Interrupt 13 Functions used in the MBR and Boot code

Interrupt 13 functions used in the Windows MBR and the operating system boot programs are briefly discussed. This is an addendum to the MBR and boot code web pages.

For additional information and other functions see the references at the end of this document.

Function 00h: Reset Disk System

This function resets both the diskette and the fixed disk controllers. It reinitializes the drive parameters and sets the Read/Write heads to cylinder 0.

On input:

• AH = function number, 00h in this case

On output:

• If CF = 0, then no error and AH = 0
• If CF = 1, then error and AH contains an error code

Function 02h: Read Disk Sectors

This function reads disk sectors using CHS format. This can be either a floppy diskette or a hard disk.

On input, the CHS values are contained in the following registers:

• AH = function number, 02h in this case
• AL = number of sectors to read (must be nonzero)
• CH = low 8 bits of cylinder number
• CL = high 2 bits of cylinder number and 6-bit sector number as follows:
  • high-order bits 6-7 (hard disk only) contain high 2 bits of cylinder number
  • low-order bits 0-5 contain sector number (1-63)
• DH = head number
• DL = drive number (bit 7 set for hard disk)
• ES:BX = data buffer

On output, the data that is read is placed in the location specified by ES:BX.

Status of the read operation is contained in the following registers:

• If CF = 0, then no error and AH = 0
• If CF = 1, then error and AH contains an error code

Function 08h: Get Disk-Drive Parameters

This function returns the disk-drive parameters of the drive number specified in DL. This can be either a floppy diskette or a hard disk. DL reports the number of disk drives attached to the disk controller.

On input:

• AH = function number, 08h in this case
• DL = drive number (bit 7 set for hard disk, i.e. DL >= 80h)

On output: (See Int 13/AH=08h for additional parameters.)

• If CF = 0, then no error and AH = 0
• If CF = 1, then error and AH contains an error code
• AL = 00h on at least some BIOSes
• CH = low eight bits of maximum cylinder number
• CL = high 2 bits of cylinder number and 6-bit sector number as follows:
  • high-order bits 6-7 (hard disk only) contain high 2 bits of cylinder number
  • low-order bits 0-5 contain sector number (1-63)
• DH = maximum head number
• DL = number of drives

Function 41h: IBM/MS INT 13 Extensions - Installation Check

This function checks whether the IBM/MS INT 13 Extensions are installed and supported.

On input:

• AH = function number, 41h in this case
• BX = 55AAh
• DL = drive number (80h-FFh)

On Output:

• If CF = 0, then successful
• If CF = 1, then extensions not supported
• BX = AA55h if extensions are installed
• AH = major version of extensions

Function 42h: Extended Read

This function reads disk sectors using LBA format and is one of the IBM/MS INT 13 Extensions.

On input, the CHS values are contained in the following registers:

• AH = function number, 42h in this case
• DL = drive number
• DS:SI = disk address packet (see below)

Status of the read operation is contained in the following registers:

• If CF = 0, then no error and AH = 0
• If CF = 1, then error and AH contains an error code

On Line References

For more information on Interrupt 13 extensions as well as other interrupts:

• Ralf Brown's Interrupt list as ASCII text files can be found at Ralf Brown's Files and can be downloaded as several ZIP files. They can also be found on Ralf Brown's Home Page
• HTML versions of Ralf Brown's Interrupt list can be found at
  Ralf Brown's Interrupt List - HTML Version and Interrupt Jump Table

If you have comments or suggestions, email me at Ray Knights

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This page was created on 18 December 2000, last updated on 2 September 2003.