CODEWITHSUNDEEP
gccassemblyx86inline-assemblyatt
octain
octain

Reputation: 964

GNU assembly Inline: what do %1 and %0 mean?

I am very new to GNU assembly inlining, I have read multiple write ups but still do not fully understand what is going on. From my understanding:

movl %eax, %ebx\n\t will move whatever is in %eax into ebx, but will not add the contents to each other

addl %eax, %ebx\n\t will add the contents of %eax with ebx and keep it at the right most register

addl %1, %0\n\t this is where i get confused, we are adding 1 and 0? why do we need to have the %0 there?

Upvotes: 3

Views: 3810

Answers (1)

NoName
NoName

Reputation: 71

The whole asm inline block looks like:

 asm [volatile] ( AssemblerTemplate
                      : OutputOperands
                      [ : InputOperands
                      [ : Clobbers ] ])

OR

 asm [volatile] ( AssemblerTemplate
                      : OutputOperands)

In the AssemblerTemplate is your assembly code, and in Output/InputOperands, you can pass variable between C and ASM.

Then in Asm, %0 refers to the first variable passed as OutputOperand or InputOperand, %1 to the second, etc.

Example:

 int32_t a = 10;
 int32_t b;
 asm volatile ("movl %1, %0" : "=r"(b) : "r"(a) : );

This asm code is equivalent to "b = a;"

A more detailed explanation is here: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

Upvotes: 6

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