Learning to Read X86 Assembly Language

Post by CalChris » Mon, Nov 28, 2016, 5:10 AM UTC

wolfgke wrote: post #50

minimax wrote: post #48

Earlier quoted context omitted.

It also didn't have memory segmentation, otherwise known as The Worst Thing. The examples in the post are clearly x86-64 running in 64 bit mode. I.e. it's running in flat model... there is no segmentation to worry about.

The segment registers fs and gs still exist in x86-64 and are used. Under Windows gs (under x86-32 fs) points to the Thread Information Block (TIB): > https://en.wikipedia.org/wiki/Win32_Thread_Information_Block Under Linux gs is used for thread-local storage (TLS). But "as a typical programmer" indeed you only have to worry about the internal details of this if you are an OS developer, otherwise you can simply use t…

Post by userbinator » Mon, Nov 28, 2016, 6:27 AM UTC

Narishma wrote: post #113

amw-zero wrote: post #88

Earlier quoted context omitted.

There's a bit of a miscommunication going on. What I meant was, when you write an assembly instruction, that maps to one machine instruction. I.e., because of things like addressing modes, different invocations of an ADD instruction can map to different machine instructions. But one ADD invocation will always map to one machine instruction. The parent comment sounded to me like one assembly instruction could map to s…

I read it as one assembly instruction can map to one of several machine instructions.

Post by MichaelMoser123 » Mon, Nov 28, 2016, 7:04 AM UTC

adamnemecek wrote: post #2

Also Matt Godbolt's gcc explorer is the the bee's knees for understanding assembly https://godbolt.org/ I think that playing around with it for 2 hours will teach you more than most classes on the topic. It really drives home why interactivity is such a bit deal in education. You should also try writing a script for counting instructions in binaries. It's pretty illuminating. Here are some sample statistics https://w…

Post by pjmlp » Mon, Nov 28, 2016, 7:24 AM UTC

MichaelBurge wrote: post #92

makmanalp wrote: post #82

Earlier quoted context omitted.

OK, so it's not a magical convention - it works it out bottom-up. First decide on registers for each parameter when you generate the code for the function, then based on that generate specific code for the instances where you call that function. Cool, thank you. Also that example, heh. I tried to go back gcc versions to see if there was a case where it didn't do TCO - nope. Also, I like how returning 0 is "xor eax, e…

Suppose you're right and the registers are arbitrary. Then how would foreign function calls work? If you're compiling Rust code that calls into a C library, how does it know what registers to use? So the choice of registers cannot be arbitrary, unless the compiler knows the function is only used within an object file. The registers are predetermined by a convention unless you use the 'static' keyword to signal that t…

Post by pjmlp » Mon, Nov 28, 2016, 7:28 AM UTC

userbinator wrote: post #70

What a train wreck! It’s hard to imagine a more confusing state of affairs. I'd say that's more attributed to someone many many years ago deciding they would not follow the official Intel syntax (for what reason I do not know), and somehow convincing the rest of the community to follow them. That's actually one of the things that could make for a very interesting article: how one processor family got two different an…