Assembly on the ebm - Part 3

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2 - Anatomy of a Function

Key to writing ebm assembly code is adhering to the programming conventions regarding registers. The registers of a well-behaved function look like this:

r0-r6 contain the parameters as are passed, starting with r0.
r7 is a scratchpad, and needn't be saved or restored.
r8-r14 is the parent's workspace, and must be saved and restored if needed.
r15 is also a scratchpad, and needn't be saved or restored.

When you call a function from your code, expect the above rules to apply in reverse:

Pass your function parameters in r0 through r6.
Expect r0-r7 and r15 to be trashed by the subroutine.
Keep variables you need preserved in r8-r14.

Variable Needs

Although register data is convenient and fast, sometimes it isn't enough. Some code only needs a few registers, while other code can use the parameter values as workspace. Then there's code that needs local storage, and for that the stack is ideal:

  ; make room on stack for local variables
  ldc -OFFSET ; constant - must be multiple of 4
  add sp      ; add stack pointer value
  mov t,sp    ; move back & protect space

Note that OFFSET is negative because the stack grows downward.

Once allocated, we can access the data:

  ldc VAR_OFFSET ; offset to variable
  add sp
  mov (t),r0     ; or some other instruction...

Finally, remember to clean up:

  ; make room on stack for local variables
  ldc OFFSET ; now positive-we are returning the space
  add sp     ; add stack pointer value
  mov t,sp   ; move back & free up

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