On Mon, 7 Sep 1998 15:41:17 -0400 Shahid Sheikh <sheikh@EROLS.COM>
writes:

>I'm having a little problem understanding how the PC on a 12C508
>works. I have a scenario where I want to skip a number of instructions
in the
>code
>and the number is specified in the variable X. So I have code that
>looks
>something like
>
>        movf    X       ;X is the number of instructions to skip
>        addlw   PCL, f  ;Add X to PC.

> In my above example, if the value of X is zero, will that cause the
program to
>freeze?

The value read from PCL by the addwf (not addlw) instruction will be the
address of the next instruction.  If you add zero, the program will
continue with the next instruction, though with an extra cycle of delay.
If you add FF (-1), the addwf PCL instruction will execute in an infinite
loop.  If you add one, the next instruction will be skipped, adding two
skips over 2 instructions, and so on.

Of course the entire part of the program involving the skipping must be
in the first 256 instruction subpage of the program space.  It is not
possible to go outside the first 256 instructions with a write to PCL on
a 12C508.  If the result of the add overflows, the program will continue
in the first part of the program space.

>Or is that considered a situation in which the "instruction does not
>change the PC."

Every instruction that writes to PCL is treated as a goto, requiring an
extra cycle.  It doesn't matter what value is written.

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