UEFI Shell Memory Commands

How memmap, mem, and mm help firmware engineers inspect memory maps, MMIO registers, and low-level platform state before OS boot.

June 10, 2026 2 min read

Sometimes the question is brutally simple:

Did the register really change?

UEFI Shell memory commands are useful when the bug sits below drivers and above hardware documentation.

memmap

Shell> memmap

Useful when investigating:

Shell> mem 0x00000000 0x100

mem dumps memory content. Depending on Shell implementation and platform policy, access may be restricted.

Shell> mm 0xFED40000

mm can read or modify memory / IO / PCI configuration depending on syntax and Shell support.

A firmware engineer may use it to inspect:

PM registers
GPIO controller MMIO
EC-related status register
PCI config space
chipset strap / lock bit

A sleep bug appears only after a BIOS change. Before diving into OS logs, a low-level check may ask:

Did firmware program the expected PM base?
Does the register range decode?
Is the lock bit already set?

Shell cannot replace a silicon debugger, but it can quickly show whether the platform state is obviously wrong.

A raw value like this is not useful by itself:

0x00000005

A useful debug note records:

Address
Access width
Expected bit definition
Original value
New value if modified
Reason for reading it
Board / BIOS version

Một số bài viết, ghi chú hoặc project có liên quan đến nội dung bạn vừa đọc.

Notes là nơi ghi nhanh khái niệm.