How to use gPXE with QEMU

Step by step

  1. Install Qemu
  2. Compile or download gpxe.pdsk.
  3. Type on the command line:
     qemu -fda gpxe.pdsk -net nic -net user -bootp 

Quick start

First make sure you have gpxe.pdsk or ns8390.pdsk. You can download them from or build them yourself.

Here is how to boot over HTTP:

qemu -bootp http://server/file gpxe.pdsk

Or to boot using PXE:

qemu -bootp tftp:// -tftp /path/to/pxedir -fda gpxe.pdsk

QEMU's built-in TFTP server serves files from /path/to/pxedir at IP address


The -bootp option was added in QEMU 0.9.1. For versions of QEMU before 0.9.1, the instructions from contrib/bochs/README.qemu can be used.

gPXE will strip one of the slashes immediately to the left of the actual filename in the -bootp URI you see above. QEMU 0.9.1 demands that there be a leading slash before the filename, so use the two slashes as you see them above unless your QEMU behaves differently.

Debugging gPXE with QEMU

The QEMU monitor (CTRL+ALT+2) supports debug commands to inspect registers and memory (try help or tab complete).

  • stop - Stops guest execution, continue using c.
  • info registers - Prints the CPU state including all registers.
  • x - Dumps memory:
x/5ih 0x10000        Disassemble 5 instructions in 16-bit mode at physical address 0x10000
x/iw 0xf600 + 0xa00  Disassemble 1 instruction in 32-bit mode at physical address 0xf600 + 0xa00 = 0x10000
x/xw 0x10000         Dump 32-bit value at physical address 0x10000
x/xh 0x10000         Dump 16-bit value at physical address 0x10000
x/xb 0x10000         Dump 8-bit value at physical address 0x10000

Address calculation

The QEMU monitor (and GDB stub) only deals with paged or physical addresses. Since gPXE does not use paging, QEMU does no address translation automatically.

Physical addresses can be used unmodified. This is what QEMU expects you to enter.

Real-mode addresses need to be translated. For example, 0400:f002 is (0x400 « 4) + 0xf002 = 0x13002 physical.

Protected-mode addresses need to be translated. For example, we want to calculate the physical address of the EIP value:

EAX=00000000 EBX=00055398 ECX=000556d4 EDX=00055398
ESI=f824d8d0 EDI=000adc58 EBP=00009ce8 ESP=00002d7e
EIP=000004f5 EFL=00000202 [-------] CPL=0 II=0 A20=1 SMM=0 HLT=1
ES =9ce8 0009ce80 0000ffff 00009309
CS =9c8b 0009c8b0 0000ffff 00009b09
SS =9ce8 0009ce80 0000ffff 00009309
DS =9ce8 0009ce80 0000ffff 00009309
FS =9ce8 0009ce80 0000ffff 00009309
GS =9ce8 0009ce80 0000ffff 00009309

Since instructions are loaded from CS:EIP, we need 0x9c8b0 + 0x4f5 = 0x9cda5.

Remember that QEMU understands expressions as addresses (e.g. 0x9c8b0 + 0x4f5).


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