I've just spent a ton of time debugging code that works great on
the PicMaster running at 4 MHz, but messes up on the actual chip (16c74a)
running at 16 MHz.  I finally got it to run by disabling interrupts in
the area where w is referenced in the main loop.  Below is the code I'm
using to store and restore context, then the ISR that uses these.  Also
shown are macros for EnableIRQ and DisableIRQ.  Where am I going wrong???

Thanks!

Harold



DisableIRQ macro
        local DIRQloop          ; Declare local symbol in macro
DIRQloop
        bcf     intcon,gie      ; disable global interrupts
        btfsc   intcon,gie      ; make sure it's clear
        goto    dirqLoop        ; if not, go clear again
        endm
EnableIRQ macro
        bsf     intcon,gie      ; set global interrupt enable
        endm

  cblock        0x20            ; reserve ram for save and restore
context
        isrW                    ; Hold registers during ISR. isrW is
also isrw! in bank 1
        isrStatus
        isrFSR
  endc
isrW1   equ     0xa0
SaveContext macro
; Save register context for isr.  Swaps swap high and low halves of f
with the result going either
; back to f or to w.  Swap is used here because they affect no status
bits.
        movwf   isrW            ; Save W
        swapf   status,w        ; get status in W (including bank
select bits) swapping hi/lo halves
        bcf     status,rp0      ; force to bank 0
        movwf   isrStatus       ; and save swapped status
        movf    fsr,0           ; Get fsr in w
        movwf   isrFSR          ; and save it
        endm
RestoreContext macro
        movf    isrFSR,0        ; get old fsr
        movwf   fsr             ; and restore it
        swapf   isrStatus,w     ; get old status (including bsr
bits) in w, swapping back to normal
        movwf   status          ; restore it
        swapf   isrw,1          ; swap halves in isrw making
them backwards
        swapf   isrw,0          ; swap back to forward and in
W without affecting status
        endm



ISR
;       Determine cause of interrupt and do it.
        saveContext             ; Save registers
        btfsc   pir1,rcif               ; check for received
character
        call    RxSerial                ; Go get byte
        btfsc   pir1,pspif              ; Parallel slave port
write?
        call    readPSP         ; Go handle it
        btfsc   intcon,t0if             ; Check for timer 0
interrupt
        call    Timer0ISR       ; go handle it (which transmits a
byte on dmx)
        restoreContext
        retfie