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Comment by Donckele | Hacker News Reader

Regarding the book, What would be the difference between 32bit and 64bit? Is it still relevant to read today?

Yes - it is one way to do it and it will work and be reasonably efficient. Everyone[1] who knows x86-64 asm today learned 32 bit first. You still have 32 bit instructions, and 16 bit, and 8 bit. The instructions will translate. You get more registers. You need to update the function and syscall calling conventions. There's no reason there can't be "learn x64 asm" books but there aren't many last I looked, but maybe someone can link us up to show how wrong I am?

https://wiki.osdev.org/Calling_Conventions

Richard Blum [2] after Dunteman.

[1] At a reasonable level of approximation.

[2] https://www.wiley.com/en-us/Professional+Assembly+Language-p...

I would doubt that, people haven't been running 32-bit x86 code for years at this point.

How exactly do you get away with not using the 32-bit subset of AMD64 ASM?

"32-bit x86 code" just means "code that does not use any of the AMD64 ISA." (And at this point, probably SSE2+ and a handful of other extensions.)

GP is correct: anyone that "knows" AMD64 assembly inherently must know 32-bit ASM because one must use the original x86 registers, instructions, etc. for the vast majority of tasks.

People are still constantly running what was formerly known as "32-bit x86 code" in their 64-bit applications.

I read “learned 32-bit first” as “learned on i386 machines”, not “learned what the 32-bit instructions do on 64-bit computers”. There’s a big difference between these two, because the way you write code for a machine that doesn’t do 64-bit is quite different, since you have many more registers, the calling convention is pretty different, etc.

From my very limited RE experience, one difference is calling convention: 32-bit passes a lot of stuffs on stack while 64-bit passes the first 8 (IIRC) through registers and the rest on stack. I could be wrong but this is one glaring difference. Another one of course is the registers them selves: EAX vs RAX for example.

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