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Building an Activation Record.

I wish to implement Euclid's GCD algorithm.

GCD(A,B) =  A if B = 0
            GCD(B,A) if B > A
            GCD(B, A%B) otherwise

Here is some c++ code.

To start with, I want to write some code that implements int GetNumber(string prompt);
- I will pass the address of the prompt in $a0
- I will return the integer in $v0
- I will use $s0 for my input.
  - This is not efficient, use a t not an s, but I want to demonstrate everything.
- I will also save $ra. This is a leaf routine and I would not need to do that but...
In GetNumber
- I will save
  - FP, RA and S1
- I will start by saving relative to the sp
  - save fp at sp-4
  - save ra at sp-8
  - save s1 at sp-12
- I will then change the sp to sp-12
- Then do the computation
- Then restore relative to the fp
  - load s1 from fp -8
  - load ra from fp - 4
  - Load fp from fp
  - add 12 to the sp to restore the sp.
To call GetNumber
- The first time, I am not using any registers, so nothing to do.
- The next time I will have a value in t0.
  - In theory, I do not know if the function will use t0 or not, so I better save it.
  - This needs to occur on the stack.
- To do this
  - Store t0 at -4 off of the stack pointer.
  - Change the stack pointer by -4
  - Call the function.
  - At this point, I could restore t0 and the stack pointer, but I am about to make another call, so I just
  - Save t1 at sp-4
  - Change the sp again by -4.