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INT 13H

BIOS interrupt call for disk access From Wikipedia, the free encyclopedia

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INT 13h is shorthand for BIOS interrupt call 13hex, the 20th interrupt vector in an x86-based (IBM PC-descended) 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. PC BIOSes also include INT 13h extension functions, originated by IBM and Microsoft in 1992, that provide those same disk access services using 64-bit LBA addressing; with minor additions, these were quasi-standardized by Phoenix Technologies and others as the EDD (Enhanced Disk Drive) BIOS extensions.[citation needed]

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Overview

All versions of MS-DOS, (including MS-DOS 7 and Windows 95) have a bug which prevents booting disk drives with 256 heads (register value 0xFF), so many modern BIOSes provide CHS translation mappings with at most 255 (0xFE) heads,[1][2] thus reducing the total addressable space to exactly 8032.5 MiB (approx 7.844 GiB).[3]

To support addressing of even larger disks, an interface known as INT 13h Extensions was introduced by IBM and Microsoft, then later re-published and slightly extended by Phoenix Technologies as part of BIOS Enhanced Disk Drive Services (EDD).[4][5]

Some cache drivers flush their buffers when detecting that DOS is bypassed by directly issuing INT 13h from applications. A dummy read via INT 13h can be used as one of several methods to force cache flushing for unknown caches (e.g. before rebooting).[1][2]

AMI BIOSes from around 1990–1991 trash word unaligned buffers. Some DOS and terminate-and-stay-resident programs clobber interrupt enabling and registers so PC DOS and MS-DOS install their own filters to prevent this.[6]

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References

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