On Mon, 14 Jul 1997 15:33:00 +0200 David BALDWIN
<baldwin@ETCA.ALCATEL.BE> writes:
>hi,
>
>        Nice to find someone that's doing PIC development in Belgium!
>What are
>you working on? For the Swapf, I don't know, but I am using it, and it
>works, but I don't understand why we need to do this...


The SWAPF instruction swaps the high and low 4 bits of a byte.  If a
memeory location contains 11100001, SWAPF  memloc,W will return 00011110
in W.  This is useful for packed BCD operations.

But why in an ISR?  The reason is that SWAPF doesn't affect the Z status
bit but MOVFW (or MOVF file,w) does.  The obvious way to save and restore
W and STATUS during an ISR would be:

int
        movwf   savew           ;Save interrupted program's W
        movfw   STATUS          ;and STATUS
        movwf   saves
[Interrupt service code here, may freely modify W and STATUS]
        movfw   saves           ;Get stored status back
        movwf   STATUS
        movfw   savew           ;Wrong.  Will affect Z.
        retfie

Unlike some processors, in a PIC the Z flag does not always correspond to
whether the W register contains zero.  The ISR above will force it to,
which may cause logical errors in the main program.

Since SWAPF doesn't affect Z, it is the less damaging way to recall from
memory to W.  Relpacing the last movfw with:
        swapf   savew,f         ;Put bits in proper order to
swap again
        swapf   savew,w         ;Restore to W without
affecting Z
will restore W and STATUS exactly as they were when the program was
interrupted.