[Mountain]

Recently, I read some books which say that R12 has some special use. It mean IP. But I think IP is the concept in Intel CUP, I did not see any concept about IP in ARM.

Dose R12 really have special use in ARM?

[Jacob]

The ARM architecture doesn't define special uses for many registers. Register "r12" is also referred to as "IP", and is used as an intra-procedure call scratch register. It is in the ARM Architecture Procedure Call Standard (AAPCS). However, it would be possible for a system to use a different procedure call standard that uses r12 for something else.

To be more descriptive, r12 can be used by the linker in the implementation of any veneers or similar generated code. It may also be used as a general-purpose scratch register between function calls, much like r0-r3.

Note: If you use assembly code, you can ignore the AAPCS for any utility functions you may define, so you can use r12 for whatever you like in that case. You only need to ensure that the externally-visible interface to your code is compliant with AAPCS.

[isogen74]

I think a good summary of AAPCS calling convention for C code is that the *called* function can safely assume that it can corrupt r0-r3 and r12. If the *calling* function needs to preserve these it must stack them before calling any function.

Note - private functions within a translation unit do not need to obey this rule, and most compilers will define an optimized calling convention for each TU local function.

This post has been edited by isogen74: 26 November 2008 - 09:35 AM