Use QEMU Monitor to Inspect BIOS Entry State

Inspecting firmware state in QEMU

QEMU's monitor lets you pause the emulated CPU right after reset and read registers before any bootloader runs. That snapshot shows the real-mode segment values and instruction pointer the BIOS entry code expects. For example, after system_reset, info registers typically shows EIP=0xFFF0 and segment registers matching legacy x86 reset behavior.

Monitor commands that matter

info registers: CS, IP, flags, and general-purpose registers at the current halt point
x /10i $cs:$pc: Disassemble the next instructions from the code segment
info mem: Overview of mapped regions (ROM, VGA, RAM)
info tlb / info mtree: Memory layout details on newer QEMU builds
system_reset: Cold reset to re-run the power-on path

c
Loading…

Launch with the monitor on stdio or a Unix socket:

bash
Loading…

Compare register values against the Intel reset documentation
Note real mode: 16-bit operations, 1 MB logical view
ROM at the top of the address space holds the first jump into BIOS

Why for this exercise

You will use the QEMU monitor to capture BIOS entry register state and disassemble the first instructions. This exercise requires correlating what you see in emulation with the documented reset vector and real-mode addressing rules.

Use QEMU to inspect the CPU state immediately after power-on.

Requirements:

Start QEMU x86-64 with monitor enabled using -monitor stdio
Pause immediately after start (QEMU pauses by default in monitor mode)
Use info registers to view all register values
Examine memory at the reset vector 0xFFFFFFF0
Document the CS and IP register values
Document the first instruction at the reset vector
Explain why the CPU starts in real mode
Print your findings in a structured format
Use system() to spawn QEMU process and capture output

Inspecting firmware state in QEMU

Monitor commands that matter

info registers: CS, IP, flags, and general-purpose registers at the current halt point
x /10i $cs:$pc: Disassemble the next instructions from the code segment
info mem: Overview of mapped regions (ROM, VGA, RAM)
info tlb / info mtree: Memory layout details on newer QEMU builds
system_reset: Cold reset to re-run the power-on path

c
Loading…

Launch with the monitor on stdio or a Unix socket:

bash
Loading…

Compare register values against the Intel reset documentation
Note real mode: 16-bit operations, 1 MB logical view
ROM at the top of the address space holds the first jump into BIOS

Why for this exercise

Use QEMU to inspect the CPU state immediately after power-on.

Requirements:

Start QEMU x86-64 with monitor enabled using -monitor stdio
Pause immediately after start (QEMU pauses by default in monitor mode)
Use info registers to view all register values
Examine memory at the reset vector 0xFFFFFFF0
Document the CS and IP register values
Document the first instruction at the reset vector
Explain why the CPU starts in real mode
Print your findings in a structured format
Use system() to spawn QEMU process and capture output

Topic Overview

Inspecting firmware state in QEMU

Monitor commands that matter

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source

Use QEMU Monitor to Inspect BIOS Entry State

Topic Overview

Inspecting firmware state in QEMU

Monitor commands that matter

Why for this exercise

Implementation Task

Progressive Hints

Resources

Open Source