BIOS
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BIOS PostCodes
ALR
AST
DTK
HP
IBM
NCR

Intel Motherboards:
T440BX
VC820

When power is applied to the computer, the BIOS Data Area is created at memory location 0040:0000h with a typical size of 255 bytes.

Offset Hex Offset Dec BIOS Service Field Size Function
00h 0 Int 14h 2 bytes Base I/O address for serial port 1 (communications port 1 - COM 1)
02h 2 Int 14h 2 bytes Base I/O address for serial port 2 (communications port 2 - COM 2)
04h 4 Int 14h 2 bytes Base I/O address for serial port 3 (communications port 3 - COM 3)
06h 6 Int 14h 2 bytes Base I/O address for serial port 4 (communications port 4 - COM 4)
08h 8 Int 17h 2 bytes Base I/O address for parallel port 1 (printer port 1 - LPT 1)
0Ah 10 Int 17h 2 bytes Base I/O address for parallel port 2 (printer port 2 - LPT 2)
0Ch 12 Int 17h 2 bytes Base I/O address for parallel port 3 (printer port 3 - LPT 3)
0Eh 14 POST 2 bytes Base I/O address for parallel port 4 (printer port 4 - LPT 4) (Only found in PC/XT systems)
10h 16 Int 11h 2 bytes Equipment Word
        Bits 15-14 indicate the number of parallel ports installed
        00b = 1 parallel port
        01b = 2 parallel ports
        03b = 3 parallel ports
        Bits 13-12 are reserved
        Bits 11-9 indicate the number of serial ports installed
        000b = none
        001b = 1 serial port
        002b = 2 serial ports
        003b = 3 serial ports
        004b = 4 serial ports
        Bit 8 is reserved
        Bit 7-6 indicate the number of floppy drives installed
        0b = 1 floppy drive
        1b = 2 floppy drives
        Bits 5-4 indicate the video mode
        00b = EGA or later
        01b = color 40x25
        10b = color 80x25
        11b = monochrome 80x25
        Bit 3 is reserved
        Bit 2 indicates if a PS/2 mouse is installed
        0b = not installed
        1b = installed
        Bit 1 indicated if a math coprocessor is installed
        0b = not installed
        1b = installed
        Bit 0 indicated if a boot floppy is installed
        0b = not installed
        1b = installed
12h 18 POST 1 byte Interrupt flag - Manufacturing test
13h 19 Int 12h 2 bytes Memory size in Kb
15h 21   2 bytes Error codes for AT+; Adapter memory size for PC and XT
17h 22 Int 16h 1 byte Keyboard shift flags 1
        Bit 7 indicates if Insert is on or off
        0b = Insert off
        1b = Insert on
        Bit 6 indicates if CapsLock is on or off
        0b = CapsLock off
        1b - CapsLock on
        Bit 5 indicates if NumLock is on or off
        0b = NumLock off
        1b = NumLock on
        Bit 4 indicates if ScrollLock is on or off
        0b = ScrollLock off
        1b = ScrollLock on
        Bit 3 indicates if the Alt key is up or down
        0b = Alt key is up
        1b = Alt key is down
        Bit 2 indicates if the Control key is up or down
        0b = Control key is up
        1b = Control key is down
        Bit 1 indicates if the Left Shift key is up or down
        0b = Left Shift key is up
        1b = Left Shift key is down
        Bit 0 indicates if the Right Shift key is up or down
        0b = Right Shift key is up
        1b = Right Shift key is down
18h 23 Int 16h 1 byte Keyboard shift flags 2
        Bit 7 indicates if the Insert key is up or down
        0b = Insert key is up
        1b = Insert key is down
        Bit 6 indicates if the CapsLock key is up or down
        0b = CapsLock is key is up
        1b = CapsLock key is down
        Bit 5 indicates if the NumLock key is up or down
        0b = NumLock key is up
        1b = Numlock key is down
        Bit 4 indicates if the ScrollLock key is up or down
        0b = ScrollLock key is up
        1b = ScrollLock key is down
        Bit 3 indicates if the Pause key is active or inactive
        0b = pause key is inactive
        1b = Pause key is active
        Bit 2 indicates if the SysReg key is up or down
        0b = SysReg key is up
        1b = SysReg key is down
        Bit 1 indicates if the Left Alt key is up or down
        0b = Left Alt key is up
        1b = Left Alt key is down
        Bit 0 indicates if the Right Alt key is up or down
        0b = Right Alt key is up
        1b = Right Alt key is down
19h 24 Int 09h 1 byte Alt Numpad work area
1Ah 26 Int 16h 2 bytes Pointer to the address of the next character in the keyboard buffer
1Ch 28 Int 16h 2 bytes Pointer to the address of the last character in he keyboard buffer
1Eh 60 Int 16h 32 bytes Keyboard buffer
3Eh 61 Int 13h 1 byte Floppy disk drive calibration status
        Bits 7-4 are reserved
        Bit 3 = floppy drive 3 (PC, XT)
        Bit 2 = floppy drive 2 (PC, XT)
        Bit 1 = floppy drive 1
        Bit 0 = floppy drive 0
        0b indicates not calibrated
        1b indicates calibrated
3Fh 62 Int 13h 1 byte Floppy disk drive motor status
        Bit 7 indicates current operation
        0b = read or verify operation
        1b = write or format operation
        Bit 6 is not used
        Bit 5-4 indicates drive select
        00b = Drive 0
        01b = Drive 1
        10b = Drive 2 (PC, XT)
        11b = Drive 4 (PC, XT)
        Bit 3 indicates drive 3 motor
        0b = motor off
        1b = motor on
        Bit 2 indicates drive 2 motor
        0b = motor off
        1b = motor on
        Bit 1 indicates drive 0 motor
        0b = motor off
        1b = motor on
        0b = motor off
        1b = motor on
40h 63 Int 13h 1 byte Floppy disk drive motor time-out
41h 64 Int 13h 1 byte Floppy disk drive status
        Bit 7 indicates drive ready status
        0b = drive ready
        1b = drive not ready (time out)
        Bit 6 indicates seek status
        0b = no seek error detected
        1b = indicates a seek error was detected
        Bit 5 indicates floppy disk controller test
        0b = floppy disk controller passed
        1b = floppy disk controller failed
        Bit 4-0 error codes
        00000b = no errors
        00001b = illegal function requested
        00010b = address mark not found
        00011b = write protect error
        00100b = sector not found
        00110b = diskette change line active
        01000b = DMA overrun
        01001b = DMA boundary error
        01100b = unknown media type
        10000b = CRC error during read
42h 65 Int 13h 1 byte Hard disk and floppy controller status register 0
        Bit 7-6 indicate the interrupt code
        00b = command completed normally
        01b = command terminated abnormally
        10b = abnormal termination, ready line on, or diskette changed
        11b = seek command not completed
        Bit 5 indicated seek command
        0b = seek command not completed
        1b = seek command completed
        Bit 4 indicated drive fault
        0b = no drive fault
        1b = drive fault
        Bit 3 indicates drive ready
        0b = drive ready
        1b = drive not ready
        Bit 2 indicates head state when interrupt occurred
        00b = drive 0
        01b = drive 1
        10b = drive 2 (PC, XT)
        11b = drive 3 (PC, XT)
        Bit 1-0 indicates drive select
        00b = drive 0
        01b = drive 1
        10b = drive 2 (PC, XT)
        11b = drive 3 (PC, XT)
43h 66 Int 13h 1 byte Floppy drive controller status register 1
        Bit 7-0 indicates no error
        Bit 7, 1b = indicates attempted access beyond last cylinder
        Bit 6, 0b = not used
        Bit 5, 1b = CRC error during read
        Bit 4, 1b = DMA overrun
        Bit 3, 0b = not used
        Bit 2, 1b = Sector not found or reading diskette ID failed
        Bit 1, 1b = medium write protected
        Bit 0, 1b = missing address mark
44h 67 Int 13h 1 byte Floppy drive controller status register 2
        Bit 7, 0b = not used
        Bit 6, 1b = deleted data address mark
        Bit 5, 1b = CRC error detected
        Bit 4, 1b = wrong cylinder
        Bit 3, 1b = condition of equal during verify
        Bit 2, 1b = sector not found during verify
        Bit 1, 1b = bad cylinder
        Bit 0, 1b = address mark not found during read
45h 68 Int 13h 1 byte Floppy disk controller: cylinder number
46h 69 Int 13h 1 byte Floppy disk controller: head number
47h 70 Int 13h 1 byte Floppy disk controller: sector number
48h 71   1 byte Floppy disk controller: number of byte written
49h 72 Int 10h 1 byte Active video mode setting
4Ah 74 Int 10h 2 bytes Number of textcolumns per row for the active video mode
4Ch 76 Int 10h 2 bytes Size of active video in page bytes
4Eh 78 Int 10h 2 bytes Offset address of the active video page relative to the start of video RAM
50h 80 Int 10h 2 bytes Cursor position for video page 0
52h 82 Int 10h 2 bytes Cursor position for video page 1
54h 84 Int 10h 2 bytes Cursor position for video page 2
56h 86 Int 10h 2 bytes Cursor position for video page 3
58h 88 Int 10h 2 bytes Cursor position for video page 4
5Ah 90 Int 10h 2 bytes Cursor position for video page 5
5Ch 92 Int 10h 2 bytes Cursor position for video page 6
5Eh 94 Int 10h 2 bytes Cursor position for video page 7
60h 96 Int 10h 2 bytes Cursor shape
62h 97 Int 10h 1 byte Active video page
63h 99 Int 10h 2 bytes I/O port address for the video display adapter
65h 100 Int 10h 1 byte Video display adapter internal mode register
        Bit 7, 0b = not used
        Bit 6, 0b = not used
        Bit 5
        0b = attribute bit controls background intensity
        1b = attribute bit controls blinking
        Bit 4, 1b = mode 6 graphics operation
        Bit 3 indicates video signal
        0b = video signal disabled
        1b = video signal enabled
        Bit 2 indicates color operation
        0b = color operation
        1b = monochrome operation
        Bit 1, 1b = mode 4/5 graphics operation
        Bit 0, 1b = mode 2/3 test operation
66h 101 Int 10h 1 byte Color palette
        Bit 7, 0b = not used
        Bit 6, 0b = not used
        Bit 5 indicates mode 5 foreground colors
        0b = green/red/yellow
        1b = cyan/magenta/white
        Bit 4 indicates background color
        0b = normal background color
        1b = intensified background color
        Bit 3 indicates intensified border color (mode 2) and background color (mode 5)
        Bit 2 indicates red
        Bit 1 indicates green
        Bit 0 indicates blue
67h 103   2 bytes Adapter ROM offset address
69h 106   2 bytes Adapter ROM segment address
6Bh 107   1 byte Last interrupt (not PC)
        Bit 7 indicates IRQ 7 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 6 indicates IRQ 6 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 5 indicates IRQ 5 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 4 indicates IRQ 4 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 3 indicates IRQ 3 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 2 indicates IRQ 2 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 1 indicates IRQ 1 hardware interrupt
        0b = did not occur
        01 = did occur
        Bit 0 indicates IRQ 0 hardware interrupt
        0b = did not occur
        01 = did occur
6Ch 111 Int 1Ah 4 bytes Counter for Interrupt 1Ah
70c 112 Int 1Ah 1 byte Timer 24 hour flag
71h 113 Int 16h 1 byte Keyboard Ctrl-Break flag
72h 115 POST 2 bytes Soft reset flag
74h 116 Int 13h 1 byte Status of last hard disk operation
        00h = no errors
        01h = invalid function requested
        02h = address mark not found
        04h = sector not found
        05h = reset failed
        06h = removable media changed
        07h = drive parameter activity failed
        08h = DMA overrun
        09h = DMA boundary overrun
        0Ah = bad sector flag detected
        0Bh = bad track detected
        0Dh = invalid number of sectors on format
        0Eh = control data address mark detected
        0Fh = DMA arbitration level out of range
        10h = uncorrectable ECC or CRC error
        11h = ECC corrected data error
        20h = general controller failure
        40h = seek operation failed
        80h = timeout
        AAh = drive not ready
        BBh = undefined error occurred
        CCh = write fault on selected drive
        E0h = status error or error register is zero
        FFh = sense operation failed
75h 117 Int 13h 1 byte Number of hard disk drives
76h 118 Int 13h 1 byte Hard disk control byte
        Bit 7
        0b = enables retries on disk error
        1b = disables retries on disk error
        Bit 6
        0b = enables reties on disk error
        1b = enables reties on disk error
        Bit 5, 0b = not used
        Bit 4, 0b = not used
        Bit 3
        0b = drive has less than 8 heads
        1b = drive has more than 8 heads
        Bit 2, 0b = not used
        Bit 1, 0b = not used
        Bit 0, 0b = not used
77h 119 Int 13h 1 byte Offset address of hard disk I/O port (XT)
78h 120 Int 17h 1 byte Parallel port 1 timeout
79h 121 Int 17h 1 byte Parallel port 2 timeout
7Ah 122 Int 17h 1 byte Parallel port 3 timeout
7Bh 123   1 byte Parallel port 4 timeout (PC, XT) support for virtual DMA services (VDS)
        Bit 7, 0b = not used
        Bit 6, 0b = not used
        Bit 5 indicates virtual DMA services
        0b = not supported
        1b = supported
        Bit 4, 0b = not used
        Bit 3 indicates chaining on interrupt 4Bh
        0b = not required
        1b = required
        Bit 2, 0b = not used
        Bit 1, 0b = not used
        Bit 0, 0b = not used
7Ch 124 Int 14h 1 byte Serial port 1 timeout
7Dh 125 Int 14h 1 byte Serial port 2 timeout
7Eh 126 Int 14h 1 byte Serial port 3 timeout
7Fh 127 Int 14h 1 byte Serial port 4 timeout
80h 129 Int 16h 2 bytes Starting address of keyboard buffer
82h 131 Int 16h 2 bytes Ending address of keyboard buffer
84h 132 Int 10h 1 byte Number of video rows (minus 1)
85h 134 Int 10h 2 bytes Number of scan lines per character
87h 135 Int 10h 1 byte Video display adapter options
        Bit 7 indicates bit 7 of the last video mode
        0b = clear display buffer when setting mode
        1b = do not clear the display buffer
        Bit 6-4 indicates the amount of memory on the video display adapter
        000b = 64Kb
        001b = 128Kb
        010b = 192Kb
        011b = 256Kb
        100b = 512Kb
        110 = 1024Kb or more
        Bit 3 indicates video subsystem
        0b = not active
        1b = active
        Bit 2 is reserved
        Bit 1 indicates monitor type
        0b = color
        1b = monochrome
        Bit 0 indicates alphanumeric cursor emulation
        0b = disabled
        1b = enabled
88h 136 Int 10h 1 byte Video display adapter switches
        Bit 7 indicates state of feature connector line 1
        Bit 6 indicates state of feature connector line 0
        Bit 5-4 not used
        Bit 3-0 indicate adapter type switch settings
        0000b = MDA/color 40x25
        0001b = MDA/color 80x25
        0010b = MDA/high-resolution 80x25
        0011b = MDA/high-resolution enhanced
        0100b = CGA 40x25/monochrome
        0101b = CGA 80x25/monochrome
        0110b = color 40x25/MDA
        0111b = color 80x25/MDA
        1000b = high-resolution 80x25/MDA
        1001b = high-resolution enhanced/MDA
        1010b = monochrome/CGA 40x25
        1011b = monochrome/CGA 80x25
89h 137 Int 10h 1 byte VGA video flags 1
        Bit 7 and 4 indicate scanline mode
        00b = 350-line mode
        01b = 400-line mode
        10b = 200-line mode
        Bit 6 indicates display switch
        0b = disabled
        1b = enabled
        Bit 5 is reserved
        Bit 3 indicates default palette loading
        0b = disabled
        1b= enabled
        Bit 2 indicates monitor type
        0b = color
        1b = monochrome
        Bit 1 indicates gray scale summing
        0b = disabled
        1b = enabled
        Bit 0 indicates VGA active state
        0b = VGA inactive
        1b = VGA active
8Ah 138 Int 10h 1 byte VGA video flags 2
8Bh 139 Int 13h 1 byte Floppy disk configuration data
        Bit 7-6 indicate last data sent to the controller
        00b = 500 Kbit/sec/sec
        01b = 300 Kbit/sec
        10b = 250 Kbit/sec
        11b = rate not set or 1 Mbit/sec
        Bit 5-4 indicate last drive steprate sent to the controller
        00b = 8ms
        01b = 7ms
        10b = 6ms
        11b = 5ms
        Bit 3-2 indicate data rate, set at start of operation (Bits 7-6)
        Bit 1-0 not used
8Ch 140 Int 13h 1 byte Hard disk drive controller status
        Bit 7 indicates controller state
        0b = controller not busy
        1b = controller busy
        Bit 6 indicates drive ready state
        0b = drive selected not ready
        1b = drive selected ready
        Bit 5 indicates write fault
        0b = write fault did not occur
        1b = write error occurred
        Bit 4 indicates seek state
        0b = drive selected seeking
        1b = drive selected seek complete
        Bit 3 indicates data request
        0b = data request is inactive
        1b = data request is active
        Bit 2 indicates data correction
        0b = data not corrected
        1b = data corrected
        Bit 1 indicates index pulse state
        0b = index pulse inactive
        1b = index pulse active
        Bit 0 indicates error
        0b = no error
        1b = error in previous command
8Dh 141 Int 13h 1 byte Hard disk drive error
        Bit 7 indicates bad sector
        0b = not used
        1b = bad sector detected
        Bit 6 indicated ECC error
        0b = not used
        1b = uncorrectable ECC error occurred
        Bit 5 indicates media state
        0b = not used
        1b = media changed
        Bit 4 indicates sector state
        0b = not used
        1b = ID or target sector not found
        Bit 3 indicates media change request state
        0b = not used
        1b = media change requested
        Bit 2 indicates command state
        0b = not used
        1b = command aborted
        Bit 1 indicates drive track error
        0b = not used
        1b = track 0 not found
        Bit 0 indicates address mark
        0b = not used
        1b = address mark not found
8Eh 142 Int 13h 1 byte Hard disk drive task complete flag
8Fh 143 Int 13h 1 byte Floppy disk drive information
        Bit 7 not used
        Bit 6 indicates drive 1 type determination
        0b = not determined
        1b = determined
        Bit 5 indicates drive 1 multirate status
        0b = no
        1b = yes
        Bit 4 indicates diskette 1 change line detection
        0b = no
        1b = yes
        Bit 3 not used
        Bit 2 indicates drive 0 type determination
        0b = not determined
        1b = determined
        Bit 1 indicates drive 0 multirate status
        0b = no
        1b = yes
        Bit 0 indicates diskette 0 change line detection
        0b = no
        1b = yes
90h 144 Int 13h 1 byte Diskette 0 media state
        Bit 7-6 indicate transfer rate
        00b = 500 Kbit/sec
        01b = 300 Kbit/sec
        10b = 250 Kbit/sec
        11b = 1 Mbit/sec
        Bit 5 indicates double stepping
        0b = not required
        1b = required
        Bit 4 indicates media in floppy drive
        0b = unknown media
        1b = known media
        Bit 3 not used
        Bit 2-0 indicates last access
        000b = trying 360k media in 360K drive
        001b = trying 360K media in 1.2M drive
        010b = trying 1.2M media in 1.2M drive
        011b = known 360K media on 360K drive
        100b = known 360K media in 1.2M drive
        101b = known 1.2M media in 1.2M drive
        110b = not used
        111b = 720K media in 720K drive or 1.44M media in 1.44M drive
91h 145 Int 13h 1 byte Diskette 1 media state
        Bit 7-6 indicate transfer rate
        00b = 500 Kbit/sec
        01b = 300 Kbit/sec
        10b = 250 Kbit/sec
        11b = 1 Mbit/sec
        Bit 5 indicates double stepping
        0b = not required
        1b = required
        Bit 4 indicates media in floppy drive
        0b = unknown media
        1b = known media
        Bit 3 not used
        Bit 2-0 indicates last access
        000b = trying 360k media in 360K drive
        001b = trying 360K media in 1.2M drive
        010b = trying 1.2M media in 1.2M drive
        011b = known 360K media on 360K drive
        100b = known 360K media in 1.2M drive
        101b = known 1.2M media in 1.2M drive
        110b = not used
        111b = 720K media in 720K drive or 1.44M media in 1.44M drive
92h 146 Int 13h 1 byte Diskette 0 operational starting state
        Bit 7 indicates data transfer rate
        00b = 500 Kbit/sec
        01b = 300 Kbit/sec
        10b = 250 Kbit/sec
        11b = 1 Mbit/sec
        Bits 5-3 not used
        Bit 2 indicates drive determination
        0b = drive type not determined
        1b = drive type determined
        Bit 1 indicates drive multirate status
        0b = drive is not multirate
        1b = drive is multirate
        Bit 0 indicates change line detection
        0b = no change line detection
        1b = change line detection
93h 147 Int 13h 1 byte Diskette 1 operational starting status
        Bit 7 indicates data transfer rate
        00b = 500 Kbit/sec
        01b = 300 Kbit/sec
        10b = 250 Kbit/sec
        11b = 1 Mbit/sec
        Bits 5-3 not used
        Bit 2 indicates drive determination
        0b = drive type not determined
        1b = drive type determined
        Bit 1 indicates drive multirate status
        0b = drive is not multirate
        1b = drive is multirate
        Bit 0 indicates change line detection
        0b = no change line detection
        1b = change line detection
94h 148 Int 13h 1 byte Diskette 0 current cylinder
95h 149 Int 13h 1 byte Diskette 1 current cylinder
96h 150 Int 16h 1 byte Keyboard status flags 3
        Bit 7, 1b = reading two byte keyboard ID in progress
        Bit 6, 1b = last code was first ID character
        Bit 5, 1b = forced Numlock on
        Bit 4 indicates presence of 101/102 key keyboard
        0b = present
        1b = not present
        Bit 3 indicates right alt key active
        0b = not active
        1b = active
        Bit 2 indicates right control key active
        0b = not active
        1b = active
        Bit 1, 1b = last scancode was E0h
        Bit 0, 1b = last scancode was E1h
97h 151 Int 16h 1 byte Keyboard status flags 4
        Bit 7, 1b = keyboard transmit error
        Bit 6, 1b = LED update in progress
        Bit 5, 1b = re-send code received
        Bit 4, 1b = acknowledge code received
        Bit 3, 1b = reserved
        Bit 2 indicates CapsLock LED state
        0b = CapsLock LED off
        1b = CapsLock LED on
        Bit 1 indicates NumLock LED state
        0b = NumLock LED off
        1b = NumLock LED on
        Bit 0 indicates ScrollLock LED state
        0b = ScrollLock LED off
        1b = ScrollLock LED on
98h 155   4 bytes Segment:Offset address of user wait flag pointer
9Ch 159   4 bytes User wait count
A0h 160   1 byte User wait flag
        Bit 7, 1b = wait time has elapsed
        Bit 6-1 not used
        Bit 0 indicates wait progress
        0b = no wait in progress
        1b = wait in progress
A1h 167   7 bytes Local area network (LAN) bytes
A8h 171   4 bytes Segment:Offset address of video parameter control block
ACh 239   68 bytes Reserved
F0h 255   16 bytes Intra-applications communications area

 


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