Hello Listmembers!

decfsz is a real nice instruction for loop counting. For example,

     movlw NumLoops
     movwf LoopCounter
Loop
     [do something]
     decfsz LoopCounter,f
     goto Loop
     return

This works GREAT for NumLoops<256 (though you can set NumLoops to 0 and get 256 loops). But, how can you initialize the loop counter if it's more than a single byte (more than 256 loops)? Here are a few problems I've found...

Let NumLoops=256

     movlw high(NumLoops)
     movwf LoopCounterHi  ; puts 1 in LoopCounterHi
     movlw low(NumLoops)
     movwf LoopCounterLo  ; puts 0 in LoopCounterLo
Loop
     [something useful]
     decfsz LoopCounterLo,f ; Drops LoopCounterLo to 0xff
     goto   Loop            ; keep looping til we hit 0
     decfsz LoopCounterHi,f ; After 256 loops, drops to 0
     goto   Loop            ; keep looping til we hit 0
     return                 ; exit

The above code DOES properly loop 256 times. Now, let's try another...

Let NumLoops=257

     movlw high(NumLoops)
     movwf LoopCounterHi  ; puts 1 in LoopCounterHi
     movlw low(NumLoops)
     movwf LoopCounterLo  ; puts 1 in LoopCounterLo
Loop
     [something useful]
     decfsz LoopCounterLo,f ; Drops LoopCounterLo to 0
     goto   Loop            ; skipped on first pass
     decfsz LoopCounterHi,f ; drops to 0
     goto   Loop            ; skipped on first pass
     return                 ; exit

So, even though we wanted 257 loops, we got 1!


The problem seems to be in setting the high byte of the loop counter. It SEEMS that if the low byte is zero, you can just use high() and low() to initialize the counter. But, if the low byte is non-zero, it SEEMS that we need to increment the high byte after initialization to get it to loop right.

Of course, we could do stuff with subtracting from the loop counter (or adding -1), dealing with carries, then checking for an overall value of zero across the multiple bytes of the loop counter, but that's far more complex than cascaded decfsz instructions. The only problem with cascaded decfsz appears to be in the initialization of the counter.

Comments????

Thanks!

Harold



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