x86 architecture 1 byte opcodes
32 comments
·October 31, 2025GuB-42
Hello sizecoders ;)
Additional resources:
http://www.sizecoding.org/wiki/DOS
A nice PDF with similar content:
https://pnx.tf/files/x86_opcode_structure_and_instruction_ov...
arjvik
Is sizecoding the same as the demoscene?
mras0
Size optimizing assembly code finds use in a variety of places. Demoscene for size constrained things is one of them, but also "hacking"/exploits and of course "whitehat" stuff (patches / compiler optimization etc).
sagacity
Relevant link to the current masters of the sizecoding niche: https://marqueedesign.demoscene.com/
classichasclass
You could call it a sub-scene of the demoscene, I suppose.
Sharlin
Need a couple of instructions for accessing memory (and possibly loading immediates) but otherwise seems like a perfectly adequate general-purpose instruction set. Might be fun (for some values of "fun") to write a compiler backend for it.
benlivengood
Push, pop, inc, and dec with a 16-bit register argument are one byte, so is ret. That technically gives you enough to do anything, but you can include jz/jnz (which do take immediate bytes, maybe cheating?), stosw, lodsw, clc, and stc to implement Brainfuck (a little harder to perform input/output with single byte instructions, but maybe pretend the OS uses int1 or int3 for calls).
sparkie
They're one byte opcodes, but not one byte ops. Most of them have operands which are encoded in a ModRM byte which follows the opcode. The ModRM may be followed by a SIB byte, and that may be followed by a a variable size immediate|displacement. There are also optional prefixes to the opcode.
themafia
You've always got the stack segment (SS) to play with and there's also:
null
GeorgeTirebiter
I don't understand, without further description of the symbols.
jcranmer
The explanation of the symbols is largely found here: https://www.sandpile.org/x86/opc_enc.htm
Essentially, the uppercase letter of an operand is a combination of the operand type (immediate, register, memory) along with how that is encoded (as ModR/M bytes have a register and a register/memory field), while the lowercase letter is the size of the operand (largely 8-bit/16-bit/32-bit/64-bit for the 1-byte opcodes).
mras0
Not sure why you're being downvoted. You need a to know quite a bit of esoteric knowledge to parse this beyond knowing x86 opcodes (even x86 assembly).
It's more or less the same information you get from the intel manuals (specifically appendix 2A of https://www.intel.com/content/www/us/en/developer/articles/t...). There you can also see what e.g. "Jb" means (a byte sized immediate following the instruction that specifies a sign-extended relative offset to the instruction).
One-byte opcodes here differs from 2 byte opcodes (386+ IIRC) prefixed by a 0F byte and even more convoluted stuff added later.
charcircuit
>Not sure why you're being downvoted.
I downvote people when they say they don't know what something is when they could have used a LLM to explain it to them.
mras0
The link is to an opcode map with strange abbreviations with no apparent explanation. Asking "What am I looking at?" without doing any research (with a LLM or otherwise) is entirely reasonable.
bigstrat2003
So you would rather people ask a machine that is known to be unreliable and have no idea what it's talking about, than ask a forum of technically skilled people who will give them a good answer. That doesn't seem very reasonable to me.
Rietty
What if the LLM gives them bad information and they don't know it? I personally would also just ask in a thread than risk the LLM info.
jrockway
I never punish people for asking a question. It's how you learn!
null
sparkie
You realize that LLMs are trained on human discussions right?
If everyone stops asking questions and asks the LLM instead, there is no new training data for future LLMs to learn from. They will stagnate, or consume their own slop, and regress.
A reverse engineer friend once taught me I could patch an x86 function with `0xEBFE` to get the CPU to spin forever. It wasn’t until much later that I understood that (IIRC) 0xEB is the “single byte” jump instruction and that of course 0xFE is -1 as a signed byte. Hence the spin.