.WARMST equ $FF7C
.OUTSTR equ $FFC7
.ENGINE equ $1004
.INPUT equ $FFAC
.STEP equ #10000
.OUTPUT equ $FFAF
    ORG $100
.loop:
    ldaa #115
    cmpa #115
    beq .speed_down
    ldd .SPEED
    addd .STEP
    std .SPEED
    bra .switch_end
.speed_down:    cmpa #119
    beq .change_direction
    ldd .SPEED
    subd .STEP
    std .SPEED
    bra .switch_end
.change_direction:  cmpa #99
    beq .switch_end
    ldaa .DIRECTION
    coma
    sta .DIRECTION
.switch_end:    jsr .rotate
    bra .loop


.rotate:    ldaa .DIRECTION
    beq .backward_rotate
    ldaa #3
    jsr .change_phase
    ldaa #6
    jsr .change_phase
    ldaa #12
    jsr .change_phase
    ldaa #9
    jsr .change_phase
    rts
.backward_rotate:
    ldaa #9
    jsr .change_phase
    ldaa #12
    jsr .change_phase
    ldaa #6
    jsr .change_phase
    ldaa #3
    jsr .change_phase
    rts

.change_phase:  staa .ENGINE
    jsr .pause
    rts

.pause: ldad .SPEED
.pause_loop:
    subd #1
    beq .pause_end_loop
    bra .pause_loop
.pause_end_loop:    rts

.SPEED: FDB #10000
.DIRECTION FCB #0