Linux Assembly language programming.

Greetings,

I am interested in learning and programming in Assembly language in Linux environment.
Are there any books that you can suggest to buy to learn it. Or any other web site or publication or whatever....etc Any ideas or helps would be appriciated.
(I have progrmmed in MVS/S390 and Windows/DOS Programming environment using Asssembly).
Thank you very much.
Regards,
Mike

Hi,

there is a wonderful page on the web called something like linuxassembly.org but I had to use a mirror:

http://linuxasm.gerf.org/

This really looks helpful with docs, tutorials, examples and other things. I never had to use a book but who knows there may be good ones out there (the above page tells you about this too).

I wonder what you would like to know beyond that, but I find it helpful to know what tools to use. Also you want to know about the ABI (Application Binary Interface) which defines your interaction with the system around your routine. I have a PowerPC based machine so I don't know how much help I can offer you . I have seen a number of methods to deal with assembly though.

I generally like dealing with GNU tools since they are identical accross the architectures except maybe for the instructions and the ABI. Some people find other assemblers better (nasm) because they may support the format they like, but I never dealt much with DOS assemblers so I went straight to 'as'.

I can never keep my ABI in my head so I generally ask gcc for help :

int add(int a, int b) {
    return a+b;
}

then call

$ gcc -O -S add.c -o add.S

that produces:

.file "add.c"
.section ".text"
.align 2
.globl add
.type add, @function
add:

       add 3,3,4
       blr
       .size   add,.-add
       .section        .note.GNU-stack,"",@progbits
       .ident  "GCC: (GNU) 3.3.3 (Yellow Dog Linux)"

Now that our curiosity is satisfied... , hold it I have to explain some more. Most necessary explanations about the assembler can be found here:

http://www.gnu.org/software/binutils/manual/gas-2.9.1/html_chapter/as_toc.html

the .section tag is important in that it defines where your assembly goes in the ELF output of the compiler. There is a nice documentation about the ELF fileformat out there, you can find that on the page mentioned above or here:

http://www.skyfree.org/linux/references/ELF_Format.pdf

Most of the tags in the file above are not required for a functioning assembly program. The page mentioned on the top gives better examples I guess. To decipher it a little bit notice the 'add' instruction. The last two registers are input registers which are used to pass the first and the second parameter from a caller to the function. The destination register 3 is used to return the result. Notice that we don't have to deal with the stack or the temporary storage of register 3 and 4 because the ABI defines that those registers will not be saved by a function. The tough stuff is described in whatever manual you can find about your architecture (try this link: http://www-106.ibm.com/developerworks/linux/library/l-ppc/ , link at top & gcc manual also helps).

Now we can fiddle with it a bit. You can use gcc to compile the assembly code and also invoke the preprocessor if you keep the .S suffix.

How about that :

// Helpful comment:
// subtract - prototype: int sub(int a, int b)
//
.section ".text"
.align 2
.globl sub
.type sub, @function
sub:

       sub 3,3,4
       blr

Now if you call:
gcc -c add.S -o add.o

you get an object file you can link into your project. If you want to disassemble the output use:

$ objdump -d add.o

add.o: file format elf32-powerpc

Disassembly of section .text:

00000000 <sub>:

  0:   7c 64 18 50     subf    r3,r4,r3
  4:   4e 80 00 20     blr

See, its really called subf which means that there are simplified mnemonics which are described right at the end of the PowerPC manual (Appendix F) just in case this ever drives somebody nuts.

Notice that you can also use inline assembly when using gcc. There is a nice manual page you can get with

$ info gcc

and then select:

C Extensions -> Extended Asm

This usually helps me.

The S390 seems to have an acceptable number of registers (16?) switching to x86/DOS must have been a pain, eh?

Now that I think about it you may have wanted to know about more involved things but this is what I have.

Regards,

K. Timmler

Plop,

As kt said, there is the absolutely wonderful linuxassembly.org website.

I also purchased (months ago) a book on Assembly programming targeted at Linux _only_.

This is:

Linux - Assembly Language Programming
by Bob Neveln
ISBN: 0-13-087940-1

Not that bad and very interesting as a "reference" book.

Regards,

Xavier Maillard

"sometimes i feel like we're making emacs better and better because we don't 
 know what to do with emacs once it is finished."

    -- AlexSchroeder on #emacs @OPN