Use BIOS Disk Services to Read Sectors from Disk

BIOS disk services (INT 0x13)

INT 0x13 reads and writes sectors through firmware abstractions. Boot sectors use it to pull the next stage from disk when 512 bytes is not enough. For example, extended read function AH=0x42 uses a Disk Address Packet (DAP) in memory to load 64 sectors starting at LBA 1 into 0x7E00.

CHS vs LBA

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• AH=0x02: Classic CHS read (up to 72 sectors, registers set cylinders/heads)
• AH=0x42: Extended LBA read via DAP (required for large disks)
• DL: Drive number (0x00 floppy A, 0x80 first hard disk)
• CF: Carry flag set on error; AH holds status

DAP layout: size byte, sector count, buffer offset/segment, starting LBA quadword.

DAP alignment

The Disk Address Packet should live in memory below 1 MB and preferably not cross a 64 KB segment boundary for oldest BIOS builds. Sector counts above what the controller supports return error 0x01. For floppy images in QEMU, DL is often 0x00; for hard disk images use -hda and expect 0x80. Logging the returned sector count field in the DAP confirms partial reads.

Why for this exercise

You will call INT 0x13 to read additional sectors into a buffer above your boot sector. This exercise requires building a valid DAP and handling carry-flag errors when the BIOS rejects the request.