Assembly on the ebm - Part 3

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3 - Skeletal View

Including any necessary register pushing and popping, a typical full function framework could look like this:

  push rp  ; r8->stack & increment rptr to point to r9
  push rp  ; r9->stack & increment rptr
  push rp  ; r10->stack...
  push rp  ; r11->stack
  push rp  ; r12->stack
  push rp  ; r13->stack
  push rp  ; r14->stack
  ldc  -100 ; set aside local storage - we need 100 bytes
  add  sp   ; decrease stack pointer value
  mov  t,sp ; and move it back

  ... do function's work

  ldc  100  ; reclaim our stack space
  add  sp
  mov  t,sp
  rptr r14  ; explicitly state the first reg to pop
  pop  rp   ; stack->r14, and rptr is decremented to point to r13
  pop  rp   ; stack->r13, etc.
  pop  rp   ; ...and so on
  pop  rp
  pop  rp
  pop  rp
  pop  rp

Of course, you can avoid the pushing and popping if you don't plan to overwrite any of these registers, and the amount of local space needed (here 100 bytes) will vary.

Time to Return

The side effects of rts are particularly interesting:

RPTR is set to r0.
r0 is copied to t.
The Z and S flags are set based on r0's (and t's) value.

This means that after a function return testing and calculation can immediately begin, since so many of the details are already taken care of. Again, the result is very compact code.

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