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Learning to Read X86 Assembly Language

patshaughnessy.net

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Re: Learning to Read X86 Assembly Language

#21

Post by wolfgke »

qwertyuiop924 wrote: post #4

x86 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…

> x86 is the worst ISA.

x86 is clearly not a beautiful ISA, but it is not as black as it is painted. The first thing that one should understand is large parts of the encoding of the instructions make a lot more sense once one writes them down in octal instead of hexadecimal (something even the Intel people who wrote the reference manual seem to have missed):

> http://www.dabo.de/ccc99/www.camp.ccc.de/radio/help.txt

Re: Learning to Read X86 Assembly Language

#22

Post by rzzzt »

School-Cotton wrote: post #6
qwertyuiop924 wrote: post #4

x86 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…

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.

My use case is inspecting a backtrace from a core dump.

Re: Learning to Read X86 Assembly Language

#23

Post by qwertyuiop924 »

blueatlas wrote: post #10
qwertyuiop924 wrote: post #4

x86 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…

Another good one for learning is the 6502. Don't forget to buy a copy of Lance A. Leventhal's "6502 Assembly Language Programming". Great book for learning not only assembly, but also the fundamentals leading up to assembly.

Of course. Who hasn't wanted to write their own chiptunes? Grab your (emulated) Ricoh 2A03 (and your emulated Konami VRC VI, for some extra fun), and get hacking.

And thanks for the book reccomendation.

Re: Learning to Read X86 Assembly Language

#25

Post by SeanDav »

qwertyuiop924 wrote: post #4

x86 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…

In my day I wrote a fair amount of x86 assembly language. I found it fairly easy and straight forward. There are probably even less reasons to learn it these days but nothing gives you a better idea of how a computer works (arguably, besides microcode, but that is a different kettle of fish).

Re: Learning to Read X86 Assembly Language

#26

Post by qwertyuiop924 »

toast0 wrote: post #13
qwertyuiop924 wrote: post #9

Earlier quoted context omitted.

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)

I haven't jumped into the kernel yet.

But I'm only 15. I still have time.

Re: Learning to Read X86 Assembly Language

#27

Post by raimue »

JoeAltmaier wrote: post #14

The 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?

The examples in the article are in AT&T syntax, you seem to be used to Intel syntax. Just keep on reading, the article will discuss the differences between AT&T and Intel syntax.

Re: Learning to Read X86 Assembly Language

#28

Post by qwertyuiop924 »

SeanDav wrote: post #25
qwertyuiop924 wrote: post #4

x86 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…

In my day I wrote a fair amount of x86 assembly language. I found it fairly easy and straight forward. There are probably even less reasons to learn it these days but nothing gives you a better idea of how a computer works (arguably, besides microcode, but that is a different kettle of fish).

The simple stuff's alright, but if you want to do anything performant, it gets hairy pretty fast, as you wade through almost 40 years of expansions and ugly hacks.

Re: Learning to Read X86 Assembly Language

#30

Post by wolfgke »

JoeAltmaier wrote: post #17
honkhonkpants wrote: post #12

Earlier 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…

> E.g. xchg ax,ax which does nothing in one byte.

Luckily in x86-16

  xchg ax,ax
simply is encoded as 0x90, which is the same as nop (the same holds for

  xchg eax, eax
in x86-32).

> But what about xchg bx,bx, xchg cx,cx and so on?

This (cleverly?) cannot be encoded in one byte. Here you have to use at least two bytes (0x87 followed by the ModR/M byte in x86-16; the same holds for their 32 bit counterparts xchg ebx, ebx; xchg ecx, ecx etc. in x86-32):

> http://x86.renejeschke.de/html/file_module_x86_id_328.html

---

ADDITION:

> 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.

The engineers of the Transmeta Crusoe/Efficion processor tried something similar:

> https://en.wikipedia.org/w/index.php?title=Transmeta_Crusoe&...

"Crusoe was notable for its method of achieving x86 compatibility. Instead of the instruction set architecture being implemented in hardware, or translated by specialized hardware, the Crusoe runs a software abstraction layer, or a virtual machine, known as the Code Morphing Software (CMS). The CMS translates machine code instructions received from programs into native instructions for the microprocessor. In this way, the Crusoe can emulate other instruction set architectures (ISAs).

This is used to allow the microprocessors to emulate the Intel x86 instruction set. In theory, it is possible for the CMS to be modified to emulate other ISAs."

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