Re: X86_64 cross compiler and generating binaries

[Daniel Kegel <dkegel at google dot com>]

wink saville wrote:
> I'm looking to create a simple kernel for the Amd64 and want to make a boot
> loader to start with. I have followed the instructions provided here,
> http://www.nondot.org/sabre/os/files/Booting/CompilingBinaryFilesUsingACompiler.pdf.
>
> I created a small program, t1.c ...
>
> When I compile this using a cross compiler created using Kegel's crosstool
> (http://kegel.com/crosstool/) using the following:
>
>   gcc -c -o t1.o t1.c
>   ld -Ttext 0x0 -e main -o t1 t1.o
>   objcopy -R .note -R .comment -S -O binary t1 t1.bin
>
> The result is:
>
>   t1.o   1256 bytes
>   t1     1049835 bytes
>   t1.bin 1048680 bytes
>
> When I compile it with gcc 3.3.2 (Debian) using the same options... The result is:
>
>   t1.o   699 bytes
>   t1     4791 bytes
>   t1.bin 51 bytes
>
> Sooooo, the 1M file is a mite big, any ideas what I'm doing wrong?

I suspect the default load address of x86_64's abi is higher than the one
in x86's abi.  Thus there are about a million zeroes at the start of
the t1.bin compiled for x86_64.

I couldn't find where it says this in http://www.x86-64.org/documentation/abi.pdf
but I bet that's it.

Running hexdump on both t1's verifies this.  The start address for x86 seems to be 0x1000 or so,
and for x86 it's 0x100000.

- Dan

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