Eric van Es <PICLIST@MITVMA.MIT.EDU> wrote:

> Would someone be so kind as to explain what exactly the significance
> of a known zero [register] is?

Eric:

If you know that a register contains 00, you can often use that
knowledge to optimize your code... For example, let's say that you
want to copy the state of the Carry flag into the W register.  Here's
one way:

    MOVLW   0               ;Assume that carry's clear, so W = 0.
    SKPNC                   ;If it IS clear, skip ahead.
    MOVLW   1               ;Otherwise, W = 1.

If you're REALLY clever, you can do this:

    MOVF    STATUS,W
    ANDLW   00000001B       ;Carry flag happens to occupy the LSB
                            ;of the STATUS register.

Both of those solutions, however, take more time and space than the
following, which depends upon the existance of a register known to
contain 00:

    RLF     KNOWNZERO,W

-Andy

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