I'm writing a NASM-compatible assembler targeting the Intel 8086, and I'm looking for an existing open source program written in assembly, with which I can showcase the capabilities (and understand the shortcomings) of my assembler. My plan with the source code is to convert it to NASM syntax manually, and then compile it with both NASM and my assembler, look at the difference, and see which subset of NASM is still missing or incorrect in my assembler.

I'm specifically looking for:

  • It should be an existing program for DOS 8086 or DOS 286, whose assembly source is available, and doesn't contain newer-than-286 instructions, protected mode instructions or floating point instructions. (Programs requiring a 386 or 8087 are not suitable.)

  • Preferably, the executable program should be a DOS .com file. My assembler doesn't support DOS .exe directly, but an .exe shorter than 64 KiB can be arranged by emitting the .exe header with dw instructions.

  • Preferably, the executable program size should be between 10 and 50 KiB, ideally near 30 KiB. There are some boot sector games by Oscar Toledo G., e.g. F-Bird and others by the same GitHub user, but they are only 512 bytes or less.

  • Any assembler dialect is fine, I will convert it to NASM manually.

  • The source code shouldn't make heavy use of assembler macros. (Conditional assembly and symbolic constants (e.g. NASM %define answer 42) are fine though.) Thus GW-BASIC dated 1983-02-10 is not a good candidate, because it uses lots of MASM macros.

  • The full source code (accounting for every single byte of the program file) should be available. It's OK if a few bytes are missing, I can add them from the disassembly.

  • Preferably the program shouldn't be a development tool (e.g. assembler, compiler, linker or interpreter), an operating system (e.g. msdos.sys, command.com, himem.sys) or a system maintenance tool (e.g. format.com, chkdsk.com, antivirus), but it should be a game or some other interactive productivity tool (e.g. text editor, picture editor, e-learning tool). This way it can be showcased in a DOS emulator as something directly useful on its own.

I was considering:

  • The game Arcade Volleyball: it's a timeless enjoyable game, it matches the desired file size), but its source code is not available, and the DOS port was written in Turbo C, not in assembly.

  • The game Paranoid: it's a timeless enjoyable game, the program file is a 26 KiB .com file, and it doesn't use 386 instructions. Unfortunately it's not open source.

  • Volkov Commander 4.05: it's very well-known, its built-in text editor can handle files larger than 64 KiB, the program file is a 47 KiB .com file, and it doesn't use 386 instructions. Unfortunately it's not open source.

As an FYI I elaborate here a bit what kind of testing I'm doing:

  • Mostly I write single-line unit tests checking whether a single assembly instruction compiles and what machine code it generates.

  • I also write multi-line unit tests with labels and various distances. Such a test checks that a short jump (with 1-byte offset) is generated for a label which is not too far away (about at most +-127 bytes). These optimizations depend on the -O... flag specified in the command-line, so I check the machine code output against the expected bytes in multiple optimization levels.

  • My goal is that my assembler to be compatible with NASM, and if different versions of NASM behave differently, then my assembler should match what NASM 0.98.39 compiled for a Linux i386 host does. Thus I also run the unit tests above with various versions of NASM and compare the machine code output. I don't compare the warning or error messages though.

  • Accepted behaviors of my assembler in general: (1) both my assembler and NASM 0.98.39 succeed for a particular input + command-line pair, and produce identical machine code in the output file; (2) either NASM 0.98.39 or my assembler fails (with non-zero exit code) for a particular input + command-line pair. An example for (2): my assembler fails if it encounters %macro. Another example for (2): NASM fails for db $*3.

  • I'd like to have as few failures (and as many matches) as possible, within my design limitations. An example design limitation of my assembler: it supports only bits 16, not bits 32 or bits 64. Sometimes I relax a small design limitation, but the one above is unlikely to change.

  • Comments are not for extended discussion; this conversation has been moved to chat.
    – Chenmunka
    Dec 12, 2022 at 8:45

4 Answers 4


The small / tiny text editors section on Free software for DOS includes a few text editors provided with source code, such as SuperTed. They are all smaller than 10KiB, so they don’t quite fit your requirements.

Any large DOS freeware / shareware compilation will have a few programs with assembly language source code; I don’t have any specific examples to highlight but a quick grep through SimtelNet archives reveals a few candidates.

A number of FreeDOS programs are written in x86 assembly language, including games such as Floppy Bird and Invaders.

A few demoscene productions for the PC include source code, usually in assembly language, but all those I’m aware of require a 386.

  • 1
    Thank you for the links! Floppy Bird program size is 8704 bytes, Invaders program size is 9194 bytes. I'm still looking for something bigger for the demo. The listings in your answer can keep me busy.
    – pts
    Dec 9, 2022 at 19:14
  • 3
    The Styx remastered source code might be worth a look, but building the executable involves linking. Dec 9, 2022 at 19:50
  • I've also looked at ibiblio.org/pub/micro/pc-stuff/freedos/files/games , and besides Invaders, there was a 2K (2048-byte .com program) Tetris which I can use, but that's still too small.
    – pts
    Dec 10, 2022 at 19:22
  • 1
    FreeDOS is a good option, there are quite a few asm programs but you have to ensure you install the source packages (or everything).
    – paxdiablo
    Dec 14, 2022 at 2:51
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    @StephenKitt: Thank you for recommending Styx Remastered! Linking wasn't a big issue, it was easy hardcode (with dw instructions) the EXE header to the NASM source. Translating from A86 to NASM syntax was also straightforward, but it needed a few hours of work. In the end, it has uncovered an optimization bug in my assembler. Translated source: github.com/pts/mininasm/tree/master/demo/styx
    – pts
    Jan 3 at 4:29

I wasn't going to post it as an answer, because I felt it exceeded the specified criteria, but I wrote a comment that the OP may enjoy testing their assembler on this actively-developed 8086 open-source assembly code: https://github.com/Joshua-Riek/x86-kernel. It's a 16-bit real mode kernel.

In response to my comment, I was pleasantly surprised that the OP responded:

Thank you for recommending github.com/Joshua-Riek/x86-kernel . By compiling it in my assembler I've discovered some useful, simple missing features (which I've implemented since then) and some machine code generation discrepancies between NASM and my assembler (which I've fixed since then). This is exactly the kind of sample assembly source code I'm looking for.

Given that the OP found it helpful, I'm posting the recommendation as an answer in the event that the comments get deleted at a future date, and that this may help others.

Another project's that has code that may be useful is an open-source assembly bootloader targeted for 8086 processors: https://github.com/Joshua-Riek/x86-bootloader. The project is designed to be assembled with NASM.

  • 2
    Thank you for both project links! After fixing some bugs in my assembler and rewriting the use of the features my assembler doesn't support, I've compiled both projects with NASM 0.98.39 and my assembler, and the resulting executable programs are identical.
    – pts
    Dec 11, 2022 at 23:17
  • 3
    You're welcome. That's very impressive, and a significant accomplishment. Well done! Do you think you may choose to release your assembler some day? Dec 12, 2022 at 7:51
  • github.com/pts/mininasm with source code and binary releases.
    – pts
    Dec 13, 2022 at 4:06

I've taken a look at all .com files of at least 6000 bytes on the FreeDOS 1.2 CD. Here is the list of programs written in assembly:

32935 NASM   devel/insight.zip.ex/progs/insight/insight.com
27566 JWasm  base/debug.zip.ex/bin/debugx.com
22476 JWasm  base/debug.zip.ex/bin/debug.com
23994 TASM   net/fdnet.zip.ex/network/pcntpk.com
16256 TASM   util/doslfn.zip.ex/bin/doslfn.com
15373 TASM32 util/wde.zip.ex/bin/wde.com
 6002 NASM   util/fdshield.zip.ex/bin/fdshield.com   
  • Source code (JWasm, MASM can also compile it, it uses macros heavily) of debug.com an debugx.com: https://github.com/Baron-von-Riedesel/DOS-debug

  • WDE, doslfn.com, debug.com and debugx.com use 32-bit registers (e.g. eax) a lot, so they won't run on a 8086, and it would be pointless to convert their source code to my assembler.

  • insight.com (within FreeDOS 1.2) uses 32-bit instructions very sparingly. Maybe it's possible to patch get rid of them. It uses multi-line macros sparingly. I'm getting about 400 errors from my assembler. Maybe I can convert it in a couple of hours, and then this modified Insight will be the largest real-world project my assembler is able to compile so far.

  • util/fdshield.zip.ex/bin/fdshield.com (6002 bytes, NASM source): After rewriting the uses of unsupported features, my assembler and NASM 0.98.39 produce an identical program file.

Please note that some .exe programs may also be written in assembly, I'll take a look at them later.

I've taken a closer look at Insight (insight.com is 32935 bytes, most of it is generated from NASM source), because that was the largest open-source DOS .com program I was able to find. Only a surprisingly few changes were necessary to avoid NASM features not supported by my assembler. I've fixed some bugs, most of them related to local labels. I've even added a few small features (such as labels starting with ..@, directive absolute $ and data instruction resw ...). Finally I discovered a serious design flaw in the jump size optimizer, at least my understanding how it converges to a fixed point was wrong. I disabled some checks, and it generated the correct insight.com file (bitwise identical to the one distributed and to the one NASM generates). This is great news: no machine code generation bugs found! However, I still had to fix the optimizer in the general case. Final case study with full source code here: https://github.com/pts/insight-mininasm

Oddly enough, games/flpybird/flpybird.com was missing from the FreeDOS 1.2 CD, even though this packages is available in FreeDOS 1.2. It's a 8704-byte DOS .com program written in NASM with only a few simple macros (which I was able to change to those supported by my asembler), and with a few 186 and 386 instructions. Since I've added 186 instruction support to my assembler recently, I only had to code the 386 instructions as db or dw when porting flpybird to my assembler. I've also discovered a parsing bug and a code generation bug in my assembler while making the port, so it was worth my time. Short case study here: https://github.com/pts/mininasm/tree/master/demo/flpybird

The game games/psrinvad/invaders.com was relatively straightforward to port, I just had to search-and-replace single-line macros (several dozen, so I've automated it) with their simple definitions in the NASM source. The final .com program file is 9194 bytes long. It's interesting that the source code of psrinvad contains autogenerated assembly source for many assemblers targeting DOS (not only NASM).

The debugger ldebug is written in NASM, it makes heavy use of macros, and it has a multistage build process, creating a compressed .exe in the end.

The game Styx Remastered was compiled using A86 and linked with TLINK. The styx.exe (without PKLITE compression) contains about 25600 bytes of 8086 machine code, the rest of the .exe file is headers, data and padding. Translating the linking was easy: I've hardcoded the DOS .exe header as dw instructions in the NASM source. Translating from A86 to NASM syntax was also straightforward, but it needed a few hours of work. In the end, it has uncovered an optimization convergence bug in my assembler (which I still have to understand and fix). Translated source: https://github.com/pts/mininasm/tree/master/demo/styx

I've also downloaded all games written in assembly from https://www.dosgamesarchive.com/related-games/open-source?r=63 . There were just 3 (other games were written in C, C++ or Pascal). I haven't checked yet how easy the conversion will be.

The largest program I was able to find written in 8086 assembly is Double Dragon II. The executable program file dragon.exe released on 1989-11-07 is 82056 bytes long. It's probably written in MASM 5.10a or earlier, the earliest .asm source file is dated 1989-08-28. Almost all of the source files were found in a deleted (but recoverable) file on a floppy disk. The files are archived in multiple places, e.g. on Internet Archive. Some source files are missing. It is unclear how much source code is missing.

  • 1
    Note that Debug/X, much like my fork lDebug, both optionally use 386+ features. They will run fine on 8086s (or eg NEC V20, as lCDebug is tested by me). The DPMI-enabled DebugX and lDebugX can also use 286+ features but only do so if Protected Mode is entered. Anyway, lDebug heavily uses the NASM preprocessor, creates an MZ .EXE output file (but as is all with two NASM assembly steps, no external linker), is generally larger than 64 KiB, and both the FreeDOS Debug/X and lDebug are "development tools".
    – ecm
    Dec 11, 2022 at 12:34
  • 1
    doslfn however does make mandatory use of 386+ registers and operations. I'm unsure about Insight, it's been a while since I have touched my Insight repo.
    – ecm
    Dec 11, 2022 at 12:43
  • 1
    Insight is documented as running on a 8086 as well. Much like lDebug it uses a patch table to change some instructions by writing an o32 prefix. (lDebug instead writes nop bytes and overwrites various o32, a32, or other instructions.) There is also two spots in its run functions where it explicitly dispatches based on a variable, eg hg.pushbx.org/ecm/insight/file/73e1b07abd73/src/trace.inc#l172 for saving/restoring fs / gs
    – ecm
    Dec 11, 2022 at 12:54
  • 1
    @ecm: I'm trying to compile your Insight repo with NASM, but the file lmacros3.mac seems to be missing. Also in tools/mkhelp.c, the hh (char) size specifiers are missing from "; HELP: %hhd %hhd %hhd %hhd %hhd.
    – pts
    Dec 11, 2022 at 23:56
  • 2
    I made a bunch of test files for my assembler and checked their output code with NASM, too. For instance addressing modes or segment override, maybe it could help.
    – vitsoft
    Dec 13, 2022 at 19:23

Might not meet this criteria, but it could be a good test anyway:

Preferably the program shouldn't be a development tool

I haven’t read through the source code, so I don’t know what kind of macros are in use, but DOS 2.0 DEBUG.COM is 11.5KB, and generated from DEBUG.ASM, which Microsoft open-sourced.

GW-BASIC is also open-sourced, but contains a dozen macros or more, so it might not be suitable for your test.

  • 1
    I also worked on Microsoft's free software Debug version a bit to add some features (recreated) from more recent (non-free) versions as well as a few extensions: hg.pushbx.org/ecm/msdebug I'm not satisfied with this as it still requires DOS to handle int 20h differently from int 21h function 4Ch, which eg FreeDOS does not implement. Also it's still in old MASM dialect.
    – ecm
    Dec 17, 2022 at 21:42

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