plain binaries with gcc

[sancho1980]

hi

i have found this pdf online

http://www.cs.york.ac.uk/rtslab/demos/r ... mpiler.pdf

it describes how to compile plain binaries with gcc...i have followed the examples in there and compiled them the way im told in there and the problem is: i dont get the same compiled results as described in the pdf! i have disassembled the compiled output and i get different output..it seems there have been too many changes to the gcc compiler since this was written...can anyone tell me where else i can find information on how to make binary files with c???
i also gave it a try by simply jumping to the compiled binaries from my assembly code, which didnt work can u help me?

thanks

martin

[AJ]

Do you use a linker script for ld? If so, just use the line:

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OUTPUT_FORMAT(binary);

If not, there is a command line switch for this too (use --help).

Cheers,
Adam

[JamesM]

I attend that university (York UK). RTS is realtime systems, and is all done in ADA95, for embedded systems. I wouldn't be surprised if the docs there are out of date. The document also isn't written by anyone at York uni, so I'm not sure of it's quality. What exactly is the difference between your results and theirs?

JamesM <[email protected]>

[sancho1980]

Do you use a linker script for ld? If so, just use the line:

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OUTPUT_FORMAT(binary);

If not, there is a command line switch for this too (use --help).

Cheers,
Adam

i dont use a linker script..just using a simple makefile

i compile with "--oformat binary"...should that be the same?

[sancho1980]

I attend that university (York UK). RTS is realtime systems, and is all done in ADA95, for embedded systems. I wouldn't be surprised if the docs there are out of date. The document also isn't written by anyone at York uni, so I'm not sure of it's quality. What exactly is the difference between your results and theirs?

JamesM <[email protected]>

there are a couple extra instructions in my output plus, im not sure, when i compile a file with several functions where main() calls one of the other functions and i specify main() as the entry functions, i get the impression that main() is not really the code that is place right at the top of the output, not so sure though..looks all a bit weird to me..ill post it when i come home from work...

[AJ]

i compile with "--oformat binary"...should that be the same?

Yes - that's the one.

ill post it when i come home from work...

Will be interesting to see. I'll also look through that pdf - I haven't really compared my code to a dissasembly before...

Adam

[sancho1980]

Okay, the simplest program ever, as discussed at the very start of the text:

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//begin test.c

int main()
{
}

//end test.c

The text recommends to compile:

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gcc -c test.c
ld test.o -o test.bin -Ttext 0x0 -e main -oformat binary

And then says

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ndisasm -b 32 test.bin

will give me:

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00000000 55 push ebp
00000001 89E5 mov ebp,esp
00000003 C9 leave
00000004 C3 ret

Now, here's what I get:

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sancho@Kiste:~$ gcc -c test.c
sancho@Kiste:~$ ld test.o -o test.bin -Ttext 0x0 -e main --oformat binary
sancho@Kiste:~$ ndisasm -b 32 test.bin 
00000000  8D4C2404          lea ecx,[esp+0x4]
00000004  83E4F0            and esp,byte -0x10
00000007  FF71FC            push dword [ecx-0x4]
0000000A  55                push ebp
0000000B  89E5              mov ebp,esp
0000000D  51                push ecx
0000000E  59                pop ecx
0000000F  5D                pop ebp
00000010  8D61FC            lea esp,[ecx-0x4]
00000013  C3                ret

What are all these extra instructions all about? Am I really supposed to be able to execute this in a no-strings-attached environment, i.e., without any operating system? Also, is there no way to specify in the compiler options that main() neednt really be compiled as a function, since I'm not technically calling it but only jmp'ing there...
Thanks for any help

martin

[frank]

All of that extra code seems to be there in order to align the stack to a 16 byte boundary. That code will run just fine without an operating system, it doesn't call any other functions, it doesn't do any privileged operations it just aligns the stack. If you don't want main to return put a for( ;; ) before the end bracket and it can't possibly return.

[sancho1980]

Yes it came to me too a little later...so it seems I'm really able to jump from my assembly boot code to my C-code *happy*