PIC Microcontoller Math Method

Increment / Decrement

Best 16 bit

From Scott Dattalo

;inc
 incf high,f
 incfsz low,f
  decf high,f


A downcounting version;
;dec
  movf    low,f
  skpnz
   decf   high,f
  decf    low,f


on the 18cxxx you can simplify these to:

  infsnz  low,f
   incf   high,f

and
  tstfsz  low,f
   decf   high,f
  decf    low,f

Al Williams of AWC Says:

Slight typos in the 18F code above for decrement. TSTFSZ doesn't put a result anywhere, so no ,F is required. Also, should be testfsnz but that instruction doesn't exist!
So in fact, you should use the 16F version for decrementing. Also, be aware that the Z flag may be set after this operation but the counter is not zero (just part of it).

• Decrement Counters at 18F:
     decf CounterLow, f
     btfss STATUS, C
        decf CounterHigh, f
     btfss STATUS, C
        decf CounterUpper, f
     and so on...
  Check for Carry-flag in Loops
  Loops will execute Counter+1 times.
  After Loop's exit, Counter == -1.
  

Questions:

• decf16  macro file    ; Decrease 16bit file, Z flag correct
      movf    file,f    ; Check low byte
      skpnz             ; Low byte zero?
      decf    file+1,f  ; Yes, need to decrement HB as well
      decf    file,f    ; Decrement low byte
      skpnz             ; Skip if no further checks needed to set Z
      movf    file+1, f ; LB is zero, now check HB
      endm