄÄÄÄ diskette DMA error
ł ł ł ŔÄÄÄÄÄÄÄ CRC check / data error
ł ł ŔÄÄÄÄÄÄÄÄ diskette controller failure
ł ŔÄÄÄÄÄÄÄÄÄ seek to track failed
ŔÄÄÄÄÄÄÄÄÄÄ diskette time-out
40:42 7 bytes NEC diskette controller status (see ~FDC~)
40:49 byte Current video mode (see ~VIDEO MODE~)
40:4A word Number of screen columns
40:4C word Size of current video regen buffer in bytes
40:4E word Offset of current video page in video regen buffer
40:50 8 words Cursor position of pages 1-8, high order byte=row
low order byte=column; changing this data isn't
reflected immediately on the display
40:60 byte Cursor ending (bottom) scan line (don't modify)
40:61 byte Cursor starting (top) scan line (don't modify)
40:62 byte Active display page number
40:63 word Base port address for active ~6845~ CRT controller
3B4h = mono, 3D4h = color
40:65 byte 6845 CRT mode control register value (port 3x8h)
EGA/VGA values emulate those of the MDA/CGA
40:66 byte CGA current color palette mask setting (port 3d9h)
EGA and VGA values emulate the CGA
40:67 dword CS:IP for 286 return from protected mode
dword Temp storage for SS:SP during shutdown
dword Day counter on all products after AT
dword PS/2 Pointer to reset code with memory preserved
5 bytes Cassette tape control (before AT)
40:6C dword Daily timer counter, equal to zero at midnight;
incremented by INT 8; read/set by ~INT 1A~
40:70 byte Clock rollover flag, set when 40:6C exceeds 24hrs
40:71 byte BIOS break flag, bit 7 is set if ~Ctrl-Break~ was
*ever* hit; set by ~INT 9~
40:72 word Soft reset flag via Ctl-Alt-Del or JMP FFFF:0
1234h Bypass memory tests & CRT initialization
4321h Preserve memory
5678h System suspend
9ABCh Manufacturer test
ABCDh Convertible POST loop
????h many other values are used during POST
40:74 byte Status of last hard disk operation (see ~INT 13,1~)
40:75 byte Number of hard disks attached
40:76 byte XT fixed disk drive control byte
40:77 byte Port offset to current fixed disk adapter
40:78 4 bytes Time-Out value for LPT1,LPT2,LPT3(,LPT4 except PS/2)
40:7C 4 bytes Time-Out value for COM1,COM2,COM3,COM4
40:80 word Keyboard buffer start offset (seg=40h,BIOS 10-27-82)
40:82 word Keyboard buffer end offset (seg=40h,BIOS 10-27-82)
40:84 byte Rows on the screen (less 1, EGA+)
40:85 word Point height of character matrix (EGA+)
byte PCjr: character to be repeated if the typematic
repeat key takes effect
40:86 byte PCjr: initial delay before repeat key action begins
40:87 byte PCjr: current Fn function key number
byte Video mode options (EGA+)
ł7ł6ł5ł4ł3ł2ł1ł0ł Video mode options (EGA+)
ł ł ł ł ł ł ł ŔÄÄ 1=alphanumeric cursor emulation enabled
ł ł ł ł ł ł ŔÄÄÄ 1=video subsystem attached to monochrome
ł ł ł ł ł ŔÄÄÄÄ reserved
ł ł ł ł ŔÄÄÄÄÄ 1=video subsystem is inactive
ł ł ł ŔÄÄÄÄÄÄ reserved
ł ŔÄÁÄÄÄÄÄÄÄ video RAM 00-64K 10-192K 01-128K 11-256K
ŔÄÄÄÄÄÄÄÄÄÄ video mode number passed to ~INT 10~, function 0
40:88 byte PCjr: third keyboard status byte
EGA feature bit switches, emulated on VGA
ł7ł6ł5ł4ł3ł2ł1ł0ł EGA feature bit switches (EGA+)
ł ł ł ł ł ł ł ŔÄÄ EGA SW1 config (1=off)
ł ł ł ł ł ł ŔÄÄÄ EGA SW2 config (1=off)
ł ł ł ł ł ŔÄÄÄÄ EGA SW3 config (1=off)
ł ł ł ł ŔÄÄÄÄÄ EGA SW4 config (1=off)
ł ł ł ŔÄÄÄÄÄÄ Input FEAT0 (ISR0 bit 5) after output on FCR0
ł ł ŔÄÄÄÄÄÄÄ Input FEAT0 (ISR0 bit 6) after output on FCR0
ł ŔÄÄÄÄÄÄÄÄ Input FEAT1 (ISR0 bit 5) after output on FCR1
ŔÄÄÄÄÄÄÄÄÄ Input FEAT1 (ISR0 bit 6) after output on FCR1
40:89 byte Video display data area (MCGA and VGA)
ł7ł6ł5ł4ł3ł2ł1ł0ł Video display data area (MCGA and VGA)
ł ł ł ł ł ł ł ŔÄÄ 1=VGA is active
ł ł ł ł ł ł ŔÄÄÄ 1=gray scale is enabled
ł ł ł ł ł ŔÄÄÄÄ 1=using monochrome monitor
ł ł ł ł ŔÄÄÄÄÄ 1=default palette loading is disabled
ł ł ł ŔÄÄÄÄÄÄ see table below
ł ł ŔÄÄÄÄÄÄÄ reserved
ł ŔÄÄÄÄÄÄÄÄ 1=display switching enabled
ŔÄÄÄÄÄÄÄÄÄ alphanumeric scan lines (see table below)
% Bit7 Bit4 Scan Lines
0 0 350 line mode
0 1 400 line mode
1 0 200 line mode
1 1 reserved
40:8A byte Display Combination Code (DCC) table index (EGA+)
40:8B byte Last diskette data rate selected
ł7ł6ł5ł4ł3ł2ł1ł0ł last diskette data rate selected
ł ł ł ł ŔÄÁÄÁÄÁÄÄÄ reserved
ł ł ŔÄÁÄÄÄÄÄÄÄÄÄÄ last floppy drive step rate selected
ŔÄÁÄÄÄÄÄÄÄÄÄÄÄÄ last floppy data rate selected
% Data Rate Step Rate
00 500K bps 00 step rate time of 0C
01 300K bps 01 step rate time of 0D
10 250K bps 10 step rate time of 0A
11 reserved 11 reserved
40:8C byte Hard disk status returned by controller
40:8D byte Hard disk error returned by controller
40:8E byte Hard disk interrupt control flag(bit 7=working int)
40:8F byte Combination hard/floppy disk card when bit 0 set
40:90 4 bytes Drive 0,1,2,3 media state
ł7ł6ł5ł4ł3ł2ł1ł0ł drive media state (4 copies)
ł ł ł ł ł ŔÄÁÄÁÄÄÄ drive/media state (see below)
ł ł ł ł ŔÄÄÄÄÄÄÄ reserved
ł ł ł ŔÄÄÄÄÄÄÄ 1=media/drive established
ł ł ŔÄÄÄÄÄÄÄ double stepping required
ŔÄÁÄÄÄÄÄÄÄ data rate: 00=500K bps 01=300K bps
10=250K bps 11=reserved
% Bits
% 210 Drive Media State
000 360Kb diskette/360Kb drive not established
001 360Kb diskette/1.2Mb drive not established
010 1.2Mb diskette/1.2Mb drive not established
011 360Kb diskette/360Kb drive established
100 360Kb diskette/1.2Mb drive established
101 1.2Mb diskette/1.2Mb drive established
110 Reserved
111 None of the above
40:94 byte Track currently seeked to on drive 0
40:95 byte Track currently seeked to on drive 1
40:96 byte Keyboard mode/type
ł7ł6ł5ł4ł3ł2ł1ł0ł Keyboard mode/type
ł ł ł ł ł ł ł ŔÄÄÄ last code was the E1 hidden code
ł ł ł ł ł ł ŔÄÄÄÄ last code was the E0 hidden code
ł ł ł ł ł ŔÄÄÄÄÄ right CTRL key depressed
ł ł ł ł ŔÄÄÄÄÄÄ right ALT key depressed
ł ł ł ŔÄÄÄÄÄÄÄ 101/102 enhanced keyboard installed
ł ł ŔÄÄÄÄÄÄÄÄ force num-lock if Rd ID & KBX
ł ŔÄÄÄÄÄÄÄÄÄ last char was first ID char
ŔÄÄÄÄÄÄÄÄÄÄ read ID in process
40:97 byte Keyboard LED flags
ł7ł6ł5ł4ł3ł2ł1ł0ł Keyboard LED flags
ł ł ł ł ł ł ł ŔÄÄÄ scroll lock indicator
ł ł ł ł ł ł ŔÄÄÄÄ num-lock indicator
ł ł ł ł ł ŔÄÄÄÄÄ caps-lock indicator
ł ł ł ł ŔÄÄÄÄÄÄ circus system indicator
ł ł ł ŔÄÄÄÄÄÄÄ ACK received
ł ł ŔÄÄÄÄÄÄÄÄ re-send received flag
ł ŔÄÄÄÄÄÄÄÄÄ mode indicator update
ŔÄÄÄÄÄÄÄÄÄÄ keyboard transmit error flag
40:98 dword Pointer to user wait complete flag
40:9C dword User wait Time-Out value in microseconds
40:A0 byte RTC wait function flag
ł7ł6ł5ł4ł3ł2ł1ł0ł ~INT 15,86~ RTC wait function flag
ł ł ł ł ł ł ł ŔÄÄÄ 1= wait pending
ł ŔÄÁÄÁÄÁÄÁÄÁÄÄÄÄ not used
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1=INT 15,86 wait time elapsed
40:A1 byte LANA DMA channel flags
40:A2 2 bytes Status of LANA 0,1
40:A4 dword Saved hard disk interrupt vector
40:A8 dword BIOS Video Save/Override Pointer Table address
(see ~VIDEO TABLES~)
40:AC 8 bytes Reserved
40:B4 byte Keyboard NMI control flags (convertible)
40:B5 dword Keyboard break pending flags (convertible)
40:B9 byte Port 60 single byte queue (convertible)
40:BA byte Scan code of last key (convertible)
40:BB byte NMI buffer head pointer (convertible)
40:BC byte NMI buffer tail pointer (convertible)
40:BD 16bytes NMI scan code buffer (convertible)
40:CE word Day counter (convertible and after)
40:F0 16bytes Intra-Applications Communications Area (IBM Technical
Reference incorrectly locates this at 50:F0-50:FF)
% Address Size Description (BIOS/DOS Data Area)
50:00 byte Print screen status byte
00 = PrtSc not active,
01 = PrtSc in progress
FF = error
50:01 3 bytes Used by BASIC
50:04 byte DOS single diskette mode flag, 0=A:, 1=B:
50:05 10bytes POST work area
50:0F byte BASIC shell flag; set to 2 if current shell
50:10 word BASICs default DS value (DEF SEG)
50:12 dword Pointer to BASIC ~INT 1C~ interrupt handler
50:16 dword Pointer to BASIC ~INT 23~ interrupt handler
50:1A dword Pointer to BASIC ~INT 24~ disk error handler
50:20 word DOS dynamic storage
50:22 14bytes DOS diskette initialization table (~INT 1E~)
50:30 4bytes MODE command
70:00 I/O drivers from IO.SYS/IBMBIO.COM
^The following map varies in size and locus
07C0:0 Boot code is loaded here at startup (31k mark)
A000:0 EGA/VGA RAM for graphics display mode 0Dh & above
B000:0 MDA RAM, Hercules graphics display RAM
B800:0 CGA display RAM
C000:0 EGA/VGA BIOS ROM (thru C7FF)
C400:0 Video adapter ROM space
C600:0 256bytes PGA communication area
C800:0 16K Hard disk adapter BIOS ROM
C800:5 XT Hard disk ROM format, AH=Drive, AL=Interleave
D000:0 32K Cluster adapter BIOS ROM
D800:0 PCjr conventionalsoftware cartridge address
E000:0 64K Expansion ROM space (hardwired on AT+)
128K PS/2 System ROM (thru F000)
F000:0 System monitor ROM
PCjr: software cartridge override address
F400:0 System expansion ROMs
F600:0 IBM ROM BASIC (AT)
F800:0 PCjr software cartridge override address
FC00:0 BIOS ROM
FF00:0 System ROM
FFA6:E ROM graphics character table
FFFF:0 ROM bootstrap code
FFFF:5 8 bytes ROM date (not applicable for all clones)
FFFF:E byte ROM machine id (see ~MACHINE ID~)
:boot sector:boot record
^Boot Sector (since DOS 2.0)
% Offset Size Description
00 3bytes jump to executable code
03 8bytes OEM name and version
0B word bytes per sector
0D byte sectors per cluster (allocation unit size)
0E word number of reserved sectors (starting at 0)
10 byte number of FAT's on disk
11 word number of root directory entries (directory size)
13 word number of total sectors (0 if partition > 32Mb)
15 byte media descriptor byte (see ~MEDIA DESCRIPTOR~)
16 word sectors per FAT
18 word sectors per track (DOS 3.0+)
1A word number of heads (DOS 3.0+)
1C word number of hidden sectors (DOS 3.0+)
20 dword (DOS 4+) number of sectors if offset 13 was 0
24 byte (DOS 4+) physical drive number
25 byte (DOS 4+) reserved
26 byte (DOS 4+) signature byte (29h)
27 dword (DOS 4+) volume serial number
2B 11bytes (DOS 4+) volume label
36 8bytes (DOS 4+) reserved
- implementation format not guaranteed in all OEM DOS releases
- BIOS expects a boot sector of 512 bytes
- DOS 3.2 began reading BIOS Parameter Block (~BPB~) information from
the boot sector, previous versions used only the media byte in FAT
- DOS 4.x added offsets 20-3Dh and offset 20h determines the number
of sectors if offset 13h is zero
- hard disks have a master boot record and partition boot records;
the master boot record and ~Disk Partition Table~ (DPT) share the
same sector
:Box Drawing Chars
^Box Drawing Characters
218 196 194 191 201 205 203 187
ÚÄÄÄÄÄÄÄÄÄÂÄÄÄÄż ÉÍÍÍÍÍÍÍÍÍËÍÍÍÍ»
179 ł 197 ł ł 179 186 ş 206 ş ş 186
ł \ ł ł ş \ ş ş
195 ĂÄÄÄÄÄÄÄÄÄĹÄÄÄÄ´ 180 204 ĚÍÍÍÍÍÍÍÍÍÎÍÍÍÍą 185
ł ł ł ş ş ş
ŔÄÄÄÄÄÄÄÄÄÁÄÄÄÄŮ ČÍÍÍÍÍÍÍÍÍĘÍÍÍÍĽ
192 196 193 217 200 205 202 188
214 196 210 183 213 205 209 184
ÖÄÄÄÄÄÄÄÄÄŇÄÄÄÄ· ŐÍÍÍÍÍÍÍÍÍŃÍÍÍ͸
186 ş 215 ş ş 186 179 ł 216 ł ł 179
ş \ ş ş ł \ ł ł
199 ÇÄÄÄÄÄÄÄÄÄ×ÄÄÄĶ 182 198 ĆÍÍÍÍÍÍÍÍÍŘÍÍÍ͵ 181
ş ş ş ł ł ł
ÓÄÄÄÄÄÄÄÄÄĐÄÄÄÄ˝ ÔÍÍÍÍÍÍÍÍÍĎÍÍÍÍľ
211 196 208 189 212 205 207 190
- see ~ASCII~
:code pages
^Code Page and Country Codes
% Country Keyboard Valid Code
% Country Code Code Pages
Arabic 785 437
Australia 061 US 437,850
Belgium 032 BE 437,850
Canada (English) 001 US 437,850
Canada (French) 002 CF 863,850
Denmark 045 DK 865,850
Finland 358 SU 437,850
France 033 FR 437,850
Germany 049 GR 437,850
Hebrew 972 437
Italy 039 IT 437,850
Latin America 003 LA 437,850
Netherlands 031 NL 437,850
Norway 047 NO 865,850
Portugal 351 PO 860,850
Spain 034 SP 437,850
Sweden 046 SV 437,850
Switzerland (French) 041 SF 437,850
Switzerland (German) 041 SF 437,850
United Kingdom 044 UK 437,850
United States 001 US 437,850
- code pages are lookup tables containing the definition
of one or more character sets
- contain country specific information
- implemented starting with DOS 3.3
:colors:color table
^Color Definitions
% Definitions found in TURBO C's "conio.h"
0 - BLACK 4 - RED 8 - DARKGRAY C - LIGHTRED
1 - BLUE 5 - MAGENTA 9 - LIGHTBLUE D - LIGHTMAGENTA
2 - GREEN 6 - BROWN A - LIGHTGREEN E - YELLOW
3 - CYAN 7 - LIGHTGRAY B - LIGHTCYAN F - WHITE
ł7ł6ł5ł4ł3ł2ł1ł0ł AL
ł ł ł ł ł ł ł ŔÄÄÄ blue component of foreground color
ł ł ł ł ł ł ŔÄÄÄÄ green component of foreground color
ł ł ł ł ł ŔÄÄÄÄÄ red component of foreground color
ł ł ł ł ŔÄÄÄÄÄÄ INTENSITY component of foreground color
ł ł ł ŔÄÄÄÄÄÄÄ blue component of background color
ł ł ŔÄÄÄÄÄÄÄÄ green component of background color
ł ŔÄÄÄÄÄÄÄÄÄ red component of background color
ŔÄÄÄÄÄÄÄÄÄÄ BLINKING of foreground character
- see ~ANSI~ for ANSI color definitions
:country codes:country info
^DOS Country Codes (DOS 2.x)
% Offset Size Description
00 word Date and time format
0 = month day year, hh:mm:ss (USA)
1 = day month year, hh:mm:ss (Europe)
2 = year month day, hh:mm:ss (Japan)
02 2bytes ASCIIZ currency symbol
04 2bytes ASCIIZ thousands separator
06 2bytes ASCIIZ decimal separator
08 18bytes Reserved
^DOS Country Codes (DOS 3.0+)
% Offset Size Description
00 word Date and time format
0 = month day year, hh:mm:ss (USA)
1 = day month year, hh:mm:ss (Europe)
2 = year month day, hh:mm:ss (Japan)
02 5bytes ASCIIZ currency symbol
07 2bytes ASCIIZ thousands separator
09 2bytes ASCIIZ decimal separator
0B 2bytes ASCIIZ date separator
0D 2bytes ASCIIZ time separator
0F byte Currency symbol format
0 = symbol leads, without space
1 = symbol follows, without space
2 = symbol leads, one space
3 = symbol follows, one space
4 = symbol replace decimal separator
10 byte Number of digits after decimal
11 byte Time format
Bit 0 = 0 12 hour clock
= 1 24 hour clock
12 dword Case map call address
16 2bytes ASCIIZ data list separator
18 10bytes Reserved
- see also ~INT 21,38~
:disk partition table:partition table
^Disk Partition Table (Fixed disk boot record)
% Offset Represents: (see format below)
01BE Partition 1 data table (16 bytes)
01CE Partition 2 data table (16 bytes)
01DE Partition 3 data table (16 bytes)
01EE Partition 4 data table (16 bytes)
01FE Signature (hex 55 AA, 2 bytes)
% Offset from beginning of partition data shown above:
% Offset Size Description
00 byte boot indicator
01 byte beginning sector head number
02 byte beginning sector (2 high bits of cylinder #)
03 byte beginning cylinder# (low order bits of cylinder #)
04 byte system indicator
05 byte ending sector head number
06 byte ending sector (2 high bits of cylinder #)
07 byte ending cylinder# (low order bits of cylinder #)
08 dword number of sectors preceding the partition
0B dword number of sectors in the partition
% Boot indicator (BYTE)
00 - non-bootable partition
80 - bootable partition (one partition only)
% System Indicator (BYTE)
00 - unknown operating system
01 - DOS with 12 bit FAT, 16 bit sector number
02 - XENIX
04 - DOS with 16 bit FAT, 16 bit sector number
05 - DOS Extended partition (DOS 3.3+)
06 - DOS 4.0 (Compaq 3.31), 32 bit sector number
51 - Ontrack extended partition
64 - Novell
75 - PCIX
DB - CP/M
FF - BBT
% Signature
Hex 55AA marks the end of valid boot sector. This is also
required in each of the partition boot records.
% Sector/Cylinder
2 bytes are combined to a word similar to INT 13:
ł7ł6ł5ł4ł3ł2ł1ł0ł 1st byte (sector)
ł ł ŔÄÁÄÁÄÁÄÁÄÁÄÄ Sector offset within cylinder
ŔÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄ High order bits of cylinder #
ł7ł6ł5ł4ł3ł2ł1ł0ł 2nd byte (cylinder)
ŔÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄ Low order bits of cylinder #
- all partitions begin on sector 1 head 0, except the first
partition which follows the disk's master boot record and begins
in sector 2
- some of this information may vary with some variants of DOS 3.2
and DOS 3.3 that use their own sectoring scheme for large disks
- see ~INT 21,32~ ~Disk Partition Table~
:device attributes
^Device Driver Attribute Values (brief)
łFłEłDłCłBłA-7ł6ł5ł4ł3ł2ł1ł0ł
ł ł ł ł ł ł ł ł ł ł ł ł ŔÄÄ 1 = character device is stdin
ł ł ł ł ł ł ł ł ł ł ł ł 1 = block dev supports generic IOCTL
ł ł ł ł ł ł ł ł ł ł ł ŔÄÄÄ 1 = character device is stdout
ł ł ł ł ł ł ł ł ł ł ł 1 = block dev supports generic IOCTL
ł ł ł ł ł ł ł ł ł ł ŔÄÄÄÄ 1 = current NUL device
ł ł ł ł ł ł ł ł ł ŔÄÄÄÄÄ 1 = current clock device
ł ł ł ł ł ł ł ŔÄÁÄÄÄÄÄÄ reserved by DOS
ł ł ł ł ł ł ŔÄÄÄÄÄÄÄÄÄ 1 = supports Get/Set logical device
ł ł ł ł ł ŔÄÄÄÄÄÄÄÄÄÄÄ reserved (must be zero)
ł ł ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄ 1 = supports removable media
ł ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄ reserved (must be zero)
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1 = non-IBM format (block device)
ł ł 1 = output until busy (char device)
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1 = supports IOCTL strings
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1 = character device, 0 = block device
- true bit values are described, a false indicates opposite
- see ~INT 21,44~ or IOCTL,n where "n" is an IOCTL function
^Device Driver Attribute Bit Values (detailed)
0 standard input device: used by character devices to
tell DOS a character device driver is the standard
input device. For block devices, a 1 indicates generic
IOCTL supported
1 standard output device: used by character devices to tell DOS a
character device driver is the standard output device. For
block devices, a 1 indicates generic IOCTL supported
2 NUL attribute: used for character devices only. Tells
DOS the character device driver is a NUL device. This bit
is used by DOS to determine if the NUL device is being used.
The NUL device cannot be reassigned.
3 clock device: set to 1 to tell DOS this is the new CLOCK$ device.
0B open/close removable media: set to 1 tells DOS the device
driver can handle removable media. (DOS 3.x)
0D non-IBM format: for block devices, indicates the method
the driver uses to determine media type. Set to 1 for
drivers that use the BPB to determine media type, set to
zero for drivers that use the media descriptor byte. For
character devices (usually printers), set to 1 if the
driver supports output until busy, set to 0 otherwise.
0E IOCTL bit: used with both character and block devices.
Indicates if the device driver can handle control strings
through the IOCTL. Zero if a device driver can't process
control strings. If an attempt to send/receive an IOCTL
control strings, is made without this bit set, an error code
is returned. The IOCTL functions allow data to be sent to
and from the driver without doing normal reads or writes.
The device driver can use the data for information. It is
up to the device to interpret the string, but the information
must not be treated as a normal I/O request. Affects
only IOCTL functions AL=2 and AL=5.
0F device type: used to indicate block or character device.
:device command codes:device codes:device commands
^Device Command Codes (Device Request Header)
% Code Function
0 INIT
1 MEDIA CHECK (block devices,character = NOP)
2 BUILD BPB (block devices,character = NOP)
3 IOCTL
4 INPUT (read)
5 NONDESTRUCTIVE INPUT NO WAIT (character devices)
6 INPUT STATUS (character devices)
7 INPUT FLUSH (character devices)
8 OUTPUT (write)
9 OUTPUT (write with verify)
10 OUTPUT STATUS (character devices)
11 OUTPUT FLUSH (character devices)
12 IOCTL OUTPUT
13 DEVICE OPEN (DOS 3.x)
14 DEVICE CLOSE (DOS 3.x)
15 REMOVABLE MEDIA (DOS 3.x)
:device header
^Device Driver Header
% Offset Size Description
00 dword pointer to next device header
04 word attribute (see ~DEVICE ATTRIBUTE~)
06 word pointer to device strategy routine
08 word pointer to device interrupt routine
0A 8bytes name/unit field
- see ~INT 21,44~ and IOCTL,n where "n" is an IOCTL function
:device request header
^Device Request Header Format
% Offset Size Description
00 byte length in bytes of the request header
01 byte unit code; the sub-unit the operation is for (block
devices); meaningless for character devices
02 word command code, (see ~DEVICE COMMANDS~)
04 8bytes reserved for DOS
0C nbytes request data (variable length)
- see ~INT 21,44~ or ~DEVICE ATTRIBUTES~ or ~DEVICE CODES~
- see also IOCTL,n where "n" is an IOCTL function
:device status
^Device Status Word
The device status word is set to zero on entry and is set by
the driver interrupt routine on return.
ł15ł14-10ł9ł8ł7-0ł STATUS word
ł ł ł ł ŔÄÄÄÄ Error return code (if bit 15=1)
ł ł ł ŔÄÄÄÄÄÄ Done bit, function completed
ł ł ŔÄÄÄÄÄÄÄ Busy bit
ł ŔÄÄÄÄÄÄÄÄÄÄ Reserved
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄ Error flag bits 0-7 have error code
% Error return codes
00 Write protect violation 01 Unknown unit
02 Device not ready 03 Unknown command
04 CRC error 05 Bad drive request structure length
06 Seek error 07 Unknown media
08 Sector not found 09 Printer out of paper
0A Write fault 0B Read fault
0C General failure 0D Reserved
0E Reserved 0F Invalid disk change
:directory format
^DIRECTORY - DOS Directory Structure
% Byte Description
00 Filename status:
00 = Filename never used
05 = First character of filename is E5
E5 = File has been erased
2E = This is a subdirectory entry
00-07 Filename, left justified
08-0A Filename extension, left justified
0B File's attribute:
ł7ł6ł5ł4ł3ł2ł1ł0ł byte 0B
ł ł ł ł ł ł ł ŔÄÄÄ read only
ł ł ł ł ł ł ŔÄÄÄÄ hidden
ł ł ł ł ł ŔÄÄÄÄÄ system
ł ł ł ł ŔÄÄÄÄÄÄ volume label
ł ł ł ŔÄÄÄÄÄÄÄ subdirectory
ł ł ŔÄÄÄÄÄÄÄÄ archive
ŔÄÁÄÄÄÄÄÄÄÄÄ unused
0C-15 Reserved by DOS
16-17 Time the file was created or last updated:
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł 17,16
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄ seconds/2
ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄ minutes
ŔÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ hours
18-19 Date the file was created or last updated:
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł 19,18
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄ day 1-31
ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄ month 1-12
ŔÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ year + 1980
1A-1B Starting cluster number of the first file cluster
1C-1F File size in bytes (low order first)
:DBT:Disk Base Table
^DBT - Disk Base Table (BIOS INT 13)
% Offset Size Description
00 byte specify byte 1; step-rate time, head unload time
01 byte specify byte 2; head load time, DMA mode
02 byte timer ticks to wait before disk motor shutoff
03 byte bytes per sector code:
0 - 128 bytes 2 - 512 bytes
1 - 256 bytes 3 - 1024 bytes
04 byte sectors per track (last sector number)
05 byte inter-block gap length/gap between sectors
06 byte data length, if sector length not specified
07 byte gap length between sectors for format
08 byte fill byte for formatted sectors
09 byte head settle time in milliseconds
0A byte motor startup time in eighths of a second
:DTA:Disk Transfer Area
^DTA - Disk Transfer Area (partially undocumented)
DTA contains data, of which the first 21 bytes (00-15h) are known
as being "reserved for DOS use on subsequent find next calls"
% Offset Size Description
00 byte attribute of search (undocumented)
01 byte drive used in search (undocumented)
02 11bytes search name used (undocumented)
0D word directory entry number (0 based, DOS 3.x+, undoc.)
0F word starting cluster number of current directory
zero for root directory (DOS 3.x+, undocumented)
11 word reserved (undocumented)
13 word starting cluster number of current directory
zero for root directory (DOS 2.x+, undocumented)
15 byte attribute of matching file
16 word file time (see ~FILE ATTRIBUTES~)
18 word file date (see FILE ATTRIBUTES)
1A word file size
1E 13bytes ASCIIZ filename and extension in the form NAME.EXT
with blanks stripped
- fields of DTA change dependent upon function call in progress
- the ~DTA~ cannot span a 64K segment boundary
- for compatibility with CP/M the default DTA is at offset 80h
in the ~PSP~; this area is also used for the command tail. To
avoid collision, set another DTA (INT 21,1A) or preserve the
command tail before using ~FCB~ function calls
- see ~INT 21,4E~ ~INT 21,1A~ ~INT 21,2F~
:DOS error codes:extended errors
^DOS Error Codes
Of the following error codes, only error codes 1-12 are
returned in AX upon exit from interrupt 21 or 24; The rest
are obtained by issuing the "get extended error" function
call; see ~INT 21,59~
01 Invalid function number
02 File not found
03 Path not found
04 Too many open files (no handles left)
05 Access denied
06 Invalid handle
07 Memory control blocks destroyed
08 Insufficient memory
09 Invalid memory block address
0A Invalid environment
0B Invalid format
0C Invalid access mode (open mode is invalid)
0D Invalid data
0E Reserved
0F Invalid drive specified
10 Attempt to remove current directory
11 Not same device
12 No more files
13 Attempt to write on a write-protected diskette
14 Unknown unit
15 Drive not ready
16 Unknown command
17 CRC error
18 Bad request structure length
19 Seek error
1A Unknown media type
1B Sector not found
1C Printer out of paper
1D Write fault
1E Read fault
1F General failure
20 Sharing violation
21 Lock violation
22 Invalid disk change
23 FCB unavailable
24 Sharing buffer overflow
25 Reserved
26 Unable to complete file operation (DOS 4.x)
27-31 Reserved
32 Network request not supported
33 Remote computer not listening
34 Duplicate name on network
35 Network name not found
36 Network busy
37 Network device no longer exists
38 NetBIOS command limit exceeded
39 Network adapter error
3A Incorrect network response
3B Unexpected network error
3C Incompatible remote adapter
3D Print queue full
3E No space for print file
3F Print file deleted
40 Network name deleted
41 Access denied
42 Network device type incorrect
43 Network name not found
44 Network name limit exceeded
45 NetBIOS session limit exceeded
46 Temporarily paused
47 Network request not accepted
48 Print or disk redirection is paused
49-4F Reserved
50 File already exists
51 Reserved
52 Cannot make directory entry
53 Fail on INT 24
54 Too many redirections
55 Duplicate redirection
56 Invalid password
57 Invalid parameter
58 Network device fault
59 Function not supported by network (DOS 4.x)
5A Required system component not installed (DOS 4.x)
^DOS Error Code/Classes
% Error Classes
01 Out of resource, out of space, channel, etc
02 Temporary situation, not an error, ex: file lock
03 Authorization, permission denied
04 Internal, system detected internal error
05 Hardware failure, serious problem related to hardware
06 System failure, ex: invalid configuration
07 Application error, inconsistent request
08 Not found, file/item not found
09 Bad format, file/item in invalid format
0A Locked, file/item interlocked
0B Media failure, ECC/CRC error, wrong or bad disk
0C Already exists, collision with existing item
0D Unknown, classification doesn't exist or is inappropriate
^DOS Error Code/Action Codes and Locus
% Error Action Codes (in BL)
01 retry, attempt a few more times and re-prompt
02 delay retry, retry a few more times after a pause
03 re-enter input, prompt user to re-enter input
04 abort with cleanup, orderly abort and shutdown
05 immediate abort, exit immediately without cleanup
06 ignore error
07 user intervention, retry after user fixes the problem
% Error Locus (in CH)
01 unknown
02 block device
03 network
04 serial device
05 memory
:DOS versions:version
^VERSION - Versions of DOS
% Version Date Changes
PC-DOS 1.0 Oct 1981 original release, single sided drive
PC-DOS 1.1 Jun 1982 bugfix, double sided drive support
MS-DOS 1.25 Jun 1982 for early compatibles
PC-DOS 2.0 Mar 1983 PC/XT, added (hard drive &UNIX features)
PC-DOS 2.1 Oct 1983 PCjr & portable mods, fixes for 2.0
MS-DOS 2.11 Oct 1983 compatible equivalent to 2.1
PC-DOS 3.0 Aug 1984 support for 1.2 Mb drive (AT)
PC-DOS 3.1 Nov 1984 added network support, fixes for 3.0
MS-DOS 2.25 Oct 1985 compatible; foreign language support
PC-DOS 3.2 Jul 1986 720k 3«" drive support for Convertible
MS-DOS 3.2 Jul 1986
MS-DOS 3.21 May 1987
MS-DOS 3.3 Jul 1987
PC-DOS 3.3 Apr 1987 PS/2, 1.44 disk support, mult. partitions
MS-DOS 3.30a Feb 1988
CPQ-DOS 3.31 Oct 1988 Compaq DOS for disk partitions > 32MB
PC-DOS 4.00 Aug 1988 Larger DOS partitions, EMS support (bugs)
MS-DOS 4.00 Oct 1988
MS-DOS 4.01 Nov 1988
MS-DOS 4.01a Apr 1989
PC-DOS 4.01 ??? ???? Fixes for major bugs in 4.0
- release dates vary between OEM versions
- IBM was supposedly responsible for most changes in DOS 4.x
- DOS 4.01 reports version 4.0, except in some vendor versions
- DOS 5.0 is primarily a Microsoft developed version
:drive parameter table:disk parameter table:DPB:DPT
^DPT/DPB - Drive Parameter Table / Disk Parameter Block
% Offset Size Description
00 byte drive (0 = A, 1 = B)
01 byte unit within device, usually equals drive (ramdisk=0)
02 word bytes per sector
04 byte sectors per cluster minus 1
05 byte sectors per cluster (times to shift left or x2)
06 word number of sectors before FAT (boot sectors)
08 byte number of ~FAT~ copies
09 word number of root directory entries
0B word number of first data sector
0D word total number of clusters plus 1
0F byte number of sectors used by first FAT
% The following fields are DOS version dependant
10 word number of first sector in root directory
12 dword far pointer to current disk device header
16 byte media descriptor byte (see ~MEDIA DESCRIPTOR~)
17 byte zero if disk accessed, (default=FF, must rebuild ~DPB~)
18 dword far pointer to next drive parameter table; offset
is set to FFFFh if last block in chain
1C word current directory cluster number; 0=root
1E 64bytes ASCIIZ current working directory
^Fields differing in DOS 3.x
% Offset Size Description
1C word starting cluster for free space search
1E 64bytes number of free clusters; FFFFh = unknown
^Fields differing in DOS 4.x
% Offset Size Description
11 word first sector of root directory
13 dword far pointer to current disk device header
17 byte media descriptor byte (see ~MEDIA DESCRIPTOR~)
18 byte zero if disk accessed, (default=FF, must rebuild ~DPB~)
19 dword pointer to next drive parameter table; offset
is set to FFFFh if last block in chain
1D word starting cluster for free space search
1F word number of free clusters, FFFFh = unknown
- ~INT 21,32~ which is used to read this data resets accessed byte at
offset 17h or 18h (depending on DOS version)
- dword at offset 12h & 13h aren't supported in OS/2 compatability box
- sector references are DOS logical sectors
- see ~Disk Base Table~ ~BPB~ ~INT 21,1F~ ~INT 21,32~
:drive status data:disk status data:diskette status data
^Drive Status Bytes Found in BIOS Data Area
% Drive recalibration status (at 40:3E)
ł7ł6ł5ł4ł3ł2ł1ł0ł byte at 40:3E
ł ł ł ł ł ł ł ŔÄÄÄ 1=recalibrate drive 0
ł ł ł ł ł ł ŔÄÄÄÄ 1=recalibrate drive 1
ł ł ł ł ł ŔÄÄÄÄÄ 1=recalibrate drive 2
ł ł ł ł ŔÄÄÄÄÄÄ 1=recalibrate drive 3
ł ŔÄÁÄÁÄÄÄÄÄÄÄ unused
ŔÄÄÄÄÄÄÄÄÄÄÄÄ 1=working interrupt flag
% Diskette motor status
ł7ł6ł5ł4ł3ł2ł1ł0ł byte at 40:3F
ł ł ł ł ł ł ł ŔÄÄÄ 1=drive 0 motor on
ł ł ł ł ł ł ŔÄÄÄÄ 1=drive 1 motor on
ł ł ł ł ł ŔÄÄÄÄÄ 1=drive 2 motor on
ł ł ł ł ŔÄÄÄÄÄÄ 1=drive 3 motor on
ł ŔÄÁÄÁÄÄÄÄÄÄÄ unused
ŔÄÄÄÄÄÄÄÄÄÄÄÄ 1=write operation
% Disk Status Byte
ł7ł6ł5ł4ł3ł2ł1ł0ł byte at 40:41
ł ł ł ł ł ł ł ŔÄÄÄ invalid diskette command
ł ł ł ł ł ł ŔÄÄÄÄ diskette address mark not found
ł ł ł ł ł ŔÄÄÄÄÄ sector not found
ł ł ł ł ŔÄÄÄÄÄÄ diskette DMA error
ł ł ł ŔÄÄÄÄÄÄÄ CRC check / data error
ł ł ŔÄÄÄÄÄÄÄÄ diskette controller failure
ł ŔÄÄÄÄÄÄÄÄÄ seek to track failed
ŔÄÄÄÄÄÄÄÄÄÄ diskette time-out
% Last diskette data rate selected
ł7ł6ł5ł4ł3ł2ł1ł0ł byte at 40:8B
ł ł ł ł ŔÄÁÄÁÄÁÄÄÄ step rate time selected (see below)
ŔÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄ data rate selected (see below)
% Bits
% 76 Diskette Data Rate Selected
00 500K bps
01 300K bps
10 250K bps
11 reserved
% Bits
% 54 Diskette Step Rate Time Selected
00 step rate time of 0C
01 step rate time of 0D
10 step rate time of 0A
11 reserved
% Media state for fixed drives (bytes at 40:90-93)
ł7ł6ł5ł4ł3ł2ł1ł0ł bytes at 40:90-40:93
ł ł ł ł ł ŔÄÁÄÁÄÄÄ reserved (set to 1)
ł ł ł ł ŔÄÄÄÄÄÄÄÄ reserved
ł ł ł ŔÄÄÄÄÄÄÄÄÄ 0=media/drive unestablished
ł ł ŔÄÄÄÄÄÄÄÄÄÄ reserved
ŔÄÁÄÄÄÄÄÄÄÄÄÄÄ data rate (see below)
% Bits
% 76 Fixed Disk Data Rate Selected
00 500K bps
01 reserved
10 250K bps
11 reserved
- not all OEM's implemented these bytes identically. Some are
also AT specific.
:EXE file header:.EXE
^.EXE - DOS EXE File Structure
% Offset Size Description
00 word "MZ" - Link file .EXE signature (Mark Zbikowski?)
02 word length of image mod 512
04 word size of file in 512 byte pages
06 word number of relocation items following header
08 word size of header in 16 byte paragraphs, used to locate
the beginning of the load module
0A word min # of paragraphs needed to run program
0C word max # of paragraphs the program would like
0E word offset in load module of stack segment (in paras)
10 word initial SP value to be loaded
12 word negative checksum of pgm used while by EXEC loads pgm
14 word program entry point, (initial IP value)
16 word offset in load module of the code segment (in paras)
18 word offset in .EXE file of first relocation item
1A word overlay number (0 for root program)
- relocation table and the program load module follow the header
- relocation entries are 32 bit values representing the offset
into the load module needing patched
- once the relocatable item is found, the CS register is added to
the value found at the calculated offset
% Registers at load time of the EXE file are as follows:
AX: contains number of characters in command tail, or 0
BX:CX 32 bit value indicating the load module memory size
DX zero
SS:SP set to stack segment if defined else, SS = CS and
SP=FFFFh or top of memory.
DS set to segment address of EXE header
ES set to segment address of EXE header
CS:IP far address of program entry point, (label on "END"
statement of program)
:EBDA:extended BIOS
^EBDA - Extended BIOS Data Area EBDA (PS/2)
% Offset Size Description
00 word number of bytes allocated to EBDA in Kbytes
01-21 21bytes reserved
22 dword device driver far call pointer
26 byte pointing device flag (1st byte, see below)
27 byte pointing device flag (2nd byte, see below)
28-2F 8 bytes reserved
% Pointing Device Flag Byte 1
ł7ł6ł5ł4ł3ł2ł1ł0ł Offset 26
ł ł ł ł ł ŔÄÁÄÁÄÄÄ index count
ł ł ł ł ŔÄÄÄÄÄÄÄÄ reserved (0)
ł ł ł ŔÄÄÄÄÄÄÄÄÄ error
ł ł ŔÄÄÄÄÄÄÄÄÄÄ acknowledge
ł ŔÄÄÄÄÄÄÄÄÄÄÄ resend
ŔÄÄÄÄÄÄÄÄÄÄÄÄ command in progress
% Pointing Device Flag Byte 2
ł7ł6ł5ł4ł3ł2ł1ł0ł Offset 27
ł ł ł ł ł ŔÄÁÄÁÄÄÄ package size
ł ŔÄÁÄÁÄÁÄÄÄÄÄÄÄÄ reserved
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ device driver far call flag
- EBDA is located in highest memory just under 640K on PS/2
- word at ~BIOS Data Area~ 40:0E is segment address of EBDA
:FAT:File Allocation Table
^FAT - File Allocation Table
% 12 Bit Meaning 16 Bit
000 free space 0000
FF1-FF7 bad track marking FFF1-FFF7
FF8-FFE may be used to mark end of a file chain FFF8-FFFE
FFF standard marker for end of a file chain FFFF
- the FAT is implemented as an array containing a linked list
for each file; the files directory entry has a pointer to the
first cluster which contains the cluster number of the next
cluster in the chain until the pointer contained is FFFh
(12 bit FAT) and FFFFh (16 bit FAT) marking end of file
- DOS maintains two copies of the FAT, but does not use the
second copy for anything other than a mirror image of the
first; CHKDSK doesn't even read the second FAT
- disks with FF1h clusters and above use 16 bit FAT tables, disk
with less use 12 bit FAT tables
- DOS 4.x did not change the size of the cluster number as some
suggest, but instead increased the size of the sector number
- bytes 0 of the FAT contains the Media Descriptor Byte
^Calculating 12 bit FAT Entries
1. Get starting cluster from directory entry.
2. Multiply the cluster number just used by 1.5
3. The whole part of the product is the offset into the FAT,
of the entry that maps to the cluster in the directory.
This entry contains the number of the next cluster.
4. Move the word at the calculated FAT into a register.
5. If the last cluster used was an even number, keep the low order
12 bits of the register, otherwise, keep the high order 12 bits.
6. If the resultant 12 bits are (0FF8h-0FFFh) no more clusters
are in the file. Otherwise, the next 12 bits contain the
cluster number of the next cluster in the file.
^Calculating 16 Bit FAT Entries
1. Get the starting cluster of the file from the directory.
2. Multiply the cluster number found by 2.
3. Load the word at the calculated FAT offset into a register.
4. If the 16 bits are (0FFF8h-0FFFFh) no more clusters are in
the file. Otherwise, the 16 bits contain the cluster number
of the next cluster in the file.
To convert the cluster to a logical sector number (relative
sector, similar to that used by DEBUG, int 25h and 26h):
1. Subtract 2 from the cluster number
2. Multiply the result by the number of sectors per cluster.
3. Add the logical sector number of the beginning of the data area.
- see ~MEDIA DESCRIPTOR~
:FCB:File Control Block
^FCB - Standard DOS File Control Block
% Offset Size Description
-7 byte if FF this is an extended FCB Ř
-6 5bytes reserved Ř
-1 byte file attribute if extended FCB Ř
00 byte drive number (0 for default drive, 1=A:, 2=B:, ...)
01 8bytes filename, left justified with trailing blanks
09 3bytes filename extension, left justified w/blanks
0C word current block number relative to beginning of the
file, starting with zero
0E word logical record size in bytes
10 dword file size in bytes
14 word date the file was created or last updated
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł 15,14 (Intel reverse order)
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄ day 1-31
ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄ month 1-12
ŔÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ year + 1980
16 word time of last write
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł 17,16 (Intel reverse order)
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄÄ secs in 2 second increments
ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄ minutes (0-59)
ŔÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ hours (0-23)
18 8bytes see below for version specific information Ř
1A dword address of device header if character device Ř
20 byte current relative record number within current BLOCK
21 dword relative record number relative to the beginning of
the file, starting with zero; high bit omitted if
record length is 64 bytes
^DOS 2.x Values for reserved fields at offsets 18h-1Ah Ř
% Offset Size Description
18 byte ł7ł6ł5ł4ł3ł2ł1ł0ł
ł ł ŔÄÁÄÁÄÁÄÁÄÁÄÄÄ unknown
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1 = open
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ 1 = logical device
19 word starting cluster number Ř
^DOS 3.x Values for reserved fields at offsets 18h-19h Ř
% Offset Size Description
18 byte System File Table (SFT) entry for file Ř
19 byte attributes Ř
ł7ł6ł5ł4ł3ł2ł1ł0ł attributes
ł ł ŔÄÁÄÁÄÁÄÁÄÁÄÄÄÄ unknown
ŔÄÁÄ share status
00 = SHARE not loaded block device
01 = SHARE not loaded characted device
10 = SHARE loaded, remote file
11 = SHARE loaded local file
^DOS 3.x with SHARE, local file reserved offsets 1Ah-1Eh Ř
% Offset Size Description
1A word starting cluster number
1C word offset within SHARE of sharing record
1E byte file attribute
^DOS 3.x with SHARE, remote file reserved offsets 1Ah-1Eh Ř
% Offset Size Description
1A word sector number containing directory entry
1C word last cluster accessed relative to beginning of file
1E byte absolute cluster number of last cluster accessed
^DOS 3.x without SHARE reserved offsets 1Ah-1Fh Ř
% Offset Size Description
1A byte ((device attribute word low byte) & 0Ch) || (open mode)
1B word starting cluster number
1D word sector number containing directory entry
1F byte number of directory entry within sector
% The following are FCB related DOS functions:
~INT 21,F~ Open file using FCB
~INT 21,10~ Close file using FCB
~INT 21,11~ Search for first entry using FCB
~INT 21,12~ Search for next entry using FCB
~INT 21,13~ Delete file using FCB
~INT 21,14~ Sequential read using FCB
~INT 21,15~ Sequential write using FCB
~INT 21,16~ Create a file using FCB
~INT 21,17~ Rename file using FCB
~INT 21,21~ Random read using FCB
~INT 21,22~ Random write using FCB
~INT 21,23~ Get file size using FCB
~INT 21,24~ Set relative record field for FCB
~INT 21,27~ Random block read using FCB
~INT 21,28~ Random block write using FCB
~INT 21,29~ Parse filename for FCB
Ř see ~Bibliography~ reference to "Undocumented DOS"
- see ~XFCB~ ~INT 21,52~
:file attributes
^DOS File Attributes
% Directory Attribute Flags
ł7ł6ł5ł4ł3ł2ł1ł0ł Directory Attribute Flags
ł ł ł ł ł ł ł ŔÄÄÄ 1 = read only
ł ł ł ł ł ł ŔÄÄÄÄ 1 = hidden
ł ł ł ł ł ŔÄÄÄÄÄ 1 = system
ł ł ł ł ŔÄÄÄÄÄÄ 1 = volume label (exclusive)
ł ł ł ŔÄÄÄÄÄÄÄ 1 = subdirectory
ł ł ŔÄÄÄÄÄÄÄÄ 1 = archive
ŔÄÁÄÄÄÄÄÄÄÄÄ unused
% Directory Time Format
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł Directory Time Format
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄÄÄ seconds (2 second increments)
ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄ minutes (0-59)
ŔÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ hours (0-23)
% Directory Date Format
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł Directory Date Format
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄÄÄ Day (1-31)
ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄ Month (1-12)
ŔÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ Year (less 1980)
- with respect to the ~findfirst~() and ~findnext~() functions normal
files are always included along with the requested attributes
except when the LABEL attribute is requested. It's up to the
programmer to determine which files are match the requested
attributes.
:file handles:default handles:handles
^DOS Default/Predefined File Handles
0 - Standard Input Device - can be redirected (STDIN)
1 - Standard Output Device - can be redirected (STDOUT)
2 - Standard Error Device - can be redirected (STDERR)
3 - Standard Auxiliary Device (STDAUX)
4 - Standard Printer Device (STDPRN)
%See the following INT 21h Handle related function calls:
~INT 21,3C~ Create file using handle
~INT 21,3D~ Open file using handle
~INT 21,3E~ Close file using handle
~INT 21,3F~ Read file or device using handle
~INT 21,40~ Write file or device using handle
~INT 21,41~ Delete file
~INT 21,42~ Move file pointer using handle
~INT 21,43~ Change file mode
~INT 21,45~ Duplicate file handle
~INT 21,46~ Force duplicate file handle
~INT 21,56~ Rename file
~INT 21,57~ Get/set file date and time using handle
~INT 21,5A~ Create temporary file (3.x+)
~INT 21,5B~ Create new file (3.x+)
~INT 21,67~ Set handle count (3.3+)
~INT 21,68~ Flush buffer (3.3+)
- STDIN, STDOUT and STDERR can be redirected
- maximum number of files available to all DOS processes at one
time is defined by the FILES=N statement of ~CONFIG.SYS~
- maximum number of files available to an application is N-3 where
N is derived from the CONFIG.SYS FILES=N statement minus the
count of handles used by other processes
- ~INT 21,67~ can be used in DOS 3.3+ to increase the number of file
handles for an application to greater than 20; the max handle
count is still limited by the value of FILES= in CONFIG.SYS file
- An application can increase the maximum allowed file handles in
DOS 3.0-3.2 by copying the open file table located through offset
34h in the ~PSP~ to another location. All unopened slots must be
initialized to FF and the values at PSP offsets 32h and 34h must
be updated to reflect the new values.
- see ~SFT~ ~INT 21~ ~INT 21,52~ ~INT 21,67~
:float formats:floating point:real numbers
^Floating Point Formats
% IEEE 4 byte real
31 30 23 22 0
ÚÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
łsł 8 bits łmsb 23 bit mantissa lsbł
ŔÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ mantissa
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ biased exponent (7fh)
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sign bit
% IEEE 8 byte real
63 62 52 51 0
ÚÄÂÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
łsł 11 bits łmsb 52 bit mantissa lsbł
ŔÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ mantissa
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ biased exponent (3FFh)
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sign bit
% Microsoft 4 byte real
31 24 23 22 0
ÚÄÄÄÄÄÄÄÄÂÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
ł 8 bits łsłmsb 23 bit mantissa lsbł
ŔÄÄÄÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ mantissa
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sign bit
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ biased exponent (81h)
% Microsoft 8 byte real (see note below)
63 56 55 54 0
ÚÄÄÄÄÄÄÄÂÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
ł 8bits łsłmsb 52 bit mantissa lsbł
ŔÄÄÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄ mantissa
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sign bit
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ biased exponent (401h, see below)
% IEEE 10 byte real (temporary real)
79 78 64 63 62 0
ÚÄÂÄÄÄÄÄÄÄÄÄÄÄÂÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
łsł 15 bits ł1łmsb 63 bit mantissa lsbł
ŔÄÁÄÄÄÄÄÄÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
ł ł ł ŔÄÄÄÄÄ mantissa
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ first mantissa bit
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ biased exponent (3FFFh)
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sign bit
% Turbo Pascal 6 byte real
47 40 39 38 0
ÚÄÄÄÄÄÄÄÄÂÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
ł 8 bits łsłmsb 39 bit mantissa lsbł
ŔÄÄÄÄÄÄÄÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
ł ł ŔÄÄÄÄÄÄÄÄÄÄÄÄ mantissa
ł ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ sign bit
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ biased exponent (80h)
% Microsoft Fortran Complex number
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄż
ł Float Real component ł Float Imaginary component ł
ŔÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄŮ
(each component is either 8 or 16 byte IEEE real)
- sign bit representation: 0 is positive and 1 is negative
- in all float formats except the IEEE 10 byte real, the
mantissa is stored without most significant bit; since
the state of this bit is known to be set, it is not
included and the exponent is adjusted accordingly
- all formats use binary float representation
- memory representation uses 80x86 reverse byte/word order.
- Microsoft languages use the IEEE real formats; BASIC is the
only normal user of the Microsoft float format
- Microsoft 8 byte real format has not been verified; several
Microsoft publications show an 8 bit exponent instead of 11 bits
and state the BIAS is 401h; the discrepancy is that 8 bits can't
hold the value 401h (requires 11 bits)
% True exponent is the exponent value minus the following bias:
81h for Microsoft 4 byte real
401h for Microsoft 8 byte real
7Fh for IEEE 4 byte real
3FFh for IEEE 8 byte real
80h for Turbo Pascal 6 byte real
% Size Range Significant digits
4 byte real 8.43x10E-37 to 3.37x10E38 6-7
8 byte real 4.19x10E-307 to 1.67x10E308 15-16
10 byte real 3.4x10E-4932 to 1.2x10E4932 19
- see ~dmsbintoieee~() ~dieeetomsbin~() ~NUMERIC RANGES~
:floppy formats:disk formats
^Common Floppy Disk Formats
% Supporting ÚÄÄÄÄÄSectorsÄÄÄÄż Entries
% Disk DOS per per per per per Total
% Type Vers Sides Trks TRK FAT DIR Cluster DIR Sectors
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
5¬ 160k (1.0) 1 (40) 8 1 4 1 64 320
5¬ 320k (1.1) 2 (40) 8 1 7 2 112 640
5¬ 180k (2.0) 1 (40) 9 2 4 1 64 360
5¬ 360k (2.0) 2 (40) 9 2 7 2 112 720
5¬ 1.2M (3.0) 2 (80) 15 7 14 1 224 2400
3« 720k (3.2) 2 (80) 9 3 7 2 112 1440
3« 1.44M (3.3) 2 (80) 18 9 14 1 224 2880
:ID bytes:machine identificatn
^Machine Identification Byte
% Model Sub-Model Machine
FF ?? Original IBM PC 4/24/81
?? IBM PC 10/19/81
?? IBM PC 10/27/82
FE ?? IBM XT (Original)
?? IBM portable PC
?? Compaq DeskPro
FD ?? PCjr
FC ?? IBM AT (6 MHz)
01 IBM AT 3x9 (8 MHz)
02 IBM XT 286
04 IBM PS/2 Model 50
05 IBM PS/2 Model 60
0B IBM PS/1
FB 00 IBM 256/640K XT (aka XT/2)
FA 00 IBM PS/2 Model 30
01 IBM PS/2 Model 25
F9 00 IBM PC Convertible
F8 00 IBM PS/2 Model 80 (16 MHz)
01 IBM PS/2 Model 80 (20 MHz)
04 IBM PS/2 Model 70 (20 MHz)
09 IBM PS/2 Model 70 (16 MHz)
B6 ?? Hewlett Packard 110
9A ?? Compaq Plus
86 XT (BIOS 11/82+) & AT (BIOS 1/84+) (see note)
80 PC & PCjr (see note)
2D ?? Compaq PC
- found at memory location F000:FFFE or via INT 15,C0
- model values of 80 & 86 are returned by BIOS versions previous
to the PS/2 but after the BIOS dates marked
- see ~INT 15,C0~
:KB flags:keyboard flags
^Keyboard Flags Bytes 0 and 1
ł7ł6ł5ł4ł3ł2ł1ł0ł 40:17 Keyboard Flags Byte 0
ł ł ł ł ł ł ł ŔÄÄÄÄ right shift key depressed
ł ł ł ł ł ł ŔÄÄÄÄÄ left shift key depressed
ł ł ł ł ł ŔÄÄÄÄÄÄ CTRL key depressed
ł ł ł ł ŔÄÄÄÄÄÄÄ ALT key depressed
ł ł ł ŔÄÄÄÄÄÄÄÄ scroll-lock is active
ł ł ŔÄÄÄÄÄÄÄÄÄ num-lock is active
ł ŔÄÄÄÄÄÄÄÄÄÄ caps-lock is active
ŔÄÄÄÄÄÄÄÄÄÄÄ insert is active
ł7ł6ł5ł4ł3ł2ł1ł0ł 40:18 Keyboard Flags Byte 1
ł ł ł ł ł ł ł ŔÄÄÄÄ left CTRL key depressed
ł ł ł ł ł ł ŔÄÄÄÄÄ left ALT key depressed
ł ł ł ł ł ŔÄÄÄÄÄÄ system key depressed and held
ł ł ł ł ŔÄÄÄÄÄÄÄ suspend key has been toggled
ł ł ł ŔÄÄÄÄÄÄÄÄ scroll lock key is depressed
ł ł ŔÄÄÄÄÄÄÄÄÄ num-lock key is depressed
ł ŔÄÄÄÄÄÄÄÄÄÄ caps-lock key is depressed
ŔÄÄÄÄÄÄÄÄÄÄÄ insert key is depressed
^Keyboard Flags Bytes 2 and 3
ł7ł6ł5ł4ł3ł2ł1ł0ł 40:97 LED Indicator Flags
ł ł ł ł ł ł ł ŔÄÄÄÄ scroll lock indicator
ł ł ł ł ł ł ŔÄÄÄÄÄ num-lock indicator
ł ł ł ł ł ŔÄÄÄÄÄÄ caps-lock indicator
ł ł ł ł ŔÄÄÄÄÄÄÄ circus system indicator
ł ł ł ŔÄÄÄÄÄÄÄÄ ACK received
ł ł ŔÄÄÄÄÄÄÄÄÄ re-send received flag
ł ŔÄÄÄÄÄÄÄÄÄÄ mode indicator update
ŔÄÄÄÄÄÄÄÄÄÄÄ keyboard transmit error flag
ł7ł6ł5ł4ł3ł2ł1ł0ł 40:96 Keyboard Mode/Type
ł ł ł ł ł ł ł ŔÄÄÄÄ last code was the E1 hidden code
ł ł ł ł ł ł ŔÄÄÄÄÄ last code was the E0 hidden code
ł ł ł ł ł ŔÄÄÄÄÄÄ right CTRL key depressed
ł ł ł ł ŔÄÄÄÄÄÄÄ right ALT key depressed
ł ł ł ŔÄÄÄÄÄÄÄÄ 101/102 enhanced keyboard installed
ł ł ŔÄÄÄÄÄÄÄÄÄ force num-lock if Rd ID & KBX
ł ŔÄÄÄÄÄÄÄÄÄÄ last char was first ID char
ŔÄÄÄÄÄÄÄÄÄÄÄ read ID in process
- see ~INT 9~ ~BDA~
:make codes:break codes
^INT 9 - Hardware Keyboard Make/Break Codes
% Key Make Break Key Make Break
Backspace 0E 8E F1 3B BB
Caps Lock 3A BA F2 3C BC
Enter 1C 9C F3 3D BD
Esc 01 81 F4 3E BE
Left Alt 38 B8 F7 41 C1
Left Ctrl 1D 9D F5 3F BF
Left Shift 2A AA F6 40 C0
Num Lock 45 C5 F8 42 C2
Right Shift 36 B6 F9 43 C3
Scroll Lock 46 C6 F10 44 C4
Space 39 B9 F11 57 D7
Sys Req (AT) 54 D4 F12 58 D8
Tab 0F 8F
% Keypad Keys Make Break
Keypad 0 (Ins) 52 D2
Keypad 1 (End) 4F CF
Keypad 2 (Down arrow) 50 D0
Keypad 3 (PgDn) 51 D1
Keypad 4 (Left arrow) 4B CB
Keypad 5 4C CC
Keypad 6 (Right arrow) 4D CD
Keypad 7 (Home) 47 C7
Keypad 8 (Up arrow) 48 C8
Keypad 9 (PgUp) 49 C9
Keypad . (Del) 53 D3
Keypad * (PrtSc) 37 B7
Keypad - 4A CA
Keypad + 4E CE
% Key Make Break Key Make Break
A 1E 9E N 31 B1
B 30 B0 O 18 98
C 2E AE P 19 99
D 20 A0 Q 10 90
E 12 92 R 13 93
F 21 A1 S 1F 9F
G 22 A2 T 14 94
H 23 A3 U 16 96
I 17 97 V 2F AF
J 24 A4 W 11 91
K 25 A5 X 2D AD
L 26 A6 Y 15 95
M 32 B2 Z 2C AC
% Key Make Break Key Make Break
1 02 82 - 0C 8C
2 03 83 = 0D 8D
3 04 84 [ 1A 9A
4 05 85 ] 1B 9B
5 06 86 ; 27 A7
6 07 87 ' 28 A8
7 08 88 ` 29 A9
8 09 89 \ 2B AB
9 0A 8A , 33 B3
0 0B 8B . 34 B4
/ 35 B5
^Enhanced Keyboard Keys (101/102 keys)
% Control Keys Make Break
Alt-PrtSc (SysReq) 54 D4
Ctrl-PrtSc E0 37 E0 B7
Enter E0 1C E0 9C
PrtSc E0 2A E0 37 E0 B7 E0 AA
Right Alt E0 38 E0 B8
Right Ctrl E0 1D E0 9D
Shift-PrtSc E0 37 E0 B7
/ E0 35 E0 B5
Pause E1 1D 45 E1 9D C5 (not typematic)
Ctrl-Pause (Ctrl-Break) E0 46 E0 C6 (not typematic)
- Keys marked as "not typematic" generate one stream of bytes
without corresponding break scan code bytes (actually the
break codes are part of the make code).
% Normal Mode or
% Shift w/Numlock
% Key Make Break ÚÄÄÄÄÄ Numlock on ÄÄÄÄÄÄż
% Make Break
Del E0 53 E0 D3 E0 2A E0 53 E0 D3 E0 AA
Down arrow E0 50 E0 D0 E0 2A E0 50 E0 D0 E0 AA
End E0 4F E0 CF E0 2A E0 4F E0 CF E0 AA
Home E0 47 E0 C7 E0 2A E0 47 E0 C7 E0 AA
Ins E0 52 E0 D2 E0 2A E0 52 E0 D2 E0 AA
Left arrow E0 4B E0 CB E0 2A E0 4B E0 CB E0 AA
PgDn E0 51 E0 D1 E0 2A E0 51 E0 D1 E0 AA
PgUp E0 49 E0 C9 E0 2A E0 49 E0 C9 E0 AA
Right arrow E0 4D E0 CD E0 2A E0 4D E0 CD E0 AA
Up arrow E0 48 E0 C8 E0 2A E0 48 E0 C8 E0 AA
% Key ÚÄÄLeft Shift PressedÄÄż ÚÄÄRight Shift PressedÄÄż
% Make Break Make Break
Del E0 AA E0 53 E0 D3 E0 2A E0 B6 E0 53 E0 D3 E0 36
Down arrow E0 AA E0 50 E0 D0 E0 2A E0 B6 E0 50 E0 D0 E0 36
End E0 AA E0 4F E0 CF E0 2A E0 B6 E0 4F E0 CF E0 36
Home E0 AA E0 47 E0 C7 E0 2A E0 B6 E0 47 E0 C7 E0 36
Ins E0 AA E0 52 E0 D2 E0 2A E0 B6 E0 52 E0 D2 E0 36
Left arrow E0 AA E0 4B E0 CB E0 2A E0 B6 E0 4B E0 CB E0 36
PgDn E0 AA E0 51 E0 D1 E0 2A E0 B6 E0 51 E0 D1 E0 36
PgUp E0 AA E0 49 E0 C9 E0 2A E0 B6 E0 49 E0 C9 E0 36
Right arrow E0 AA E0 4D E0 CD E0 2A E0 B6 E0 4D E0 CD E0 36
Up arrow E0 AA E0 48 E0 C8 E0 2A E0 B6 E0 48 E0 C8 E0 36
/ E0 AA E0 35 E0 B5 E0 2A E0 B6 E0 35 E0 B5 E0 36
- The PS/2 models have three make/break scan code sets. The first
set matches the PC & XT make/break scan code set and is the one
listed here. Scan code sets are selected by writing the value F0
to the keyboard via the ~8042~ (port 60h). The following is a brief
description of the scan code sets (see the PS/2 Technical Reference
manuals for more information on scan code sets 2 and 3):
ů set 1, each key has a base scan code. Some keys generate
extra scan codes to generate artificial shift states. This
is similar to the standard scan code set used on the PC and XT.
ů set 2, each key sends one make scan code and two break scan
codes bytes (F0 followed by the make code). This scan code
set is available on the IBM AT also.
ů set 3, each key sends one make scan code and two break scan
codes bytes (F0 followed by the make code) and no keys are
altered by Shift/Alt/Ctrl keys.
ů typematic scan codes are the same as the make scan code
- Some Tandy 1000's do not handle Alt key combinations when multiple
shift keys are pressed. The Alt-Shift-H combination loses the Alt.
- extended keys like (F11, F12) can only be read with systems that
have extended keyboard BIOS support (or ~INT 9~ extensions); to
read these special keys on these systems ~INT 16,10~ must be used
- see ~SCAN CODES~ ~KB FLAGS~ ~KEYBOARD COMMANDS~
:GDT:Global Descriptor Table
^GDT - Global Descriptor Table
% Offset Size Description
00 8bytes dummy, set to 0
08 8bytes ~GDT~ data segment location, set to 0
10 8bytes source GDT pointer
18 8bytes target GDT pointer
20 8bytes pointer to BIOS code segment, set to 0. used by
the BIOS to create protected mode code segment
28 8bytes pointer to BIOS stack segment, set to 0. Used by
the BIOS to create protected mode stack segment
30 8bytes user code segment
38 8bytes temporary BIOS code segment
% Source/Target GDT use the following format (offsets 10h and 18h):
% Offset Size Description
00 word Segment limit
02 3bytes 24 bit segment physical address
05 byte data access rights, set to 93h
06 word reserved word, must be 0
- see ~LGDT~ ~SGDT~
:media descriptor byte
^Media Descriptor Byte
% Media Descriptor Byte Layout:
ł7ł6ł5ł4ł3ł2ł1ł0ł IBM Media Descriptor Byte
ł ł ł ł ł ł ł ŔÄÄÄ 1 = 2 sided, 0 = not 2 sided
ł ł ł ł ł ł ŔÄÄÄÄ 1 = 8 sector, 0 = not 8 sector
ł ł ł ł ł ŔÄÄÄÄÄ 1 = removable, 0 = not removable
ŔÄÁÄÁÄÁÄÁÄÄÄÄÄÄ must be set to 1
F8 Hard disk
Double sided 18 sector diskette PS/2 1.44 Mb. DSQD
F9 5¬ Double sided, High Density diskette (15 sector)
3« Double Sided, Double High Density diskette (9 sector)
FA Ram disk (not all Ramdisks use this)
FC 5¬ Single Sided, Double Density diskette (9 sector)
8 inch Double Sided, Single Density diskette
FD 5¬ Double Sided, Double Density (9 sector)
8 inch Double Sided Single Density (26 sector) (IBM 3740
format) 128 bytes per sector, soft sector, 4 sectors
cluster, 4 reserved sectors, 2 FATs, 68 directory entries,
77*26*2 sectors.
FE 5¬ Single Sided, Double Density diskette (8 sector)
8 inch Single Sided, Single Density diskette (26 sector)
(IBM 3740 format) 128 bytes per sector, soft sector,
4 sectors per cluster, 1 reserved sector, 2 FATs. 68
directory entries, 77*26 sectors.
8 inch Double Sided, Double Density (8 sector), 1024 bytes
per sector, soft sector, 1 sector per cluster, 1 reserved
sector, 2 FATs, 192 directory entries, 77*8*2 sectors. To
distinguish from 8 inch SS/SD attempt read of side 2.
FF 5¬ Double Sided, Double Density diskette (8 sector)
- see also ~INT 21,1B~ ~INT 21,1C~ ~FAT~
:Memory Control Block:MCB
^MCB - DOS Memory Control Block Format
% Offset Size Description
00 byte 'M' 4Dh member of a MCB chain, (not last)
'Z' 5Ah indicates last entry in MCB chain
other values cause "Memory Allocation Failure" on exit
01 word ~PSP~ segment address of MCB owner (Process Id)
possible values:
0 = free
8 = Allocated by DOS before first user pgm loaded
other = Process Id/PSP segment address of owner
03 word number of paras related to this MCB (excluding MCB)
05 11bytes reserved
08 8bytes ASCII program name, NULL terminated if less than max
length (DOS 4.x+)
10 nbytes first byte of actual allocated memory block
- to find the first MCB in the chain, use ~INT 21,52~
- DOS 3.1+ the first memory block contains the DOS data segment
ie., installable drivers, buffers, etc
- DOS 4.x the first memory block is divided into subsegments,
with their own memory control blocks; offset 0000h is the first
- the 'M' and 'Z' are said to represent Mark Zbikowski
- the MCB chain is often referred to as a linked list, but
technically isn't
^DOS 4.x Initial Data Segment Subsegment Control Blocks:
% Offset Size Description
00 byte subsegment type
'D' device driver
'E' device driver appendage
'I' Installable File System driver
'F' FILES= control block storage area (for FILES>5)
'X' FCBS= control block storage area, if present
'C' BUFFERS EMS workspace area if BUFFERS /X is used
'B' BUFFERS= storage area
'L' LASTDRIVE= current directory structure array
'S' STACKS= code/data area, if present (see below)
01 word paragraph of subsegment start
03 word subsegment size in paragraphs
05 3bytes unused
08 types "D" and "I", filename of driver loaded driver
- see ~INT 21,48~ ~INT 21,49~ ~INT 21,4A~
:Program Segment Prefix:PSP
^PSP - DOS Program Segment Prefix Layout
% Offset Size Description
00 word machine code ~INT 20~ instruction (CDh 20h)
02 word top of memory in segment (paragraph) form
04 byte reserved for DOS, usually 0
05 5bytes machine code instruction long call to the DOS
function dispatcher (obsolete CP/M)
06 word .COM programs bytes available in segment (CP/M)
0A dword ~INT 22~ terminate address; DOS loader jumps to this
address upon exit; the EXEC function forces a child
process to return to the parent by setting this
vector to code within the parent (IP,CS)
0E dword ~INT 23~ Ctrl-Break exit address; the original INT 23
vector is NOT restored from this pointer (IP,CS)
12 dword ~INT 24~ critical error exit address; the original
INT 24 vector is NOT restored from this field (IP,CS)
16 word parent process segment addr (Undoc. DOS 2.x+)
COMMAND.COM has a parent id of zero, or its own PSP
18 20bytes file handle array (Undocumented DOS 2.x+); if handle
array element is FF then handle is available. Network
redirectors often indicate remotes files by setting
these to values between 80-FE.
2C word segment address of the environment, or zero (DOS 2.x+)
2E dword SS:SP on entry to last INT 21 function (Undoc. 2.x+) Ř
32 word handle array size (Undocumented DOS 3.x+)
34 dword handle array pointer (Undocumented DOS 3.x+)
38 dword pointer to previous PSP (deflt FFFF:FFFF, Undoc 3.x+) Ř
3C 20bytes unused in DOS before 4.01 Ř
50 3bytes DOS function dispatcher CDh 21h CBh (Undoc. 3.x+) Ř
53 9bytes unused
5C 36bytes default unopened ~FCB~ #1 (parts overlayed by FCB #2)
6C 20bytes default unopened FCB #2 (overlays part of FCB #1)
80 byte count of characters in command tail; all bytes
following command name; also default ~DTA~ (128 bytes)
81 127bytes all characters entered after the program name followed
by a CR byte
- offset 5 contains a jump address which is 2 bytes too low for
PSP's created by the DOS EXEC function in DOS 2.x+ Ř
- program name and complete path can be found after the environment
in DOS versions after 3.0. See offset 2Ch.
Ř see ~Bibliography~ for reference to "Undocumented DOS"
:scan codes
^INT 16 - Keyboard Scan Codes
% Key Normal Shifted w/Ctrl w/Alt
A 1E61 1E41 1E01 1E00
B 3062 3042 3002 3000
C 2E63 2E42 2E03 2E00
D 2064 2044 2004 2000
E 1265 1245 1205 1200
F 2166 2146 2106 2100
G 2267 2247 2207 2200
H 2368 2348 2308 2300
I 1769 1749 1709 1700
J 246A 244A 240A 2400
K 256B 254B 250B 2500
L 266C 264C 260C 2600
M 326D 324D 320D 3200
N 316E 314E 310E 3100
O 186F 184F 180F 1800
P 1970 1950 1910 1900
Q 1071 1051 1011 1000
R 1372 1352 1312 1300
S 1F73 1F53 1F13 1F00
T 1474 1454 1414 1400
U 1675 1655 1615 1600
V 2F76 2F56 2F16 2F00
W 1177 1157 1117 1100
X 2D78 2D58 2D18 2D00
Y 1579 1559 1519 1500
Z 2C7A 2C5A 2C1A 2C00
% Key Normal Shifted w/Ctrl w/Alt
1 0231 0221 7800
2 0332 0340 0300 7900
3 0433 0423 7A00
4 0534 0524 7B00
5 0635 0625 7C00
6 0736 075E 071E 7D00
7 0837 0826 7E00
8 0938 092A 7F00
9 0A39 0A28 8000
0 0B30 0B29 8100
% Key Normal Shifted w/Ctrl w/Alt
- 0C2D 0C5F 0C1F 8200
= 0D3D 0D2B 8300
[ 1A5B 1A7B 1A1B 1A00
] 1B5D 1B7D 1B1D 1B00
; 273B 273A 2700
' 2827 2822
` 2960 297E
\ 2B5C 2B7C 2B1C 2600 (same as Alt L)
, 332C 333C
. 342E 343E
/ 352F 353F
% Key Normal Shifted w/Ctrl w/Alt
F1 3B00 5400 5E00 6800
F2 3C00 5500 5F00 6900
F3 3D00 5600 6000 6A00
F4 3E00 5700 6100 6B00
F5 3F00 5800 6200 6C00
F6 4000 5900 6300 6D00
F7 4100 5A00 6400 6E00
F8 4200 5B00 6500 6F00
F9 4300 5C00 6600 7000
F10 4400 5D00 6700 7100
F11 8500 8700 8900 8B00
F12 8600 8800 8A00 8C00
% Key Normal Shifted w/Ctrl w/Alt
BackSpace 0E08 0E08 0E7F 0E00
Del 5300 532E 9300 A300
Down Arrow 5000 5032 9100 A000
End 4F00 4F31 7500 9F00
Enter 1C0D 1C0D 1C0A A600
Esc 011B 011B 011B 0100
Home 4700 4737 7700 9700
Ins 5200 5230 9200 A200
Keypad 5 4C35 8F00
Keypad * 372A 9600 3700
Keypad - 4A2D 4A2D 8E00 4A00
Keypad + 4E2B 4E2B 4E00
Keypad / 352F 352F 9500 A400
Left Arrow 4B00 4B34 7300 9B00
PgDn 5100 5133 7600 A100
PgUp 4900 4939 8400 9900
PrtSc 7200
Right Arrow 4D00 4D36 7400 9D00
SpaceBar 3920 3920 3920 3920
Tab 0F09 0F00 9400 A500
Up Arrow 4800 4838 8D00 9800
- Some key combinations are not available on all systems. The PS/2
includes many that aren't available on the PC, XT and AT.
- To retrieve the character from a scan code logical AND the word
with 0x00FF.
- see ~INT 16~ ~MAKE CODES~
:SFT:file table:System File Table
^System File Table (Undocumented)
^DOS 2.x System File Table Format
% Offset Size Description
00 dword pointer to next system file table; offset of ffffh
indicates last table in chain
04 word number of file descriptors in table
06 nbytes file descriptor table of the format (40 bytes each):
% Offset Size Description
00 byte count of handles referring to this file or
zero if file is no longer open
01 byte open mode
02 byte file attribute
03 byte drive (1=A, 2=B:, ..., 0=char device)
04 11bytes filename in ~FCB~ format
0F word unknown
11 word unknown
13 dword file size
17 word file date
19 word file time
1B byte device attribute
1C word starting cluster of file (block device)
dword pointer to device driver (if char device)
1E word if block device, relative cluster within file of
the last cluster read; zero if file has never
been read or written
20 word absolute cluster number of current cluster
22 word unknown
24 dword current file position
^DOS 3.x System File Table and FCB Table Format
% Offset Size Description
00 dword pointer to next system file table; offset of ffffh
indicates last table in chain
04 word number of file descriptors in table
06 nbytes file descriptor table of the format (53 bytes each):
% Offset Size Description
00 word count of handles referring to this file or
zero if file is no longer open
02 word open mode, bit 15 set if file id opened via FCB
04 byte file attribute
05 word device info word
07 dword if char device pointer to device driver header
if block device pointer to DOS Device Control Block
0B word starting cluster of file
0D word file time
0F word file date
11 dword file size
15 dword current file position
19 word if block device, relative cluster within file of
the last cluster read
1B word absolute cluster number of last cluster read
zero if file has never been read or written
1D word sector number containing the directory entry
1F byte number of dir entry within sector
20 11bytes filename in ~FCB~ format
2B dword SHARE pointer to previous ~SFT~ sharing same file
2F word SHARE number of network machine opening file
31 word PSP segment of file owner
33 word offset within SHARE code segment of sharing record
0000h = none
^DOS 4.0+ System File Table and FCB Table
% Offset Size Description
00 dword pointer to next system file table; offset of ffffh
indicates last table in chain
04 word number of file descriptors in table
06 nbytes file descriptor table of the format (59 bytes each):
% Offset Size Description
00 word count of handles referring to this file or
zero if file is no longer open
02 word open mode, bit 15 set if file id opened via FCB
04 byte file attribute
05 word device info word
07 dword if char device pointer to device driver header
if block device pointer to DOS Device Control Block
or REDIR data
0B word starting cluster of file
0D word file time
0F word file date
11 dword file size
15 dword current file position
19 dword if network redirector, pointer to REDIRIFS record
word if local block dev, relative cluster within file
of the last cluster read
1B dword if local, sector number of directory entry
1F byte if local, number of directory entry within sector
20 11bytes filename in ~FCB~ format
2B dword SHARE pointer to previous ~SFT~ sharing same file
2F word SHARE number of network machine opening file
31 word PSP segment of file owner
33 word offset within SHARE code seg of sharing record
0000h = none
35 word absolute cluster number of last cluster read
zero if file has never been read or written
37 dword pointer to file IFS driver, NULL if native DOS
- these structures are allocated by DOS during the scanning of the
CONFIG.SYS file
- ~INT 21,67~ can be used to allow an application to have more than
20 file handles; the total number of handles is limited still to
the value of FILES=n in CONFIG.SYS
- the structure of these fields is undocumented; I do not recommend
relying on any of this information, instead use it as an aid in
debugging
- see ~Bibliography~ references to Bernd Schemmer and "Data Structures
Used in IBM PC Compatibles and the PS/2".
- see ~INT 21,52~
:system descriptor
^BIOS System Descriptor Table (PS/2)
% Offset Size Description
00 word length of descriptor (8 minimum)
02 byte model byte (same as FFFF:E, not reliable)
03 byte secondary model byte
04 byte BIOS revision level (zero based)
05 byte feature information (see table)
06 dword reserved
ł7ł6ł5ł4ł3ł2ł1ł0ł feature information at offset 5
ł ł ł ł ł ł ł ŔÄÄÄ reserved
ł ł ł ł ł ł ŔÄÄÄÄ 0=PC bus, 1=Micro Channel
ł ł ł ł ł ŔÄÄÄÄÄ Extended BIOS Data Area (~EBDA~) allocated
ł ł ł ł ŔÄÄÄÄÄÄ wait for external event supported
ł ł ł ŔÄÄÄÄÄÄÄ ~INT 15,4F~ used (kbd intercept)
ł ł ŔÄÄÄÄÄÄÄÄ RTC present
ł ŔÄÄÄÄÄÄÄÄÄ 2nd ~8259~ present
ŔÄÄÄÄÄÄÄÄÄÄ DMA channel 3 used by fixed disk BIOS
- see also ~MACHINE IDENT~ and ~INT 15,C0~
:video information:video tables
^Video Information Tables
% BIOS Data Area Fields
% Address Size Description
40:49 byte Current video mode (see ~VIDEO MODE~)
40:4A word Number of screen columns
40:4C word Size of video regen buffer in bytes
40:4E word Starting address in video regen buffer (offset)
40:50 8 words Cursor position of pages 1-8, high order
byte=row, low order byte=column
40:60 byte Ending (bottom) scan line for cursor
40:61 byte Starting (top) scan line for cursor
40:62 byte Active display page number
40:63 word Base port address for active ~6845~ CRT controller
3B4h = mono, 3D4h = color
40:65 byte 6845 CRT mode control register value (port 3x8h)
EGA/VGA values emulate those of the MDA/CGA
40:66 byte CGA current color palette setting (port 3d9h)
EGA and VGA values emulate the CGA
40:84 byte Rows on the screen (less 1, EGA+)
40:85 word Point height of character matrix (EGA+)
40:87 byte Video mode options (EGA+)
ł7ł6ł5ł4ł3ł2ł1ł0ł Video mode options (EGA+)
ł ł ł ł ł ł ł ŔÄÄ 1=alphanumeric cursor emulation enabled
ł ł ł ł ł ł ŔÄÄÄ 1=video subsystem attached to monochrome
ł ł ł ł ł ŔÄÄÄÄ reserved
ł ł ł ł ŔÄÄÄÄÄ 1=video subsystem is inactive
ł ł ł ŔÄÄÄÄÄÄ reserved
ł ŔÄÁÄÄÄÄÄÄÄ video RAM 00-64K 10-192K 01-128K 11-256K
ŔÄÄÄÄÄÄÄÄÄÄ video mode number passed to ~INT 10~, function 0
40:88 byte EGA feature bit switches, emulated on VGA
ł7ł6ł5ł4ł3ł2ł1ł0ł EGA feature bit switches (EGA+)
ł ł ł ł ł ł ł ŔÄÄ EGA SW1 config (1=off)
ł ł ł ł ł ł ŔÄÄÄ EGA SW2 config (1=off)
ł ł ł ł ł ŔÄÄÄÄ EGA SW3 config (1=off)
ł ł ł ł ŔÄÄÄÄÄ EGA SW4 config (1=off)
ł ł ł ŔÄÄÄÄÄÄ Input FEAT0 (ISR0 bit 5) after output on FCR0
ł ł ŔÄÄÄÄÄÄÄ Input FEAT0 (ISR0 bit 6) after output on FCR0
ł ŔÄÄÄÄÄÄÄÄ Input FEAT1 (ISR0 bit 5) after output on FCR1
ŔÄÄÄÄÄÄÄÄÄ Input FEAT1 (ISR0 bit 6) after output on FCR1
40:89 byte Video display data area (MCGA and VGA)
ł7ł6ł5ł4ł3ł2ł1ł0ł Video display data area (MCGA and VGA)
ł ł ł ł ł ł ł ŔÄÄ 1=VGA is active
ł ł ł ł ł ł ŔÄÄÄ 1=gray scale is enabled
ł ł ł ł ł ŔÄÄÄÄ 1=using monochrome monitor
ł ł ł ł ŔÄÄÄÄÄ 1=default palette loading is disabled
ł ł ł ŔÄÄÄÄÄÄ see table below
ł ł ŔÄÄÄÄÄÄÄ reserved
ł ŔÄÄÄÄÄÄÄÄ 1=display switching enabled
ŔÄÄÄÄÄÄÄÄÄ alphanumeric scan lines (see table below)
% Bit7 Bit4 Scan Line information
0 0 350 line mode
0 1 400 line mode
1 0 200 line mode
1 1 reserved
40:8A byte Display Combination Code (DCC) table index (EGA+)
40:A8 dword BIOS Video Save/Override Pointer Table address
^EGA / VGA Related Tables
% Video Save/Override Pointer Table (pointer at 40:A8):
00 dword Video Parameter Table pointer
04 dword Dynamic Parameter Save Area pointer (EGA, VGA)
08 dword Alphanumeric Character Set Override pointer
0C dword Graphics Character Set Override pointer
10 dword Secondary Save Pointer Table pointer (VGA)
14 dword reserved, set to 0000:0000
18 dword reserved, set to 0000:0000
% Video Parameter Table
00 byte number of displayed character columns
01 byte number of displayed screen rows minus 1
02 byte character matrix height in points
03 word video buffer size in bytes
05 dword contents of sequencer registers 1-4
09 byte misc. output register values
0A 25bytes contents of CRTC registers 0-18h
23 20bytes contents of attribute controller regs 0-13h
37 9 bytes contents of graphics controller regs 0-8
% Dynamic Parameter Save Area
00 16bytes contents of graphics controller pallette regs
10 byte contents of graphics controller overscan reg
11 239bytes reserved
% Alphanumeric Character Set Override
00 byte length of each character definition in bytes
01 byte character generator RAM bank
02 word count of characters defined
04 word first character code in table
06 dword pointer to character font definition table
0A byte number of character rows displayed
0B nbytes array of applicable video modes
0B+n byte FFh end of mode list marker
% Graphics Character Set Override
00 byte count of displayed character rows
01 word length of each character definition in bytes
03 dword pointer to character font definition table
07 nbytes array of applicable video modes
07+n byte FFh end of mode list marker
% Secondary Save Pointer Table
00 word length of table in bytes
02 dword pointer to display combination code table
06 dword pointer to secondary alphanumeric char set override
0A dword pointer to user palette profile table (VGA)
0E dword reserved
12 dword reserved
16 dword reserved
% Display Combination Code Table
00 byte number of table entries
01 byte DCC table version number
02 byte maximum display type code
03 byte reserved
04 n words array valid display combinations:
0,0 entry 0 no display
0,1 entry 1 MDPA
0,2 entry 2 CGA
2,1 entry 3 MDPA + CGA
0,4 entry 4 EGA
4,1 entry 5 EGA + MDPA
0,5 entry 6 MEGA
2,5 entry 7 MEGA + CGA
0,6 entry 8 PGC
1,6 entry 9 PGC + MDPA
5,6 entry 10 PGC + MEGA
0,8 entry 11 CVGA
1,8 entry 12 CVGA + MDPA
0,7 entry 13 MVGA
2,7 entry 14 MVGA + CGA
2,6 entry 15 MVGA + PGC
% Secondary Alpha Mode Auxillary Character Generator Table
00 byte bytes per character
01 byte block to load
02 byte reserved
03 dword font table pointer
07 nbytes array of mode values for this font
07+n byte FFh end of mode list marker
% Palette Profile Table (VGA only)
00 byte 1 - enable underlining in all alphanumeric modes
0 - enable underlining in monochrome alpha modes
-1 - disable underlining in all alpha modes
01 byte reserved
02 word reserved
04 word count of attribute controller regs in table
06 word first attribute controller register number
08 dword pointer to attribute controller reg table
0C word count of video DAC color registers in table
0E word first video DAC color register number
10 dword video DAC color register table pointer
14 nbytes array of applicable video modes for this font
14+n byte FFh end of video mode list marker
- see also ~BIOS Data Area~
- see ~INT 10,1B~ for Dynamic Video State Table and Video Static
Functionality Table
:video pages
^Video modes and corresponding number of pages
% Mode Pages Adapters
00 pages 0-7 (CGA,EGA,MCGA,VGA)
01 pages 0-7 (CGA,EGA,MCGA,VGA)
02 pages 0-3 (CGA)
pages 0-7 (EGA,MCGA,VGA)
03 pages 0-3 (CGA)
pages 0-7 (EGA,MCGA,VGA)
07 pages 0-7 (EGA,VGA)
no pages (MDA)
0D pages 0-7 (EGA,VGA)
0E pages 0-4 (EGA,VGA)
0F pages 0-1 (EGA,VGA)
10 pages 0-1 (EGA,VGA)
:XFCB:Extended FCB:extended file ctlblk
^XFCB - Extended DOS File Control Block
% Offset Size Description
00 byte flag containing FF if this is an extended ~FCB~
01 5bytes reserved
06 byte directory attribute byte (see ~FILE ATTRIBUTE~)
07 byte drive number (0 for default drive, 1 = A:, 2 = B:)
08 8bytes filename, left justified with trailing blanks
16 3bytes filename extension, left justified w/blanks
19 word current block number relative to file start (0 based)
21 word logical record size in bytes
23 dword file size in bytes
27 word date the file was created or last updated:
łFłEłDłCłBłAł9ł8ł7ł6ł5ł4ł3ł2ł1ł0ł 21,20
ł ł ł ł ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÁÄ day 1-31
ł ł ł ł ł ł ł ŔÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄ month 1-12
ŔÄÁÄÁÄÁÄÁÄÁÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ year + 1980
29 16bytes reserved for system use
39 byte relative record number within current BLOCK
40 4bytes relative record number from start of file (0 based)