x86 architecture 1 byte opcodes • Hacker News Board

Re: x86 architecture 1 byte opcodes

#31

Post by benlivengood » Sat, Nov 01, 2025, 12:09 AM UTC

Sharlin wrote: post #4

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.

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).

Re: x86 architecture 1 byte opcodes

#32

Post by ryanschneider » Sat, Nov 01, 2025, 12:18 AM UTC

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.

Re: x86 architecture 1 byte opcodes

#33

Post by charcircuit » Sat, Nov 01, 2025, 12:34 AM UTC

mras0 wrote: post #22

charcircuit wrote: post #18

Earlier quoted context omitted.

It is entirely reasonable, but these kind of comments are essentially wishing sites could cater to their knowledge level. It's like complaining that the article is not written in French. It's noise in the comment section of an article. If someone wants such a thing, browsers have functionality to translate pages to French. Not every site needs to have their own French translation to suit such a person.

I understand what you're getting at, but in this case even I (who know what most things on that page means) struggle to understand why it was submitted. Are we looking for the 0E opcode? New optimization opportunities? Genuinely asking, for this post did you click on the link and say "yeah, I got the point" or did you involve an LLM? If you did, what did you ask it? I'm asking because I want to get better at LLM use…

I didn't initially use an LLM, but when drafting my original post I did double check that Grok was able to explain it to ensure I want demanding the impossible.

I asked it "Explain the syntax on the page https://www.sandpile.org/x86/opc_1.htm"

Re: x86 architecture 1 byte opcodes

#34

Post by GeorgeTirebiter » Sat, Nov 01, 2025, 12:54 AM UTC

charcircuit wrote: post #21

wewtyflakes wrote: post #19

Earlier quoted context omitted.

They were not asking for the website to change; they were asking for context so that they can appreciate the website.

In this case the person was not asking anything. The person was stating they didn't understand. The equivalent in my analogy is a French speaker commenting that they don't understand English without further translation into French.

Geez. I was the first one to comment. It was "This may be great, but... would you please give us more explanations / context." It's not "laziness" but trying to understand how this table is useful / teaches us something. And, to the OP, that a 'typical' HN reader didn't get it.

I know 8008, 8080, z80, 80186, 80286, 80386, 80486, and some fancy opcodes for SSEx. The table still, IMHO, needs further explanation. Some have provided pointers to more info; thank you.

Re: x86 architecture 1 byte opcodes

#35

Post by 9rx » Tue, Nov 04, 2025, 10:18 PM UTC

bigstrat2003 wrote: post #24

charcircuit wrote: post #11

Earlier quoted context omitted.

>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.

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.

Why's that? Its is most advantageous to ensure that other people are kept in the dark.

If they are willing to pay to level the playing field perhaps it might be worth your while to fill them in. The old scholastic business model — gotta pay to play. But to take precious time out your day to fill them in to your own personal disadvantage...?

In other circles where people have well-rounded feelings you might find someone willing to do it just for the warm fuzzies it gives them. But technically skilled people are generally void of such emotion. That is often what compels them towards technology in the first place.