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Self-hosting Real Mode x86 Assembler
Assembly C Other
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Latest commit 342f575 Aug 23, 2019



SASM is a self-hosting assembler for real mode x86 for use in sufficiently DOS-like environments(*). The supported syntax is (meant to be) a subset of what NASM accepts(*).

To be self-hosting a very limited O/S is provided that can be used to develop the assembler on a clean system (requiring only an 8086/8088 processor, approximately 256K of RAM, a CGA video card, a floppy drive and limited IBM PC compatible BIOS functionality(*)).

*: See/use the code to get a better idea of what the requirements/limitations actually are.


Don't run this software in environments where you care about the consequences (this includes virtual environments where other files could be corrupted, e.g. under DOSBox). While the code wasn't written with ill intent in mind, it's only been lightly tested and most likely has severe disk (and otherwise) corruption bugs, which can potentially render your system unusable, so please test responsibly (and report any bugs you find).


Download a release or build the software yourself (see "Building"). can be run on most DOS compatible systems (this should include 32-bit Windows 10, but I don't currently have such a system to test) like DOSBox or FreeDOS.

A disk image (disk.img) containing the bootstrapping environment can also be downloaded and used with your favorite virtual machine (e.g. Bochs, QEMU, VirtualBox or PCem).

The disk image needs to be mounted in floppy drive 0 (A:) and a CGA compatible video adapter (e.g. VGA) should be present. 256K RAM is necessary to be useful (though the bootstrapping process runs with around 100K you won't be able to edit the larger files).

You can also try out the disk image online at PCjs Machines (Tested 2019-08-23).

  • Find a compatible configuration (e.g. IBM PC (Model 5150), 256Kb RAM, Color Display)
  • In the control panel (bottom) select "Browse..." and select the disk image.
  • Press "Mount" and then the "Reset" or "Ctrl-Alt-Del" button.
  • Hint: Press the "4.77 MHz" button a couple of times to get a less authentic but more enjoyable experience.


Modern Systems

Ensure you have the following installed and in your PATH:

  • CMake (3.7 or later, might work with earlier versions is you tweak CMakeLists.txt)
  • A C compiler that CMake can recognize (might need to set CC)
  • Optionally QEMU. In particular qemu-system-i386 should be in PATH.
  • Optionally DOSBox

Run the following commands in the source directory:

mkdir build
cd build
cmake ..
cmake --build .

If everything ran without errors there should be a disk.img file containing the bootstrapping environment as well as that can be used directly (it's been assembled by the C version of the assembler though, so while it should be equivalent to the result of self-hosting it might differ).

Assuming QEMU support is available, the bootstrapping environment can be tested:

cmake --build . --target qemu_test

The CMake variable QEMU_EXTRA_ARGS can be used to provide extra command line arguments to QEMU (e.g. -curses).


First make sure you've read the warning section. The bootstrapping process will use int 13h/ah=03, so make sure you're not going to regret this :)

(Note: This has only been tested with MS-DOS 5.0 and FreeDOS 1.2)

Assemble sasm.asm using either the provided C version of SASM or NASM:

nasm sasm.asm -o

Format a disk for use (drive 0 = A: must be used):

format /q a:

Copy required files to the disk:

copy *.asm a:
copy *.bat a:
copy a:
copy a:

Perform bootstrap:


The disk should now be ready, reboot and enjoy the same experience as on a modern, emulated system.

Supporting Utilities

C Version of the Assembler

This repository also contains a C version of the assembler, which is used for bootstrapping on modern systems and testing out implementation ideas in a rapid(er) development environment. It gives slightly better error messages than the ASM version and can warn about unused labels (-Wunused-label) and give hints on where to manually insert short for jmp (-Wshort).


Disktool can be used to create FAT12 floppy disk images, install bootloaders to them and insert / extract files. Call it without arguments to see a list of command line options.


Simple disassembler. Serves as test bed for the disassembler in debug.asm.

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