Parse the BIOS Memory Map (E820)

E820 Memory Map

Firmware INT 15h AX=E820 returns address ranges and types (usable, reserved, ACPI). Kernels must not place page tables in reserved regions including MMIO holes.

Entry Types

Common ACPI types:

• Type 1: usable RAM
• Type 2: reserved
• Type 3: ACPI reclaimable

For example, a machine might report usable 0x00000000-0x0009FFFF and 0x00100000-0x7FEFFFFF with a hole for VGA framebuffer.

Why This Matters in Production

The E820 memory map you're parsing here is exactly what GRUB passes to every multiboot-compliant kernel, and it's the only way an OS learns which physical addresses are real RAM versus reserved for the VGA buffer, ACPI tables, or memory-mapped PCI devices. Allocating a page from a region you mistakenly marked usable is how kernels corrupt firmware data structures or write straight into device registers.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will call the BIOS hook in your bootloader and store an array of regions for the kernel allocator. This exercise requires marking which ranges are safe for buddy allocation.