Unexpected linker section output location

Question

I'm trying to use the ld command in linux on an assembly file for a kernel. For it to boot with grub, it needs to be after the 1Mb address. So my link script has the text going to the address 0x00100000.

Here's the linker script I'm using:

    SECTIONS {
       .text 0x00100000 :{
            *(.text)
       }
       textEnd = .;
       .data :{
            *(.data)
            *(.rodata)
       }
       dataEnd = .;
       .bss :{
            *(.common)
            *(.bss)
       }
       bssEnd = .;
    }

My question is about the output file. When I look at the binary of the file, text section starts at 0x1000. When I change the text location in the script and use addresses lower than 0x1000, such as 0x500, the text will start there. But whenever I go above 0x1000, it rounds it (0x2500 will put the text at 0x500).

When I specify that the text should be at 0x100000, shouldn't it be there in the output file? Or is there another part of the binary that specifies that there's more moving to do. I'm asking because there's a problem booting my kernel, but for now I'm just simply trying to understand the linker output.

Answer 1

You are referring to two different address spaces. The addresses you refer to within the linked file (such as 0x1000 and 0x500) are just the file offsets. The addresses specified in the linker script, such as 0x00100000, are with respect to computer memory (i.e. RAM).

In the case of the linker script, the linker is being told that the .text section of the binary/executable file should be loaded at the 1MiB point in RAM (i.e. 0x00100000). This has less to do with the layout of the file output by the linker and more to do with how the file is to be loaded when executed.

The section locations in the actual file have to do with alignment. That is, your linker appears to be aligning the first section at a 4096-byte boundary. If, for example, each section is less than 4096 bytes in size and each placed at 4096-byte boundary, their respective offsets in the file would be 0x1000, 0x2000, 0x3000, etc. By default, this alignment would also hold once the file is loaded into RAM such that the previous example would yield sections located at 0x00100000, 0x00101000, 0x00102000, etc.

And it appears that when you change the load location to a small enough number, the linker automatically changes the alignment. However, the 'ALIGN' function can be used if you wanted to manually specify the alignment.

For a short & sweet explanation of the linker (describing all of the above in more detail) I recommend:

http://www.math.utah.edu/docs/info/ld_3.html

or

http://sourceware.org/binutils/docs-2.15/ld/Scripts.html