http://stanislavs.org/helppc/bios_data_area.html

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)

		|7|6|5|4|3|2|1|0| 40:10 (value in INT 11 register AL)
		 | | | | | | | `- IPL diskette installed
		 | | | | | | `-- math coprocessor
		 | | | | |-+--- old PC system board RAM < 256K
		 | | | | | `-- pointing device installed (PS/2)
		 | | | | `--- not used on PS/2
		 | | `------ initial video mode
		 `--------- # of diskette drives, less 1

		|7|6|5|4|3|2|1|0| 40:11  (value in INT 11 register AH)
		 | | | | | | | `- 0 if DMA installed
		 | | | | `------ number of serial ports
		 | | | `------- game adapter
		 | | `-------- not used, internal modem (PS/2)
		 `----------- 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)

		|7|6|5|4|3|2|1|0| keyboard flag byte 0
		 | | | | | | | `--- right shift key depressed
		 | | | | | | `---- left shift key depressed
		 | | | | | `----- CTRL key depressed
		 | | | | `------ ALT key depressed
		 | | | `------- scroll-lock is active
		 | | `-------- num-lock is active
		 | `--------- caps-lock is active
		 `---------- insert is active

	40:18	byte	Keyboard flag byte 1 (see KB FLAGS)

		|7|6|5|4|3|2|1|0| keyboard flag byte
		 | | | | | | | `--- left CTRL key depressed
		 | | | | | | `---- left ALT key depressed
		 | | | | | `----- system key depressed and held
		 | | | | `------ suspend key has been toggled
		 | | | `------- scroll lock key is depressed
		 | | `-------- num-lock key is depressed
		 | `--------- caps-lock key is depressed
		 `---------- 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

		|7|6|5|4|3|2|1|0| drive recalibration status
		 | | | | | | | `-- 1=recalibrate drive 0
		 | | | | | | `--- 1=recalibrate drive 1
		 | | | | | `---- 1=recalibrate drive 2
		 | | | | `----- 1=recalibrate drive 3
		 | `---------- unused
		 `----------- 1=working interrupt flag

	40:3F	byte	Diskette motor status

		|7|6|5|4|3|2|1|0| diskette motor status
		 | | | | | | | `-- 1=drive 0 motor on
		 | | | | | | `--- 1=drive 1 motor on
		 | | | | | `---- 1=drive 2 motor on
		 | | | | `----- 1=drive 3 motor on
		 | `---------- unused
		 `----------- 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)

		|7|6|5|4|3|2|1|0| status of last diskette operation
		 | | | | | | | `--- invalid diskette command
		 | | | | | | `---- diskette address mark not found
		 | | | | | `----- sector not found
		 | | | | `------ diskette DMA error
		 | | | `------- CRC check / data error
		 | | `-------- diskette controller failure
		 | `--------- seek to track failed
		 `---------- 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+)

		|7|6|5|4|3|2|1|0| Video mode options (EGA+)
		 | | | | | | | `-- 1=alphanumeric cursor emulation enabled
		 | | | | | | `--- 1=video subsystem attached to monochrome
		 | | | | | `---- reserved
		 | | | | `----- 1=video subsystem is inactive
		 | | | `------ reserved
		 | `--------- video RAM  00-64K  10-192K  01-128K  11-256K
		 `---------- video mode number passed to INT 10, function 0

	40:88	byte	PCjr: third keyboard status byte
			EGA feature bit switches, emulated on VGA

		|7|6|5|4|3|2|1|0| EGA feature bit switches (EGA+)
		 | | | | | | | `-- EGA SW1 config (1=off)
		 | | | | | | `--- EGA SW2 config (1=off)
		 | | | | | `---- EGA SW3 config (1=off)
		 | | | | `----- EGA SW4 config (1=off)
		 | | | `------ Input FEAT0 (ISR0 bit 5) after output on FCR0
		 | | `------- Input FEAT0 (ISR0 bit 6) after output on FCR0
		 | `-------- Input FEAT1 (ISR0 bit 5) after output on FCR1
		 `--------- Input FEAT1 (ISR0 bit 6) after output on FCR1

	40:89	byte	Video display data area (MCGA and VGA)

		|7|6|5|4|3|2|1|0| Video display data area (MCGA and VGA)
		 | | | | | | | `-- 1=VGA is active
		 | | | | | | `--- 1=gray scale is enabled
		 | | | | | `---- 1=using monochrome monitor
		 | | | | `----- 1=default palette loading is disabled
		 | | | `------ see table below
		 | | `------- reserved
		 | `--------  1=display switching enabled
		 `--------- 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

		|7|6|5|4|3|2|1|0| last diskette data rate selected
		 | | | | `--------- reserved
		 | | `------------ last floppy drive step rate selected
		 `-------------- 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

		|7|6|5|4|3|2|1|0| drive media state (4 copies)
		 | | | | | `------- drive/media state (see below)
		 | | | | `------- reserved
		 | | | `------- 1=media/drive established
		 | | `------- double stepping required
		 `--------- 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

		|7|6|5|4|3|2|1|0| Keyboard mode/type
		 | | | | | | | `--- last code was the E1 hidden code
		 | | | | | | `---- last code was the E0 hidden code
		 | | | | | `----- right CTRL key depressed
		 | | | | `------ right ALT key depressed
		 | | | `------- 101/102 enhanced keyboard installed
		 | | `-------- force num-lock if Rd ID & KBX
		 | `--------- last char was first ID char
		 `---------- read ID in process

	40:97	byte	Keyboard LED flags

		|7|6|5|4|3|2|1|0| Keyboard LED flags
		 | | | | | | | `--- scroll lock indicator
		 | | | | | | `---- num-lock indicator
		 | | | | | `----- caps-lock indicator
		 | | | | `------ circus system indicator
		 | | | `------- ACK received
		 | | `-------- re-send received flag
		 | `--------- mode indicator update
		 `---------- 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

		|7|6|5|4|3|2|1|0| INT 15,86 RTC wait function flag
		 | | | | | | | `--- 1= wait pending
		 | `-------------- not used
		 `--------------- 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)


