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Comment by Grom_PE | Hacker News Reader

As AT&T syntax is still being used, at this point I'm willing to believe it's to purposely make x86 assembly hard and unpleasant to read and write. Perhaps so people will want to stay away from it, and in a way, to reduce the amount of code that is tied to the x86 platform. It spreads the thinking x86 assembly is terrible and ugly.

Intel syntax is much cleaner, in particular, Intel Ideal (as opposed to MASM), and specifically, FASM (flat assembler). FASM makes it as clean as possible and turns writing assembly into a joy.

Compare:

    movl %fs:-10(%ebp), %eax       ; AT&T
    mov eax, dword ptr fs:[ebp-10] ; MASM
    mov eax, [fs:ebp-10]           ; FASM

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As I recall, the "dword ptr" stuff was only necessary if the instruction was otherwise ambiguous. Using EAX means you are using a 32-bit destination. But something like:

    move fs:[ebp-10],5

is ambiguous. Is that an 8-bit constant? 16 bits? 32-bits?

I believe MASM-style assemblers require "dword ptr" at all times and many disassemblers keep outputting that in unambiguous situations.

Your example won't assemble because size can't be guessed, but this will:

    mov dword [fs:ebp-10], 5

All of those are misleading because the FS segment override isn't specific to an operand. It applies to the whole instruction, which commonly has one place (a memory reference) for the override to take effect. You can have more than one override, but only the last one remains active. Normally you can have an override even if it isn't used. There are a few instructions with more than one memory access; the override only applies to one access and you don't get to choose which one.

No. This is misleading:

    fs movs byte [edi], [esi]

This is a syntax error:

    movs byte [fs:edi], [esi]

And this is a valid override with sensible syntax:

    movs byte [edi], [fs:esi]

How would you disassemble that instruction with more than one segment prefix in front of it? The hardware accepts this by ignoring all but the final segment prefix. For example, the prefixes might be: FS, REP, GS, FS, FS

Note that code can jump past some of the prefixes. The C library on Linux does this to bypass prefixes. Reasonable assembly syntax needs to be able to describe this. You need to be able to put a label right after a prefix.

Ideally, like this:

    fs rep gs fs ; Note: extraneous prefix
    mov eax, [fs:ebx]

Some of disassemblers may put them as raw bytes (db 0x63, 0xf3, 0x65, 0x64).

> Note that code can jump past some of the prefixes. The C library on Linux does this to bypass prefixes

What's the use case for this?

I wouldn't split the prefix from the instruction and would rather use label+1 where it's absolutely required.

An example is bypassing a LOCK prefix when there is only 1 thread or only 1 CPU.

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