BIOS interrupt call
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BIOS Interrupt Calls are a facility that DOS programs, and some other software such as boot loaders, use to invoke the BIOS's facilities. Some operating systems also use the BIOS to probe and initialise hardware resources during their early stages of booting.
[edit] Interrupt Table
| Interrupt | Description | ||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| INT 00h | CPU: Division by Zero | ||||||||||||||||||||||||||||||
| INT 01h | CPU: Single Step for debugging | ||||||||||||||||||||||||||||||
| INT 02h | CPU: NMI, used e.g. by POST for memory errors | ||||||||||||||||||||||||||||||
| INT 03h | CPU: Breakpoint for debugging | ||||||||||||||||||||||||||||||
| INT 04h | CPU: Numeric Overflow | ||||||||||||||||||||||||||||||
| INT 05h | Print Screen | ||||||||||||||||||||||||||||||
| INT 08h | IRQ0: Called by system timer 18.2 times per second | ||||||||||||||||||||||||||||||
| INT 09h | IRQ1: Called by keyboard | ||||||||||||||||||||||||||||||
| INT 0Bh | IRQ3: Called by 2nd serial port COM2 | ||||||||||||||||||||||||||||||
| INT 0Ch | IRQ4: Called by 1st serial port COM1 | ||||||||||||||||||||||||||||||
| INT 0Dh | IRQ5: Called by hard disk controller (PC/XT) or 2nd parallel port LPT2 (AT) | ||||||||||||||||||||||||||||||
| INT 0Eh | IRQ6: Called by floppy disk controller | ||||||||||||||||||||||||||||||
| INT 0Fh | IRQ7: Called by 1st parallel port LPT1 (printer) | ||||||||||||||||||||||||||||||
| INT 10h | Video Services
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| INT 11h | Equipment Installed | ||||||||||||||||||||||||||||||
| INT 12h | Get Conventional Memory Size | ||||||||||||||||||||||||||||||
| INT 13h | Low Level Disk Services
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| INT 14h | Serial I/O
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| INT 15h | Miscellaneous
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| INT 16h | Keyboard
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| INT 17h | Print Services
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| INT 18h | This interrupt will be called if INT 19h fails, in early IBM BIOS version then ROM Basic was loaded | ||||||||||||||||||||||||||||||
| INT 19h | After POST this interrupt is used by BIOS to load the operating system | ||||||||||||||||||||||||||||||
| INT 1Ah | Real Time Clock Services
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| INT 1Bh | Called by Break key | ||||||||||||||||||||||||||||||
| INT 1Ch | Software Timer Interrupt, called by INT 08h | ||||||||||||||||||||||||||||||
| INT 1Dh | Address pointer: VPT = Video Parameter Table | ||||||||||||||||||||||||||||||
| INT 1Eh | Address pointer: DPT = Diskette Parameter Table | ||||||||||||||||||||||||||||||
| INT 1Fh | Address pointer: VGCT = Video Graphics Character Table | ||||||||||||||||||||||||||||||
| INT 41h | Address pointer: FDPT = Fixed Disk Paramter Table (1st hard drive) | ||||||||||||||||||||||||||||||
| INT 46h | Address pointer: FDPT = Fixed Disk Paramter Table (2nd hard drive) | ||||||||||||||||||||||||||||||
| INT 4Ah | Called by RTC for alarm | ||||||||||||||||||||||||||||||
| INT 70h | IRQ8: Called by RTC | ||||||||||||||||||||||||||||||
| INT 74h | IRQ12: Called by mouse | ||||||||||||||||||||||||||||||
| INT 75h | IRQ13: Called by math coprocessor | ||||||||||||||||||||||||||||||
| INT 76h | IRQ14: Called by primary IDE controller | ||||||||||||||||||||||||||||||
| INT 77h | IRQ15: Called by secondary IDE controller |
[edit] INT 13h: Low Level Disk Services
[edit] Drive Table
| DL = 00h | 1st floppy disk ( "drive A:" ) |
| DL = 01h | 2nd floppy disk ( "drive B:" ) |
| DL = 80h | 1st hard disk |
| DL = 81h | 2nd hard disk |
[edit] Function Table
| AH = 00h | Reset Disk Drives | |
| AH = 01h | Check Drive Status | |
| AH = 02h | Read Sectors From Drive | |
| AH = 03h | Write Sectors To Drive | |
| AH = 04h | Verify Sectors | |
| AH = 05h | Format Track | |
| AH = 08h | Read Drive Parameters | |
| AH = 09h | HD | Initialize Disk Controller |
| AH = 0Ah | HD | Read Long Sectors From Drive |
| AH = 0Bh | HD | Write Long Sectors To Drive |
| AH = 0Ch | HD | Move Drive Head To Cylinder |
| AH = 0Dh | HD | Reset Disk Drives |
| AH = 0Eh | PS/2 | Controller Read Test |
| AH = 0Fh | PS/2 | Controller Write Test |
| AH = 10h | HD | Test Whether Drive Is Ready |
| AH = 11h | HD | Recalibrate Drive |
| AH = 12h | PS/2 | Controller RAM Test |
| AH = 13h | PS/2 | Drive Test |
| AH = 14h | HD | Controller Diagnostic |
| AH = 15h | Read Drive Type | |
| AH = 16h | FD | Detect Media Change |
| AH = 17h | FD | Set Media Type For Format ( used by DOS versions <= 3.1 ) |
| AH = 18h | FD | Set Media Type For Format ( used by DOS versions >= 3.2 ) |
| AH = 41h | EXT | Test Whether Extensions Are Available |
| AH = 42h | EXT | Read Sectors From Drive |
| AH = 43h | EXT | Write Sectors To Drive |
| AH = 44h | EXT | Verify Sectors |
| AH = 45h | EXT | Lock/Unlock Drive |
| AH = 46h | EXT | Eject Drive |
| AH = 47h | EXT | Move Drive Head To Sector |
| AH = 48h | EXT | Read Drive Parameters |
| AH = 49h | EXT | Detect Media Change |
Second column is empty == function may be used both for floppy and hard disk.
"FD" == for floppy disk only.
"HD" == for hard disk only.
"PS/2" == for hard disk on PS/2 system only.
"EXT" == part of the Int 13h Extensions which were written in the 1990s to support hard drives with more than 8 GBytes.
[edit] INT 13h AH=00h: Reset Disk Drives
Parameters:
| AH | 00h |
| DL | Drive Index |
[edit] INT 13h AH=01h: Check Drive Status
Parameters:
| AH | 01h |
Results:
| AL | Return Code
|
[edit] INT 13h AH=02h: Read Sectors From Drive
Parameters:
| AH | 02h |
| AL | Sectors To Read Count |
| CX | Track + Sector / See remark |
| DH | Head |
| DL | Drive |
| ES:BX | Buffer Address Pointer |
Results:
| CF | Set On Error, Clear If No Error |
| AH | Return Code |
| AL | Actual Sectors Read Count |
Remarks:
Register CX contains both the cylinder number (10 bits, possible values are 0 to 1023) and the sector number (6 bits, possible values are 1 to 63):
CX = ---CH--- ---CL--- cylinder : 76543210 98 sector : 543210
Examples of translation:
Turbo Pascal: CX := ( ( cylinder and 255 ) shl 8 ) or ( ( cylinder and 768 ) shr 2 ) or sector; cylinder := hi ( CX ) or ( ( lo ( CX ) and 192 ) shl 2 ); sector := CX and 63;
Addressing of Buffer should guarantee that the complete buffer is inside the given segment, i.e. ( BX + size_of_buffer ) <= 10000h. Otherwise the interrupt may fail with some BIOS or hardware versions.
Example: Assume you want to read 16 sectors (= 2000h Bytes) and your buffer starts at memory address 4FF00h. There are different ways to calculate the register values, e.g.:
ES = segment = 4F00h BX = offset = 0F00h sum = memory address = 4FF00h would be a good choice because 0F00h + 2000h = 2F00h <= 10000h ES = segment = 4000h BX = offset = FF00h sum = memory address = 4FF00h would be no good choice because FF00h + 2000h = 11F00h > 10000h
Function 02h of interrupt 13h may only read sectors of the first 16,450,560 sectors of your hard drive, to read sectors beyond the 8 GByte limit you should use function 42h of Int 13h Extensions. Another alternate may be DOS interrupt 25h which reads sectors within a partition.
[edit] INT 13h AH=08h: Read Drive Parameters
Parameters:
| Registers | |
|---|---|
| AH | 08h = function number for read_drive_parameters |
| DL | drive index (e.g. 1st HDD = 80h) |
Results:
| CF | Set On Error, Clear If No Error |
| AH | Return Code |
| DL | number of hard disk drives |
| DH | logical last index of heads = number_of - 1 (because index starts with 0) |
| CX | logical last index of cylinders = number_of - 1 (because index starts with 0) logical last index of sectors per track = number_of (because index starts with 1) |
Remarks:
Logical values of function 08h may/should differ from physical CHS values of function 48h.
Result register CX contains both cylinders and sector/track values, see remark of function 02h.
[edit] INT 13h AH=0Ah: Read Long Sectors From Drive
The only difference between this function and function 02h (see above) is that function 0Ah reads 516 bytes per sector instead of only 512. The last 4 bytes contains the Error Correction Code ECC, a checksum of sector data.
[edit] INT 13h AH=41h: Check Extensions Present
Parameters:
| Registers | |
|---|---|
| AH | 41h = function number for extensions check |
| DL | drive index (e.g. 1st HDD = 80h) |
| BX | 55AAh |
Results:
| CF | Set On Not Present, Clear If Present |
| AH | Error Code or Major Version Number |
| BX | AA55h |
| CX | Interface support bitmask: 1 - Device Access using the packet structure |
[edit] INT 13h AH=42h: Extended Read Sectors From Drive
Parameters:
| Registers | |
|---|---|
| AH | 42h = function number for extended read |
| DL | drive index (e.g. 1st HDD = 80h) |
| DS:SI | segment:offset pointer to the DAP, see below |
| DAP : Disk Address Packet | ||
|---|---|---|
| offset range | size | description |
| 00h | 1 byte | size of DAP = 16 = 10h |
| 01h | 1 byte | unused, should be zero |
| 02h | 1 byte | number of sectors to be read, 0..127 (= 7Fh) |
| 03h | 1 byte | unused, should be zero |
| 04h..07h | 4 bytes | segment:offset pointer to the memory buffer to which sectors will be transferred |
| 08h..0Fh | 8 bytes | absolute number of the start of the sectors to be read (1st sector of drive has number 0) |
Results:
| CF | Set On Error, Clear If No Error |
| AH | Return Code |
[edit] INT 13h AH=48h: Extended Read Drive Parameters
Parameters:
| Registers | |
|---|---|
| AH | 48h = function number for extended_read_drive_parameters |
| DL | drive index (e.g. 1st HDD = 80h) |
| DS:SI | segment:offset pointer to Result Buffer, see below |
| Result Buffer | ||
|---|---|---|
| offset range | size | description |
| 00h..01h | 2 bytes | size of Result Buffer = 30 = 1Eh |
| 02h..03h | 2 bytes | information flags |
| 04h..07h | 4 bytes | physical number of cylinders = last index + 1 (because index starts with 0) |
| 08h..0Bh | 4 bytes | physical number of heads = last index + 1 (because index starts with 0) |
| 0Ch..0Fh | 4 bytes | physical number of sectors per track = last index (because index starts with 1) |
| 10h..17h | 8 bytes | absolute number of sectors = last index + 1 (because index starts with 0) |
| 18h..19h | 2 bytes | bytes per sector |
| 1Ah..1Dh | 4 bytes | optional pointer to Enhanced Disk Drive (EDD) configuration parameters
which may be used for subsequent interrupt 13h Extension calls (if supported) |
Results:
| CF | Set On Error, Clear If No Error |
| AH | Return Code |
Remark: Physical CHS values of function 48h may/should differ from logical values of function 08h.
[edit] INT 18h: Execute BASIC
Description:
This interrupt traditionally jumped to an implementation of BASIC stored in ROM. This call would typically be invoked if the BIOS was unable to identify any bootable volumes on startup. (At the time the original IBM PC was released in 1981, the BASIC in ROM was a key feature.) As time went on and BASIC was no longer shipped on all PCs, this interrupt would simply display an error message indicating that no bootable volume was found (famously, "No ROM BASIC", or more self-explanatory messages in later BIOS versions); in other BIOS versions it would prompt the user to insert a bootable volume and press a key, and then after the user did so it would loop back to the bootstrap loader to try booting again.
