Get Memory Map via BIOS INT 0x15 E820

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.

Implement BIOS E820 memory map retrieval in boot sector code.

Requirements:

Use INT 0x15, AX=0xE820 to query memory map
Store entries in a buffer at a known address
Handle the continuation mechanism (EBX)
Print each entry: base, length, type
Show type names (Usable, Reserved, ACPI, etc.)
Count total usable RAM
Handle errors (CF set)
Use 'SMAP' signature
Call until EBX=0
Print summary statistics

Test:

Show E820 call initiated
Show memory entries found
Show usable RAM total
Show reserved regions identified

E820 memory map via INT 0x15

Iterating the map

nasm
Loading…

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

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.

Implement BIOS E820 memory map retrieval in boot sector code.

Requirements:

Use INT 0x15, AX=0xE820 to query memory map
Store entries in a buffer at a known address
Handle the continuation mechanism (EBX)
Print each entry: base, length, type
Show type names (Usable, Reserved, ACPI, etc.)
Count total usable RAM
Handle errors (CF set)
Use 'SMAP' signature
Call until EBX=0
Print summary statistics

Test:

Show E820 call initiated
Show memory entries found
Show usable RAM total
Show reserved regions identified

Topic Overview

E820 memory map via INT 0x15

Iterating the map

Passing the map onward

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Get Memory Map via BIOS INT 0x15 E820

Topic Overview

E820 memory map via INT 0x15

Iterating the map

Passing the map onward

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source