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Learning to Read X86 Assembly Language

patshaughnessy.net

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Re: Learning to Read X86 Assembly Language

#41

Post by qwertyuiop924 »

lgeek wrote: post #37
qwertyuiop924 wrote: post #18

Earlier quoted context omitted.

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.

> 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. I very much prefer ARM myself, but you can probably apply the same rule of thumb to it. There are upwards of 400 instructions both in AArch32 and AArch64, with a fair number of differences between the two. Edit: I've also posted a breakdown of the immediate li…

...But how many of those instructions are just variants of other instructions, but with a different addressing mode?

Re: Learning to Read X86 Assembly Language

#42

Post by qwertyuiop924 »

wolfgke wrote: post #40
qwertyuiop924 wrote: post #33

Earlier quoted context omitted.

...that really doesn't help with the issues I have: the instruction set is an absolutely ugly jumble of almost 40 years of religious backwards compatability, odd hacks, and various extensions.

> an absolutely ugly jumble of [...] religious backwards compatability, odd hacks, and various extensions Like POSIX, MS-Windows, X11, WinAPI, C++, HTML5, ... ;-)

Hey, I didn't say it was the only one.

Although POSIX and HTML5 hold up a bit better than Win32 and C++, IMHO. Especially POSIX (it's not great, but it works pretty well).

But I've heard X11 is absolutely miserable, so the windows folks don't have a monopoly on satanically evil APIs with religious backwards-compatability.

Re: Learning to Read X86 Assembly Language

#43

Post by lgeek »

qwertyuiop924 wrote: post #41
lgeek wrote: post #37

Earlier quoted context omitted.

> 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. I very much prefer ARM myself, but you can probably apply the same rule of thumb to it. There are upwards of 400 instructions both in AArch32 and AArch64, with a fair number of differences between the two. Edit: I've also posted a breakdown of the immediate li…

...But how many of those instructions are just variants of other instructions, but with a different addressing mode?

Fair enough, some instructions have many variants. But I work with ARM assembly almost daily and I still wouldn't remember that, for example, 'VQRDMULH' is a real instruction and it stands for 'Vector Saturating Rounding Doubling Multiply Returning High Half'.

Re: Learning to Read X86 Assembly Language

#45

Post by ris »

honkhonkpants wrote: post #12
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…

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.

> There's a reason that x86 won and continues to win.

It is definitely, definitely not the ISA.

Re: Learning to Read X86 Assembly Language

#47

Post by sigjuice »

ycmbntrthrwaway wrote: post #15
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?

gdb uses AT&T syntax by default. [0] [0] https://en.wikipedia.org/wiki/X86_assembly_language#Syntax

This article uses lldb, and not gdb. Both default to AT&T syntax, I guess.

Re: Learning to Read X86 Assembly Language

#48

Post by minimax »

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…

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.

Re: Learning to Read X86 Assembly Language

#49

Post by ivanhoe »

To understand assembly it really helps to know at least something about how computers work on a low level. When I first tried learning it (long time ago, in a high school) I had no idea how computers really work on such a low level, how CPU's addressing registers and that kind of stuff, and while I managed to learn the syntax, even write some asm code, it was all really confusing to me. And only few years later on University, after I've learned in details about the cpu architecture, registers, buses, DMA, etc, it suddenly all started to make perfect sense and became 100x more clear and easier. So if you're interested in this, it will save you a lot of effort to invest some time first to learn the computer architecture basics, and then from there go to learn the assembly lang. Just my $0.02

Re: Learning to Read X86 Assembly Language

#50

Post by wolfgke »

minimax wrote: post #48
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…

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 the appropriate segment override prefix and otherwise don't care about it.

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