INT 13H

INT 13h is shorthand for BIOS interrupt call 13_hex, the 20th interrupt vector in an x86-based computer system. The BIOS typically sets up a real mode interrupt handler at this vector that provides sector-based hard disk and floppy disk read and write services using cylinder-head-sector (CHS) addressing.

INT is an x86 instruction that triggers a software interrupt, and 13_hex is the interrupt number (as a hexadecimal value) being called.

Modern computers come with both BIOS int 0x13 and UEFI functionality that provides the same services and more, with the exception of UEFI Class 3 that completely removes CSM thus lacks INT 13h and other interrupts . Typically, UEFI drivers use LBA-addressing instead of CHS-addressing.

Overview

Under real mode operating systems, such as DOS, calling INT 13h would jump into the computer's ROM-BIOS code for low-level disk services, which would carry out physical sector-based disk read or write operations for the program. In DOS, it serves as the low-level interface for the built-in block device drivers for hard disks and floppy disks. This allows INT 25h and INT 26h to provide absolute disk read/write functions for logical sectors to the FAT file system driver in the DOS kernel, which handles file-related requests through DOS API (INT 21h) functions.

Under protected mode operating systems, such as Microsoft Windows NT derivatives (e.g. NT4, 2000, XP, and Server 2003) and Linux with dosemu, the OS intercepts the call and passes it to the operating system's native disk I/O mechanism. Windows 9x and Windows for Workgroups 3.11 also bypass BIOS routines when using 32-bit File Access.

The original BIOS real-mode INT 13h interface supports drives of sizes up to about 504 MB using what is commonly referred to as physical CHS addressing. This limit originates from the hardware interface of the IBM PC/XT disk hardware. The BIOS used the cylinder-head-sector (CHS) address given in the INT 13h call, and transferred it directly to the hardware interface.

This interface was later extended to support addressing of up to exactly 8064 MB using what is commonly referred to as logical CHS addressing. This limit originates from a combination of the register value based calling convention used in the INT 13h interface, and the goal of maintaining backward compatibility. There were originally a number of BIOSes that offered incompatible versions of this interface, but eventually the computer industry standardized on the interface developed in the Microid Research ("MR BIOS") in 1989. This limit uses 1024 cylinders, 256 heads, 63 sectors, and 512 byte blocks, allowing exactly 7.875 GiB of addressing (1024 * 256 * 63 * 512 bytes).

To support even larger addressing modes, an interface known as INT 13h Extensions was introduced by Western Digital and Phoenix Technologies as part of BIOS Enhanced Disk Drive Services (EDD).^[1][2] It uses 64-bit logical block addressing (LBA) which allows addressing up to 8 ZiB (the drive can also support 28-bit or 48-bit LBA which allows up to 128 GiB or 128 PiB respectively, assuming a 512-byte sector/block size). This is a packet interface, because it uses a pointer to a packet of information rather than the register based calling convention of the original INT 13h interface. This packet is a data structure that contains an interface version, data size, and LBAs.

"[Most] versions of MS-DOS, (including MS-DOS 7, and Windows 95) have a bug which prevents booting hard disks with 256 heads (register value 0xFF), so many modern BIOSes provide mappings with at most 255 (FEh) heads",^[3][4] thus reducing the total addressable space to exactly 8032.5 MiB (approx 7.844 GiB).^[5] Some cache drivers flush their buffers when detecting that DOS is bypassed by directly issuing INT 13h from applications. A dummy read can be used as one of several methods to force cache flushing for unknown caches (e.g. before rebooting).^[3][4] AMI BIOSes from around 1990-1991 trash word unaligned buffers. Some DOS and TSR programs clobber interrupt enabling and registers so PC DOS and MS-DOS install their own filters to prevent this.^[6]

AWARD AT BIOS and AMI 386sx BIOS have been extended to handle more than 1024 cylinders by placing bits 10 and 11 of the cylinder number into bits 6 and 7 of DH

List of INT 13h services

CX =       ---CH--- ---CL---
cylinder : 76543210 98
sector   :            543210

CX := ( ( cylinder and 255 ) shl 8 ) or ( ( cylinder and 768 ) shr 2 ) or sector;
cylinder := ( (CX and 0xFF00) shr 8 ) or ( (CX and 0xC0) shl 2)
sector := CX and 63;

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 not be a good choice because FF00h + 2000h = 11F00h > 10000h

See also

• INT 10H
• BIOS interrupt call
• Cylinder-head-sector
• INT (x86 instruction)
• DPMI (DOS Protected Mode Interface)

References

External links

• BIOS Interrupt 13h Extensions
• Ralf Brown's comprehensive Interrupt List
• Norton Guide about int 13h, ah = 00h .. 1ah