What do these ARM assembly instructions do?

Question

I came across these ASM instructions recently and am trying to figure out what they do. I went through a few ARM assembly books (such as THIS) and was able to figure out what a couple of instructions do.

For instance, MRS (Move to Register from Status) copies status from the PSR (Program Status Register) register to %0 (which I believe is register 0).

I am not able to understand what : "=r" (Cs) and other similar instructions do.

UINT32 Cp;
if((Cp & 0x1) == 0)
{
   UINT32 Cs;
   __asm
   (
     " MRS %0, PSR\n"
     "BIC %1, %2, #0x80\n"
     "cpsie i"
           : "=r" (Cs), "=r" (Cp)
           : "r" (Cs)
   );
}

Can someone please explain?

Edit 1: This is inside a GlobalLock release function. cpsie i enables the interrupts.

Useless Useless · Accepted Answer · 2015-03-09T16:09:09

That isn't (just) assembler, it's inline assembler embedded in C code. Presumably there's some context around this code, or where you found it, which should suggest this.

See, for example, the GCC Extended Asm docs.

This builtin emits the assembler (which you've already identified) in the middle of some C code, and also tells the compiler what the assembler did: specifically, which registers were used or damaged.

The last two lines indicate which variables are used in the assembler, and how they're affected

"=r" (Cs) - the variable Cs is used as an output register, called %0 in the assembly code, and is overwritten
"=r" (Cp) - the variable Cp is used as an output register, called %1 in the assembly code, and is overwritten
"r" (Cs) - Cs is used as an input register, called %2 in the assembly code

Note that %0/1/2 are just the positions of the entries in the input/output list, starting from zero.

What do these ARM assembly instructions do?

2 Answers