Here are a couple of MPALC macros I found useful for storage
allocation on PICs.  They're for when you wat a register allocated
but don't care which; you can choose whether to allocate from the
top or bottom end of the register set.  The parameters given here
do for 16C71 and 16C84 PICs, for others change RAM_Start and MAX_RAM.

If you like these, I also have some code for implementing timed finite
state machine dispatch (a machine is a set of states;  the current state
is executed after a delay specified as a number of calls of the dispatcher;
states can set the next state and next delay to dispatch to;  for N instances
of machines, it uses 2N+2 registers, 2N of them permanently and 2 as temps
for dispatch).  I also have a master transmitter and slave receiver I^2 C
implementation for PICs (16C84 or 16C71) running at > 6MHz that supports
multimaster operation (in theory).  If you are interested, say so, and I'll
post the code.

John Hallam, AI Dept., Edinburgh University. 

-------------------cut here for MPALC code---------------------------

;******************************** Register File Store Allocation
;
; These macros allocate store from the register file.  If there is
; enough store they succeed, defining the specified label.  If not,
; they fail, leaving the label undefined and so inducing errors.

        RAM_Start       equ     0Ch             ; First GP Register
        MAX_RAM         equ     30h             ; Last GP Register + 1

        RAM_Lo          set     RAM_Start       ; Current low water mark
        RAM_Hi          set     MAX_RAM         ; Current high water mark

        AllocHi macro   label,amount            ; Allocate store called <label>

                if      RAM_Lo+amount > RAM_Hi  ; Not enough registers  
                exitm                           ; ERROR EXIT
                else
                RAM_Hi  set     RAM_Hi-amount   ; Adjust mark
                label   equ     RAM_Hi          ; Allocate at high end
                endif
                endm

        AllocLo macro   label,amount            ; Allocate store called <label>

                if      RAM_Lo+amount > RAM_Hi  ; Not enough registers  
                exitm                           ; ERROR EXIT
                else
                label   equ     RAM_Lo          ; Allocate at low end
                RAM_Lo  set     RAM_Lo+amount   ; Adjust mark
                endif
                endm