Use BIOS INT 0x13 for Disk Operations

INT 0x13 disk access

Disk services load kernels and stage-two boot code. Classic CHS reads use AH=0x02 with cylinder in CH, sector in CL (bits 0-5), head in DH, and count in AL. For example, reading sector 1, head 0, cylinder 0 from drive 0x80 pulls the next boot block after the MBR.

Extended LBA read

nasmCopy
Loading…

• DL: Boot drive number firmware passes at 0x7C00 entry
• Reset disk: AH=0x00 clears controller errors
• Status in AH when carry set: 0xAA means drive not ready
• Prefer LBA when CHS geometry exceeds firmware limits

Geometry and retries

INT 0x13 returns detailed status in AH when CF is set: 0x01 is bad command, 0xAA is drive not ready. Floppy reads may need multiple attempts with controller reset between tries. Always preserve DL across calls if your code uses multiple routines; the BIOS does not restore it for you. Logging AH on failure speeds up debugging in QEMU compared to silent hangs.

Why for this exercise

You will read sectors with INT 0x13 and handle errors via the carry flag. This exercise requires building CHS or DAP requests and placing data at a known segment:offset buffer.