Multi-byte Increment & Decrement

This page was originally published by me on a site called pic-projects.net.

All Microchip devices can increment and decrement an 8-bit file register in a single instruction. This article examines how a value spread over multiple file registers can be efficiently adjusted. Examples are included for 16-, 24- and 32-bit values.

Incrementing

The key to multi-byte incrementing is spotting when a value rolls over from h'ff' to h'00' as this is the trigger to increment the next most significant byte of the value. This turns out to be very easy as the INCF instruction automatically sets the Z bit in the STATUS register to reflect the result of the increment and when used in conjunction with BTFSC instruction to test the flag

 ; (All devices)
        incf    FILE0,F         ; Increment 16-bits in FILE0 & FILE1
        btfsc   STATUS,Z
        incf    FILE1,F

        incf    FILE0,F         ; Increment 24-bits in FILE0,FILE1 & FILE2
        btfsc   STATUS,Z
        incf    FILE1,F
        btfsc   STATUS,Z
        incf    FILE2,F

        incf    FILE0,F         ; Increment 32-bits in FILE0, FILE1, FILE2 and FILE3
        btfsc   STATUS,Z
        incf    FILE1,F
        btfsc   STATUS,Z
        incf    FILE2,F
        btfsc   STATUS,Z
        incf    FILE3,F

The pattern here pretty obvious and can be extended for values of greater length. High performance processors have an INFSNZ (increment file register and skip if not zero) instruction that can be used to replace the first increment and test to shorten the sequences slightly.

; (High Performance devices)
        infsnz  FILE0,F         ; Increment 16-bits in FILE0 & FILE1
        incf    FILE1,F

        infsnz  FILE0,F         ; Increment 24-bits in FILE0,FILE1 & FILE2
        incf    FILE1,F
        btfsc   STATUS,Z
        incf    FILE2,F

        infsnz  FILE0,F         ; Increment 32-bits in FILE0, FILE1, FILE2 and FILE3
        incf    FILE1,F
        btfsc   STATUS,Z
        incf    FILE2,F
        btfsc   STATUS,Z
        incf    FILE3,F

On high performance and enhanced mid-range devices the ADDWFC instruction offers an alternative means to adjust the value which is as fast for 32-bits and faster for larger values.

; (Enhanced Mid-Range and High Performance devices)
        movlw   .0             ; Load zero into WREG
        bsf     STATUS,C       ; And set the carry
        addwfc  FILE0,F        ; Then add to FILE0..FILE3
        addwfc  FILE1,F
        addwfc  FILE2,F
        addwfc  FILE3,F

Decrementing

When decrementing a value we need to know when a value will change from h'00' to h'ff' but the DECF instruction sets the Z bit in the status on a transition from h'01' to h'00' so instead we must test the value to see if it is zero before decrementing.

; (All devices)
        movf    FILE0,F         ; Decrement 16-bits in FILE0 & FILE1
        btfsc   STATUS,Z
        decf    FILE1,F
        decf    FULE0,F

For longer values this pattern can only be used with the last two bytes. Lower significance bytes must be tested and a goto or branch used to skip to the corresponding DECF when a non-zero value is found.

; (All devices)
        movf    FILE0,F         ; Decrement 24-bits in FILE0, FILE1 & FILE2
        btfss   STATUS,Z
        goto    DEC0
        movf    FILE1,F
        btfsc   STATUS,Z
        decf    FILE2,F
        decf    FILE1,F
DEC0:   decf    FILE0,F

        movf    FILE0,F         ; Decrement 32-bits in FILE0, FILE1, FILE2 & FILE3
        btfss   STATUS,Z
        goto    DEC0
        movf    FILE1,F
        btfss   STATUS,Z
        goto    DEC1
        movf    FILE2,F
        btfsc   STATUS,Z
        decf    FILE3,F
        decf    FILE2,F
DEC1:   decf    FILE1,F
DEC0:   decf    FILE0,F

; (High performance devices)
        movf    FILE0,F         ; Decrement 24-bits in FILE0, FILE1 & FILE2
        bnz     DEC0
        movf    FILE1,F
        btfsc   STATUS,Z
        decf    FILE2,F
        decf    FILE1,F
DEC0:   decf    FILE0,F

        movf    FILE0,F         ; Decrement 32-bits in FILE0, FILE1, FILE2 & FILE3
        bnz     DEC0
        movf    FILE1,F
        bnz     DEC1
        movf    FILE2,F
        btfsc   STATUS,Z
        decf    FILE3,F
        decf    FILE2,F
DEC1:   decf    FILE1,F
DEC0:   decf    FILE0,F

On the enhanced mid-range and high performance devices the SUBFWB offers a more efficient solution.

; (Enhanced Mid-Range and High Performance devices)
        movlw   .0             ; Load zero into WREG
        bcf     STATUS,C       ; And clear the carry
        subfwb  FILE0,F        ; Then subtract from FILE0..FILE3
        subfwb  FILE1,F
        subfwb  FILE2,F
        subfwb  FILE3,F

This page was last updated on 2nd February 2018