p22a.asm

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