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by hilarious 6081 days ago
Writing device drivers in C, and then using the -S flag ... have you seen the output gcc -S on Linux (Ubuntu) returns for a trivial hello world program? ;)
1 comments
jacquesm 6081 days ago
Sure. And for non-trivial programs as well.

Low level assembly is what you want, that presumably includes interfacing with you favorite C code.

Calling conventions, stack frames and so on.

Besides, the boilerplate runtime stuff has nothing to do with writing kernel code, so a 'trivial hello world' program will have a lot of cruft added to it:

        .file   "hello.c"
        .section        .rodata
  .LC0:
        .string "helo, world!"
        .text
  .globl main
        .type   main, @function 
  main:
        leal    4(%esp), %ecx
        andl    $-16, %esp

        pushl   -4(%ecx)
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %ecx

        subl    $20, %esp
        movl    $.LC0, (%esp)
        call    puts
        addl    $20, %esp

        popl    %ecx
        popl    %ebp

        leal    -4(%ecx), %esp
        ret
        .size   main, .-main
        .ident  "GCC: (Ubuntu 4.3.3-5ubuntu4) 4.3.3"
        .section        .note.GNU-stack,"",@progbits
that's not that bad.

Note the stack frame alignment trick, the fact that 'printf' was specified in the source but puts is being called!

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ismarc 6081 days ago
The -S flag was invaluable for me when I was moving from C to assembly. Then again, I used it in an iterative fashion, do something simple -S, do the same thing -S -O1, -S -O2, -S O3, and then a spattering of the different optimization techniques to see how it converted the C to assembly. I miss those days (did I just admit to being old?).
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jacquesm 6081 days ago
Well I'll admit it right along with you, I miss those days too.

I think I must have coded myself to the moon and back by now if you'd print it all out on fanfold paper (in C, mostly), but the joy of getting a little OS to boot up from nothing is never going to pale in comparison to installing framework X and building some web-app. No matter how successful.

Web-apps make substantially more $ though...

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