I was recently working on some x86 emulation code. This is one of the best links that I found to summarize how it works, skipping the giant Intel instruction set references.
I spent some time last weekend on a small side project which involves JIT encoding ARM64 instructions to run them on Apple Silicon.
I’ve written assembly before, but encoding was always kind of black magic.
How surprised was I to learn how simple instruction encoding is on arm64! Arguably simpler than implementing encoding wasm to byte code, which I played with a while ago.
If you want to play with this, based on my very very limited experience so far, I’d suggest starting with arm - fixed length 4 byte instructions, nice register naming scheme, straightforward encoding of arguments, make it very friendly.
X86 feels painful. So many instructions that wind up being decoded by physical hardware, eating up unnecessary die space and electricity, all to save ram and storage space which is now abundant and cheap compared to when x86 was designed
This reminds me that at some point I should write up my exploration of the x86 encoding scheme, because a lot of the traditional explanations tend to be overly focused on how the 8086 would have decoded instructions, which isn't exactly the same way you look at them for a modern processors.
I actually have a tool I wrote to automatically derive on x86 decoder from observing hardware execution (based in part on sandsifter, so please don't ask me if I've heard of it), and it turns out to largely be a lot simpler than people make it out to be... if you take a step back and ignore some of what people have said about the role of various instruction prefixes (they're not prefixes, they're extra opcode bits).
(FWIW, this is fairly dated in that it doesn't cover the three-byte opcodes, or the 64-bit prefixes that were added, like the REX and VEX prefixes).
I would absolutely love to read about that. Please, if you have the time to do that, do it.
also do you have a blog or a place where I can follow you and read your articles/code?
12 comments
[ 262 ms ] story [ 1477 ms ] threadhttps://youtube.com/playlist?list=PLJRRppeFlVGIvcTQNISPTxvNm...
I spent some time last weekend on a small side project which involves JIT encoding ARM64 instructions to run them on Apple Silicon.
I’ve written assembly before, but encoding was always kind of black magic.
How surprised was I to learn how simple instruction encoding is on arm64! Arguably simpler than implementing encoding wasm to byte code, which I played with a while ago.
If you want to play with this, based on my very very limited experience so far, I’d suggest starting with arm - fixed length 4 byte instructions, nice register naming scheme, straightforward encoding of arguments, make it very friendly.
I actually have a tool I wrote to automatically derive on x86 decoder from observing hardware execution (based in part on sandsifter, so please don't ask me if I've heard of it), and it turns out to largely be a lot simpler than people make it out to be... if you take a step back and ignore some of what people have said about the role of various instruction prefixes (they're not prefixes, they're extra opcode bits).
(FWIW, this is fairly dated in that it doesn't cover the three-byte opcodes, or the 64-bit prefixes that were added, like the REX and VEX prefixes).
concise & complete
1996 - 2025
with APX