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get_byte:                       ; Bit 7 is the Empty Flag of the Receiving (RX) FIFO
    BBS7 STATUS, get_byte       ; Loop for as long as it's 1 (ie FIFO has no Data in it)
    LDA RX_FIFO                 ; Once it's 0, read a Byte from it
RTS

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    ; User input has changed
    AND     <ButtonNew      ; Now A contains '1' for Positive Edge Button events only.
    STA     <ButtonOld      ; ButtonOld is out of date anyway, so re-use it to store above value.
    
    ; Take Heap 1
    BBR0    <ButtonOld, ?SkipTakeHeap1  ; If bit 0 clear, user has not pressed Take on Heap 1
    LDA     <HeapSelect     ; If HeapSelect = 2 or 3, don't decrement.
    CMP     #2
    BEQ     ?SkipTakeHeap1
    CMP     #3
    BEQ     ?SkipTakeHeap1
    LDA     <Heap1          ; Don't decrement or record selection if Heap 1 isn't zero.
    BEQ     ?SkipTakeHeap1
    LDA     #1              ; Record that player has selected heap 1
    STA     <HeapSelect
    DEC     <Heap1          ; Decrement Heap.
?SkipTakeHeap1:

    ; Take Heap 2
    BBR1    <ButtonOld, ?SkipTakeHeap2  ; If bit 1 clear, user has not pressed Take on Heap 2
    LDA     <HeapSelect     ; If HeapSelect = 1 or 3, don't decrement.
    CMP     #1
    BEQ     ?SkipTakeHeap2
    CMP     #3
    BEQ     ?SkipTakeHeap2
    LDA     <Heap2          ; Don't decrement or record selection if Heap 2 isn't zero.
    BEQ     ?SkipTakeHeap2
    LDA     #2              ; Record that player has selected heap 2
    STA     <HeapSelect
    DEC     <Heap2          ; Decrement Heap.
?SkipTakeHeap2:

    ; Take Heap 3
    BBR2    <ButtonOld, ?SkipTakeHeap3  ; If bit 2 clear, user has not pressed Take on Heap 3
    LDA     <HeapSelect     ; If HeapSelect = 1 or 2, don't decrement.
    CMP     #1
    BEQ     ?SkipTakeHeap3
    CMP     #2
    BEQ     ?SkipTakeHeap3
    LDA     <Heap3          ; Don't decrement or record selection if Heap 3 isn't zero.
    BEQ     ?SkipTakeHeap3
    LDA     #3              ; Record that player has selected heap 3
    STA     <HeapSelect
    DEC     <Heap3          ; Decrement Heap.
?SkipTakeHeap3:

    ; Done Button
    BBR3    <ButtonOld, ?SkipDone  ; If bit 3 clear, user has not pressed Done
    LDA     <HeapSelect     ; Check if player has done at least one by checking HeapSelect.
    BEQ     ?SkipDone       ; If HeapSelect=0, cannot go to next player's turn.
    JSR     CheckVictory    ; Check if the player has won. If they have, CheckVictory does not return.
    LDA     <Turn
    BEQ     ?TurnIsGreen    ; If Z=1 then Turn=0, hence Green.
    LDA     #$FF            ; If turn was 1 (red), overwrite A with FF so that INC A gives 0 (green).
?TurnIsGreen:
    INC     A               ; If TurnIsGreen, A now=1 hence RED. If not, A now=0, hence GREEN.
    STA     <Turn
    STZ     <HeapSelect     ; Clear HeapSelect
?SkipDone: