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                    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)