Low level programming assembly/machine code question!

[s7r1k3r]

Assalam-O-Alaekum!

Assembly language is processor dependant not assembler dependent, right? Although, the difference between assemblers in linux and windows make the assembly code quite different but would the machine code also differ? Shouldn't machine code be the same because the OS is running on the same architecture?

[farhanksa]

yes assembly laguage is processor dependant..not the assembler...but the point is assebmler gives us the way to write different syntax .
the assemlby code do differ in the syntax and other matters..but the machine code is the same.. for specific processor architecture

[s7r1k3r]

Assalam-O-Alaekum!

Thanks for the reply. I have another question on the same topic. Is there a COM file equivalent for linux? Meaning, just like COM files don't have big headers with them, is there any such format for linux?

Also, what are ELF binaries and how do they differ from the older binaries?

[slick]

I dont know what youre asking about the COM file stuff. For ELF, you can search google or intels website. or take a look at this

http://www.muppetlabs.com/~breadbox/software/ELF.txt

[s7r1k3r]

By COM issue I meant that in DOS/Windows, COM files contain no extra header info except the PSF while EXEs contain a lot of extra assembly code not written by the programmer. I wanted to know if there was an equivivalent of this in Linux.

[slick]

hmm....i cant really think of anything that would do that...

[s7r1k3r]

Assalam-O-Alaekum!

Well, I found something. If we don't use the libc wrapper calls in assembly (meaning calling the syscalls directly) this reduces the size of elf binaries quite dramatically. I got a hello world program binary only in 488 bytes.

[lambda]

Thanks for the reply. I have another question on the same topic. Is there a COM file equivalent for linux? Meaning, just like COM files don't have big headers with them, is there any such format for linux?

pretty much all binaries you execute on linux are elf binaries. if you write your entire app in assembly and never call out to any libraries, you can use the a.out format instead. you might have to load a kernel module or recompile your kernel to make it support a.out, though.

Also, what are ELF binaries and how do they differ from the older binaries?

elf binaries can have multiple sections of data, code (called "text"), and comments. a.out just has text, data, bss (unless hacked for more sections, which pretty much everyone did before switching to elf). a binary built with debugging info can have another section just for debugging foo; the advantage is that this section doesn't have to be paged in when the binary is run. also, a.out has to be hacked to support shared libraries; elf supports it natively.

elf is also cross-platform, and most x86 (for example) operating systems use the same binary format for their executables.

[s7r1k3r]

Thanks for the informative reply. Does the PE (Portable Executable) file format used in Windows OS a variation of the ELF standard or is it something different?

[lambda]

it's a variant of COFF. see http://www.theparticle.com/cs/bc/os/elfpecoff.html.