Write a Minimal Boot Sector that Prints Message

Anatomy of a boot sector

The BIOS loads the first 512-byte sector of the boot disk to 0x7C00 and jumps there in real mode. Byte 0 starts with a short jump, bytes 510-511 must be 0x55 0xAA, and everything between is your code and data. For example, a teletype loop using INT 0x10 with AH=0x0E can print Hello before the sector hits the mandatory padding.

Minimal structure

nasmCopy
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• ORG 0x7C00: Assembler offset matches load address
• Stack: Set SS:SP (often 0x0000:0x7C00 or 0x9000:0xFFFF)
• Size limit: 510 bytes for code+data before signature
• Position independent: No hard-coded absolute addresses for code

Build with nasm -f bin and test in QEMU with -fda disk.img.

QEMU and disk images

dd if=boot.bin of=disk.img bs=512 count=1 conv=notrunc writes the sector at LBA 0. If QEMU prints no output, verify the boot signature with hexdump -C boot.bin | tail and confirm [ORG 0x7C00] matches the load address. Some BIOS builds leave DL as the boot drive; preserving it in memory helps later disk reads even if this task only prints text.

Why for this exercise

You will write a 512-byte boot sector that prints a multi-line message and halts. This exercise requires the 0xAA55 signature, BIOS teletype output, and exact sector padding.