assembly - What's the real meaning of $$ in nasm

Question

$$ is defined as current segment address in NASM. But what's the real meaning of it? I wrote two asm files to test it:

a.asm

extern another

[section .text]
global _start
_start:
    mov ebx, $$
    call another

b.asm

[section .text]
global another
another:
    mov eax, $$
    ret

compile

nasm -f elf a.asm -g
nasm -f elf b.asm -g
ld -o test a.o b.o

Using gdb to debug the final file test, I found that though I defined two sections with the same name, the $$ is different in both file. So I guess that:

1. Once I defined a section in a file, the value of $$ is the starting address of that section. And $$ has nothing to do with the so-called segment registers(cs,ss, fs, gs, .etc).

2. If I have defined the another section with the same name in other file, it's interpreted as a different section. But if the two sections with the same name are defined in the same file, whether there are other section definitions between them, it's always interpreted as the same section, with the same $$ value. Such as follows, the two .text sections are just the same.

   [section .text]
   global _start
   _start:
       mov ebx, $$
   
   [section .d]
   d:
       mov ecx, $$ 
   
   [section .text]
   another:
       mov eax, $$
       ret
   

3. I guess there are some section names that NASM can recognize and put them to the right place when compiling. Such as .data, then what are these section names that NASM can recognize and do something with them? Thanks a lot!

Answer 1

$$ is the address of the beginning of the current section. It is a relocatable value (not "scalar" - a word you will find in an error message, but not in the Manual). It is an offset, so doesn't care what's in a segment register.

• Documentation: https://www.nasm.us/doc/nasmdoc3.html#section-3.5
• example use case for a boot sector: https://www.nasm.us/doc/nasmdo13.html#section-13.1.3
• Related: How does $ work in NASM, exactly? (since $$ is usually used with $).

About the only thing it's useful for is $ - $$, the length of the section so far. $ - $$ is a "scalar" (as is any difference between labels) and can be used in expressions which would otherwise cause Nasm to whine about "not a scalar value".

The section names "known" to Nasm depend on the output format - "-f obj" doesn't know any at all. .text, .data, and .bss are pretty universal - some output formats know others. Best place to find 'em is in the "output format" chapters in the Manual. http://www.nasm.us if you didn't get the Manual with your download. These names are case sensitive, and the leading '.' is required.

I have the feeling that there's a "question" in here that I'm missing. What are you actually trying to do?