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Posted: **Mon Dec 09, 2024 11:52 am**

In my continuing of implementing paging for my kernel, I've decided to load .text at 0xc0000000, .rodata at 0xc0040000, .data at 0xc0080000 and .bss at 0xc00c0000. I have the following section in my linker script:

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.multiboot.text : ALIGN(4K)
{
	*(.multiboot.text)
	_mbtext_end = .;
	ASSERT(_mbtext_end < 0x202000, "Paging setup code exceeds page boundary!");
}

. = 0xC0000000;
_kernel_base = .;

.text ALIGN(4K) : AT (_mbtext_end)
{
	*(.text)
	*(.text.*)
	_text_end_virt = .;
	_text_end = _text_end_virt - 0xC0000000 + _mbtext_end;
}
[...]

When I test the linker script, QEMU significantly lags my computer. When I pass the image through

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nm

, I notice _text_end is 0xc020514b, which means QEMU was probably trying to allocate 3 GiB memory. How can this be? The values of _text_end_virt and _mbtext_end are 0xc000412e and 0x0020101d, so _text_end should be 0x20514b. Does LD somehow not respect operator precedence when calculating values?

Posted: **Mon Dec 09, 2024 8:57 pm**

restingwitchface wrote: ↑Mon Dec 09, 2024 11:52 amwhich means QEMU was probably trying to allocate 3 GiB memory.

QEMU uses the program headers to decide how to load your binary. Try "objdump -h" or "readelf -l" or "readelf -aW" to see the program headers.

restingwitchface wrote: ↑Mon Dec 09, 2024 11:52 amDoes LD somehow not respect operator precedence when calculating values?

You're running into something funky with relative symbols. I'm not sure exactly what, but moving the definition of _text_end outside the .text section turns it into an absolute symbol that has the value you're expecting:

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    _text_end_virt = .;
}
_text_end = _text_end_virt - 0xC0000000 + _mbtext_end;

It seems like this behavior is for backwards-compatibility. The manual tries to explain it here.

OSDev.org

Linker script confuses physical and virtual addresses

Linker script confuses physical and virtual addresses

Re: Linker script confuses physical and virtual addresses