Read Additional Sectors from Disk in Boot Sector

Reading past sector one

A single boot sector cannot hold a full loader. INT 0x13 extended read (AH=0x42) loads contiguous LBAs into memory above 0x7E00 so your stage-one code stays put. For example, a DAP requesting 32 sectors from LBA 1 fills 0x7E00-0xBE00 with stage-two code you then jump to.

Disk Address Packet

nasmCopy
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• Set DL to the BIOS boot drive number (often passed in DL at entry)
• Check carry flag after int 0x13; reset disk with AH=0x00 on failure
• Keep total bytes read within conventional memory free areas

Buffer placement

Loading to 0x7E00 places 512 bytes immediately above the boot sector, minimizing overlap risk. Larger reads must not stomp the stack you placed near 0x90000. The BIOS returns the number of sectors actually read in AH on success for CHS reads; for DAP, check the updated count field. Reset disk on error 0xAA before retrying on real hardware.

Why for this exercise

You will extend your boot sector to load additional sectors via a DAP. This exercise requires constructing the packet layout and branching to the loaded code at the buffer address.