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Concept of Scratch and Preserved Registers

Muhammad Ahsan
Hi,

I am confused in understanding the difference between scratch and preserved registers. I have read the development guide of IAR Embedded Workbench but the topic was written in such a way that I could not get the real sence of it.
Any help or if possible some example that can help me differenciating between the both will be regarded.

Thanks in advance
Parents
  • Note: This was originally posted on 5th April 2011 at http://forums.arm.com


    @ttfn thanks for your detailed reply. Actually what I am unable to understand is that we have two groups of registers r0-r3 and r4-r11. I know that r0-r3 can be used for passing parameters while the remaining cann't. I am passing 4 arguments to the function foo called by main as in your example. Inside function foo I am doing some calculations which involve r0-r6. after return I want to have the same values in all registers as they were before calling foo. Do I need to push r4-r6 on stack before calling foo and then pop them back after return to have the same register values or it will be done automatically as they are called preserved registers. I have seen in the stack window that r0-r3 are pushed to stack, when they are used as parameters.
    I hope you got my point.


    Confusion, confusion...
    Question #1: are you programming in 'C' (or any other high level programming language)? In this case, you don't have to take care of any registers - the compiler is designed to do this all for you. You're supposed to use variables, not registers.

    Question #2: are you programming in 'C' but want to manipulate CPU registers directly. In this case - think twice. This is a very bad thing to do, very rarely necessary and it's a road to hell 
    unless you know exactly what you're doing.

    Question #3: are you using a mixture of C functions and assembly functions? This is where you have to read AAPCS and write
    your assembly code accordingly.


    Do I need to push r4-r6 on stack before calling foo and then pop them back after return to have


    The compiler must do this if the function foo() code _generated by the compiler_ happens to use these registers or the foo()
    function calls other functions which, in their turn, might use these registers.


    I have seen in the stack window that r0-r3 are pushed to stack, when they are used as parameters.


    The compiler should not be expected to save/restore r0-r3 (according to AAPCS), it might push it (temporary) if it  needs to
    but you should not rely on them being preserved. If foo() returns any value - it will be returned in r0 (or r0:r1 pair for 64 bit values).
Reply
  • Note: This was originally posted on 5th April 2011 at http://forums.arm.com


    @ttfn thanks for your detailed reply. Actually what I am unable to understand is that we have two groups of registers r0-r3 and r4-r11. I know that r0-r3 can be used for passing parameters while the remaining cann't. I am passing 4 arguments to the function foo called by main as in your example. Inside function foo I am doing some calculations which involve r0-r6. after return I want to have the same values in all registers as they were before calling foo. Do I need to push r4-r6 on stack before calling foo and then pop them back after return to have the same register values or it will be done automatically as they are called preserved registers. I have seen in the stack window that r0-r3 are pushed to stack, when they are used as parameters.
    I hope you got my point.


    Confusion, confusion...
    Question #1: are you programming in 'C' (or any other high level programming language)? In this case, you don't have to take care of any registers - the compiler is designed to do this all for you. You're supposed to use variables, not registers.

    Question #2: are you programming in 'C' but want to manipulate CPU registers directly. In this case - think twice. This is a very bad thing to do, very rarely necessary and it's a road to hell 
    unless you know exactly what you're doing.

    Question #3: are you using a mixture of C functions and assembly functions? This is where you have to read AAPCS and write
    your assembly code accordingly.


    Do I need to push r4-r6 on stack before calling foo and then pop them back after return to have


    The compiler must do this if the function foo() code _generated by the compiler_ happens to use these registers or the foo()
    function calls other functions which, in their turn, might use these registers.


    I have seen in the stack window that r0-r3 are pushed to stack, when they are used as parameters.


    The compiler should not be expected to save/restore r0-r3 (according to AAPCS), it might push it (temporary) if it  needs to
    but you should not rely on them being preserved. If foo() returns any value - it will be returned in r0 (or r0:r1 pair for 64 bit values).
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