Get Memory Map via BIOS INT 0x15 E820

Topic Overview

E820 memory map via INT 0x15

Function AX=0xE820 returns an array of memory regions usable for planning page tables and kernel placement. Each call fills one 20-byte record: base, length, type, ACPI attributes. For example, type 1 means usable RAM; type 2 is reserved for firmware or ACPI.

Iterating the map

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• SMAP signature in EDX is mandatory on first call
• Store the map in a region stage two or the kernel will parse later
• Avoid overwriting the buffer while still executing from low memory

Passing the map onward

Kernels expect a known structure like Multiboot mmap entries or a custom array tagged with a signature. Place the buffer above 0x7E00 and record the count and pointer in registers or fixed cells for stage two. Type 3 ACPI reclaim and type 2 reserved must not be treated as free RAM. If E820 returns CF on first call, your buffer or signature is wrong; verify ECX is at least 20.

Why for this exercise

You will call E820 repeatedly and store region type, base, and length records. This exercise requires handling the continuation value in EBX and distinguishing usable RAM from reserved areas.

Parse a captured E820 memory map and compute the usable-RAM summary.

Input (stdin): one region record per line, three whitespace-separated fields (base (hex), length (hex), type (decimal 1-5)) as returned by INT 0x15, AX=0xE820.

Types: 1 usable RAM, 2 reserved, 3 ACPI reclaimable, 4 ACPI NVS, 5 bad memory; anything else prints as "Unknown" and counts as reserved.

Requirements:

• Echo each record in order as: region <n> base=0x<16 hex digits, uppercase> len=0x<hex> type=<t> <type name> where <n> counts records from 1
• Count only type 1 length toward usable_bytes; every other type (including Unknown) counts toward reserved_bytes
• Track the largest single usable region (first one wins on ties); with no usable region at all, largest_usable_base/len are 0
• entries = number of records read; a zero-length record still counts

Output exactly these lines after the region lines: entries=<decimal> usable_bytes=<decimal> reserved_bytes=<decimal> largest_usable_base=0x<16 hex digits, uppercase> largest_usable_len=0x<hex>