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Asked: 2026-06-17T06:05:02+00:00

I am trying to write a multi stage boot loader using NASM and gcc.

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I am trying to write a multi stage boot loader using NASM and gcc.
For this purpose I am referring grub boot loader source.

I have written a stage1 loader, but stuck at over writing partition
tables of MBR.

In grub stage1.S, they are using code like this to skip partition tables:

. = _start + STAGE1_PARTEND
.word   STAGE1_SIGNATURE

How can I do that in NASM?.

using times it will over write the partition tables.

So please help me.

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1 Answer

  1. Editorial Team

    Assigning a value to . is treated the same as a .org directive

    (see this page).

    Thus, the code you pasted is changing the current origin to _start + STAGE1_PARTEND and placing a word with the value STAGE1_SIGNATURE there.

    So in NASM code you should be able to do something like:

    fill: times _start+STAGE1_PARTEND-$ db 0
dw    STAGE1_SIGNATURE
                     ; .word is 16 bits on x86 (regardless of .code16 / .code64)

    Also see this example in the NASM manual showing how to pad a BIOS boot sector this way, as a replacement for MASM-style org or GAS-style . = new_position actually seeking and filling with padding in a flat binary output file.

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