Register Usage

[John Farmer]

Hi,

I am trying to understand how the general purpose registers are used by the
GCC compiler. Most I have been able to work out but have these questions.

Register R12 is initialised in my startup code to the same value as the SP
(R13) but it never appears to be used for anything. Whats the purpose of
this register?

I have looked at the ARM and THUMB procedure call standard (briefly) and it
looks as though R7 is the Thumb-state Work register or variable register.
However, looking at the assembly code produced for functions it is clearly
being used as the frame pointer. Why is there a difference?

Is there a defining text somewhere that details GCC register usage when
compiling for a Cortex-M3 chip?

I am using GCC V4.1.1 and a STM32F103RB (Cortex-M3) microcontroller.

Regards


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