gcc - What does the instruction mrc p15 do in ARM inline assembly?

Question

What does this line in ARM assembly do?

mrc p15, 0, %0, c9, c13, 0" : : "r" (counter)

What is p15? Usually registers are prefixed with r such as in r15.

What is the symbol :: and what are the roles or c9, c1?

Answer 1

Whilst MRC is a generic co-processor inter-op instruction, cp15 is the control processor - which all modern ARM CPUs have and this has been used by ARM was a means of extending the instruction set for on-chip units such as the cache, MMU, performance monitoring and lots else besides.

Taking your instruction a bit at a time:

mrc p15, 0, %0, c9, c13, 0" : : "r" (counter)

According to the ARM Cortex A7 MPCore Reference the instruction format is:

MRC{cond} P15, <Opcode_1>, <Rd>, <CRn>, <CRm>, <Opcode_2>

And on Page 4-11 this is described as a transfer of a CPU register to the performance monitor count register (I guess count=0 and this is a reset of the performance counter).

As for the syntax of inline assembler. refer to this for a x86 overview - which is probably similar to ARM.

The : : "r" (counter) means that the instruction has:

• No output in a register that needs to end up in a local variable
• Takes input from variable counter, and the register this is in should be used as %0.
• There are no side effects the compiler ought to be aware of (clobbers)