Just about done with a job but I need a routine to check that a HEX digit is
valid, and if so convert it to binary, and if not.... here comes the tricky
part... retain the original value. And... (what am I going to say next?) I'm
totally out of registers and the client just dropped the "retain the value"
thing on me. I'm also really pressed for space.

This is for a 16C54 (well, actually, an SX) and the best I can do is very SX
specific:

        mov w, #':'
        mov w, temp-w   ;'9' and lower sets the carry
        inc WReg                ;Make 'A'-':' = 8.
;Yes, on an SX W is a file register (replacing the counter) if OPTIONX
; is specified and you clear the RTW bit in the OPTION register (which
; I always do)
        sc                      ;Skip if Carry
         setb WReg.3    ;If it was '9' or lower, avoid this next test
        sb WReg.3               ;Skip if Bit 3. If the difference + 1 isn't 8 or
         jmp :Bad               ; more then it was between '9' and 'A'
        mov w, #'0'
        snc                     ;skip no carry
         mov w, #'A'-10
        mov w, temp-w   ;w = temp - w
        sb WReg.4               ;Skip if Bit 4. No good if we overflow 16 values
         sc                     ; or if we underflow '0'
          jmp :Bad

which in MChip speak is something like:

        MOVLW ':'
        SUBWF temp,w    ;'9' and lower sets the carry
        movwf WReg
        INCF WReg               ;Make 'A'-':' = 8.
        BTFSS status,c
         BSF WReg,d'3'  ;If it was '9' or lower, avoid this next test
        BTFSS WReg,d'3' ;If the difference + 1 isn't 8 or more then
         GOTO Bad               ; it was between '9' and 'A'
        MOVLW '0'
        BTFSC status,c
         MOVLW 'A'-10
        SUBWF temp,w    ;w = temp - w
        movwf WReg
        BTFSS WReg,d'4' ;No good if we overflow 16 values
         BTFSS status,c ; or if we underflow '0'
          GOTO Bad

where we had to use a second file register called WReg.

Any other ideas? Remember... no second register, but if you like, you can
use W as a file register.

---
James Newton (PICList Admin #3)
mailto:jamesnewton@piclist.com 1-619-652-0593
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