chktimer ;Timer vector cannot be in ZP.
         ;If timer vector MSB is 0, RTS.
         ;Checks timer, JSR if reached.

         lda timervec+2
         beq timerexit

         lda jiffyhi
         cmp timer+2
         bcc timerexit ;Not reached
         bne timervec  ;Reached

         lda jiffymi
         cmp timer+1
         bcc timerexit ;Not reached
         bne timervec  ;Reached

         lda jiffylo
         cmp timer+0
         bcc timerexit ;Not reached

timervec jsr $00        ;Self-modifying
         bcs timerexit
         lda #0
         sta timervec+2 ;Clear the timer
timerexit
         rts

timer    .byte 0,0,0

settimer ;Copy ZP Vector to JSR's addr
         lda $fb
         sta timervec+1
         lda $fc
         sta timervec+2

         ;Write .X.Y relative time.
         stx timer+0
         sty timer+1
         clc

         ;Add current Jiffy Time.
         lda jiffylo
         adc timer+0
         sta timer+0

         lda jiffymi
         adc timer+1
         sta timer+1

         lda jiffyhi
         adc #0
         sta timer+2
         rts

;AMAZINGLY, Assembles to just 88 bytes.
;abcdefghijklmnopqrstuvwxyz
;abcdefghijklmnopqrstuvwxyz
;abcdefghijklmnopqrstuvwxyz
;abcdefghi