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dax86

x86 (i386) Emulator in C

Why:

  • I wanted to trace how an OS runs on CPUs at archtecture / instruction level.

What:

  • Runs vanilla xv6 (memfs) image from boot.
  • Each instruction is manually implemented and executed sequentially without binary translation or OoOE.
  • Representation of logic is prioritized over the performance. Codes have bunch of comments covering the instructions and the hardware mechanism as well.

Done:

  • Instructions required for running xv6
  • Real mode
  • Protected mode
  • Paging
  • MP configuration
  • Software/Hardware interrupts
  • Device emulation (disk, keyboard, APIC timer, local APIC, IO APIC, UART etc)

To do:

  • FPU-related instructions
  • Exception
  • Virtual 8086 mode
  • TLB

System requirements:

  • Memory: 512MB
  • CPU: scheduler loop will occupy CPU resource. nice command, cgroups or docker resource setting might help here.
Build dax86
make
Run
# basic use
./dax86 [binary_file]

# run xv6 (ctrl + c to stop)
./dax86 xv6memfs.img

# verbose run (prints each op)
./dax86 [binary_file] -v
Setup Environment using Docker

Though dax86 can be built for different targets and run, there's a docker image in case Debian's build-essential package is preferred to the build environment of your host. Mini Debian Jessie is used for the base image.

The command below will build image, run a container with the image and execute shell in interactive mode.

make create-docker

To clean created container and image, run the command below.

make clean-docker
Test
# test all
./test.sh

# test specific one in tests/exec directory
./test.sh [test_name]

# directory test binary (stops at EIP: 0x0)
./dax86 test [binary_file]
Commands to Analyze Test Cases
  • Disassemble Binary
# 32 bit all the way 
ndisasm -b 32 [binary_file]

# 32 bit after 0xFF bytes of real mode instructions
ndisasm -b 32 [binary_file] -k 0,0xFF
  • View Binary
# hex
xxd [bin_file]

# bin
xxd -b [binary_file]

Referenced these publications with many thanks: