Hi Stewart,

I've included this on the PICList because there are probably a few people
who would be interested in what is going on here.

>I have a current interest in 16 bit up/down counting. Referring to page 298
>of your book, you handle separately the required increment/decrement
>functions and the zero sensing functions. Is there a reason why you have not
>used incfsz and decfsz, which combine these functions, and would seem to
>offer a more direct and symmetrical way to deal with this issue?

I have not included "incfsz" and "decfsz" instructions in the 16 bit
increment/decrement because they won't work in these applications.

the instructions:

  incfsz  Count
   incf   Count+1

appears to do a 16 bit increment, but actually increments "Count+1" every
time _except_ when the result of incrementing "Count" is equal to zero.

Now, on page 243, I show how this is used as a short timer:

Loop
  incfsz  Count
   incf   Count
  btfsc   PORTA,bit             ;  Wait for the Bit
   goto   Loop

  movf    Counthi,w
  subwf   Count,w
  movwf   Counthi

To properly increment a 16 bit number, you have to use the "incf" and
"btfsc" as I outlined in the book.

  incf    Count
  btfsc   STATUS, Z
   incf   Count+1


Decrementing a 16 bit value is a bit more of a problem, because "decfsz"
only skips when the result is equal to zero.

If you were to make the code:

  decfsz  Count
   decf   Count+1

Like, the two instruction sixteen bit increment, you would end up
decrementing the high value everytime except when the result of decrementing
"Count" is equal to zero.

Decrement is further complicated by the fact that decrementing to zero
doesn't mean anything (unlike incrementing to zero, which means you actually
have reached 0x0100).

So, to get the sixteen bit decrement to work properly, you have to
_subtract_ by one, to get to 0x0FF, which is where you want to decrement the
high byte.

This is why I use the code:

  movlw   1
  subwf   Count
  btfss   STATUS, C             ;  If Carry Not Set, Then Gone from 0 to 0x0FF
   decf   Count+1


Increment and decrement are shown in more detail on pages 77-78.

Good luck,

myke

>Stewart Krakauer
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"If people don't know what you're doing, they don't know what you're doing
wrong." - Sir Humphrey Appleby K.C.B