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Master the fundamental concepts of the boot process — theory through this focused micro-challenge.
Before an OS owns memory, firmware and PC architecture conventions carve the low megabyte into fixed regions. Bootloaders must avoid clobbering the IVT, BDA, and video areas. For example, the boot sector lands at 0x7C00, while the Interrupt Vector Table occupies 0x00000 through 0x003FF.
| Region | Range | Role |
|---|---|---|
| IVT | 0x00000-0x003FF | 256 real-mode interrupt vectors |
| BDA | 0x00400-0x004FF | BIOS data (equipment, timers) |
| Boot sector | 0x07C00 | 512-byte MBR/VBR load address |
| VGA text | 0xB8000 | Color text framebuffer |
| BIOS ROM | 0xC0000-0xFFFFF | Option ROMs and system firmware |
The A20 line gates address bit 20. When disabled, accesses wrap in the first megabyte (0x100000 wraps to 0x00000), which breaks loaders that place data above 1 MB. Enabling A20 is mandatory before protected-mode kernels use extended memory.
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You will map reserved regions and explain when the A20 gate must be enabled. This exercise requires showing why a bootloader cannot blindly use every address below 1 MB without hitting firmware-owned structures.
Create a program that documents and displays the BIOS memory map.
Requirements:
Three hints are available for this task, revealed one at a time inside the code workspace so you can struggle productively before seeing them.
All starter code and reference implementations are available for your local setup.
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