How to create an image file with a specific size from a linked absolute code

How to create an image file with a specific size from a linked absolute code

[ali hagigat]

I am using two assembly files and a linker script like what follows:

asm2.s:

#comment1
.comm dovom, 10
.byte 12,15,28
aval:
movl %edx, 32(%edi)

asm3.s:

.section .text
.comm dovom, 10
aval:
movl $26, %eax

asm2.ld:
SECTIONS
{
	section5 0 :
	{
		*(.data);
		*(.bss);
	} = 0xffffffff

	.text 0x400000 :
	{
		*(.text);
		BYTE(0x57);
		BYTE(0x58);
		BYTE(0x59);
	} = 0xffffffff
}

It means that between between section5 and .text there is a huge gap!
Then I compile with:
ld asm2.o asm3.o -T asm2.ld -o asm2
objcopy -O binary asm2 asm2.img

I want my final image, means asm2.img, becomes 4 mega bytes and the
space between section5 and .text gets filled by 0xff.
I wonder if anybody knows how to do.
Thank you.

Re: How to create an image file with a specific size from a linked absolute code

[Nick Clifton]

Hi Ali,

[Note your asm2.s file needs a .data directive at the start in order to 
ensure that the .byte directive puts values into the data section and 
not the text section].

> It means that between between section5 and .text there is a huge gap!
> Then I compile with:
> ld asm2.o asm3.o -T asm2.ld -o asm2
> objcopy -O binary asm2 asm2.img
>
> I want my final image, means asm2.img, becomes 4 mega bytes and the
> space between section5 and .text gets filled by 0xff.

Are you sure that the space is filled with 0xff ?  In the test I ran it 
was filled with 0x00.

> I wonder if anybody knows how to do.

What exactly is it that you want to happen ?

The "binary" file format does not support gaps, so you are not going to 
be able to create a binary file without some padding between the two 
sections.  On the other hand if you are using a modern file system (eg 
ext3, but not NTFS) then the gap will not actually take up a large 
amount of disc space.  The file system detects the pages full of zeroes 
and just does not bother to store them:

   % ls -l asm asm.img
   -rwxrwx---. 1 nickc nickc    8656 May 17 16:06 asm*
   -rwxrwx---. 1 nickc nickc 4194316 May 17 16:06 asm.img*

   % du -s -k asm asm.img
   12      asm
   12      asm.img

Cheers
   Nick