Working out how to advance to the next row on a commodore 64 bitmap

c64maththing.asm

;
; in an atari 8-bit routine that deals with bitmaps on a 320x192 screen, this is all you need
; to advance to the next row
        lda ptr1
        clc
        adc #40
        sta ptr1
        bcc :+
        inc ptr1+1
;
; Since ptr1 is in zero-page, this will be 12 cycles if the branch is taken, 14 if it isn't.
; there is a 27/32 chance it will take 12 cycles.
; there is a 5/32 chance it will take 14 cycles.
; expect it to average to 12 5/16 cycles.
;
; for the commodore 64, the routine might go a little more like this:
;
;       * add 1 to ptr1
;       * if ptr1 is now a multiple of 8, then add 312 (which is 39 * 8)
;
        inc ptr1
        lda #7
        bit ptr1
        bne DontAdd314
        clc
        lda ptr1
        adc #56
        sta ptr1
        lda ptr1+1
        adc #1    ; add 256, plus the carry bit from when 56 was added
        sta ptr1+1
DontAdd314

; that is a beast.  this takes 12 cycles if the branch is taken, but a whopping 27 if it is not.
; there is a 7/8 chance it will take 12 cycles.
; there is a 1/87 chance it will take 27 cycles.
; so expect it to average out to 13 7/8 cycles.