Excellent article.
Learning to Read X86 Assembly Language
11–20 of 238 posts
Re: Learning to Read X86 Assembly Language
#12x86 is the worst ISA. If you want to play with assembler without feeling a desire to stab yourself and end it all, I recommend ARM. Or go learn Z80, x86's weird, 8-bit cousin (it had a 16-bit version, but it sold poorly), which had a greater emphasis on backwards compatability (you can run code from the original 8080 on a Z80, unchanged), and is nicer to work with (because it wasn't extended in unticipated directions…
I never felt more like stabbing myself than when trying to cipher out exactly which immediate values are possible on ARM, and which are not. X86 I happen to enjoy. It is not "the worst ISA" by any means. It has wonderful code density, which turns out the be very important. There's a reason that x86 won and continues to win.
Re: Learning to Read X86 Assembly Language
#13Earlier quoted context omitted.
This is about learning to read, not learning to write. If you're ever in a situation where you need to read assembly, it's typically not up to you what ISA it'll be in. There are two situations where I've had to read assembly: either reverse-engineering a compiled binary or trying to understand the compiler output for a small piece of a program I'm working on in a higher-level language.
Oh. Well then, yeah. Besides, you don't have to worry about the worst of x86 in those cases. I'm pretty sure everyone has to learn at least that much x86 at some point, like it or not.
I'm not sure everyone has to learn even that much. I've managed to avoid it until very recently and I'm more willing to dive into underlying code than most people I know (I've contributed to a FreeBSD kernel patch in TCP and an OpenSSL key exchange interoperability fix)
Re: Learning to Read X86 Assembly Language
#14The examples seem odd to me: the argument order is reversed from every Intel disassembly (or assembler) I've ever used. Addi edi, 43 is the normal way to say "Add 43 to edi". The destination register is normally first; the source register 2nd in the disassembly, right?
Re: Learning to Read X86 Assembly Language
#15The examples seem odd to me: the argument order is reversed from every Intel disassembly (or assembler) I've ever used. Addi edi, 43 is the normal way to say "Add 43 to edi". The destination register is normally first; the source register 2nd in the disassembly, right?
gdb uses AT&T syntax by default. [0]
[0] https://en.wikipedia.org/wiki/X86_assembly_language#Syntax
Re: Learning to Read X86 Assembly Language
#16The examples seem odd to me: the argument order is reversed from every Intel disassembly (or assembler) I've ever used. Addi edi, 43 is the normal way to say "Add 43 to edi". The destination register is normally first; the source register 2nd in the disassembly, right?
In the examples given in the article it is written
addl $42, %edi
add edi, 42Re: Learning to Read X86 Assembly Language
#17x86 is the worst ISA. If you want to play with assembler without feeling a desire to stab yourself and end it all, I recommend ARM. Or go learn Z80, x86's weird, 8-bit cousin (it had a 16-bit version, but it sold poorly), which had a greater emphasis on backwards compatability (you can run code from the original 8080 on a Z80, unchanged), and is nicer to work with (because it wasn't extended in unticipated directions…
I never felt more like stabbing myself than when trying to cipher out exactly which immediate values are possible on ARM, and which are not. X86 I happen to enjoy. It is not "the worst ISA" by any means. It has wonderful code density, which turns out the be very important. There's a reason that x86 won and continues to win.
When originally invented the x86 instruction set was efficient - the most-used instructions had shorter byte code sequences. But eventually some instructions got 'left behind' by the compilers. There are a whole host of single-byte instructions that are never, ever used by a compiler - the register exchange instructions for instance (xchg eax, ebx). Compilers just schedule destination registers carefully, never need to swap them around.
Also the whole set of exchange-register-with-itself were defined but never used. E.g. xchg ax,ax which does nothing in one byte. In fact that one was considered useful, its used as the 'no-op' instruction (0x90) right? But what about xchg bx,bx, xchg cx,cx and so on? Just wasted single-byte opcodes. Leaving actual common instructions to use longer bytecode sequences.
So maybe an executable should begin with an opcode-decode-table that is loaded with the code, that tells the hardware what byte sequences mean what instructions. So each executable code can be essentially compressed, using optimum coding for exactly the instructions that code uses most often. Just thinking out loud.
Re: Learning to Read X86 Assembly Language
#18x86 is the worst ISA. If you want to play with assembler without feeling a desire to stab yourself and end it all, I recommend ARM. Or go learn Z80, x86's weird, 8-bit cousin (it had a 16-bit version, but it sold poorly), which had a greater emphasis on backwards compatability (you can run code from the original 8080 on a Z80, unchanged), and is nicer to work with (because it wasn't extended in unticipated directions…
I never felt more like stabbing myself than when trying to cipher out exactly which immediate values are possible on ARM, and which are not. X86 I happen to enjoy. It is not "the worst ISA" by any means. It has wonderful code density, which turns out the be very important. There's a reason that x86 won and continues to win.
Well, yes. That part sucks. But on x86, everything is like that. I'd rather have one weird, inconsistant thing than an amorphous, ever-shifting mass of them. Which is x86 in a nutshell.
Here's a handy heuristic: if somebody claims to know every x86-64 instruction (or even every x86-32 instruction), you can be at least 90% sure they're lying.
Re: Learning to Read X86 Assembly Language
#19Earlier quoted context omitted.
I never felt more like stabbing myself than when trying to cipher out exactly which immediate values are possible on ARM, and which are not. X86 I happen to enjoy. It is not "the worst ISA" by any means. It has wonderful code density, which turns out the be very important. There's a reason that x86 won and continues to win.
When originally invented the x86 instruction set was efficient - the most-used instructions had shorter byte code sequences. But eventually some instructions got 'left behind' by the compilers. There are a whole host of single-byte instructions that are never, ever used by a compiler - the register exchange instructions for instance (xchg eax, ebx). Compilers just schedule destination registers carefully, never need…
That would be cool, but I would not want to hand-code assembly on such a platform. That makes memory segmentation look fun.
Re: Learning to Read X86 Assembly Language
#20Earlier quoted context omitted.
When originally invented the x86 instruction set was efficient - the most-used instructions had shorter byte code sequences. But eventually some instructions got 'left behind' by the compilers. There are a whole host of single-byte instructions that are never, ever used by a compiler - the register exchange instructions for instance (xchg eax, ebx). Compilers just schedule destination registers carefully, never need…
That would be cool, but I would not want to hand-code assembly on such a platform. That makes memory segmentation look fun.
The table could be created from your assembly automatically? I would never want to code the hex directly in any case.