WATCH_ACIA:
        LDA  RX_STATE
        CASE  ACCUM

            CASE_OF  0                             ; In the case of RX_STATE being 0, an $FF is required to increment it to 1.
                IF_BIT   ACIA_STAT_REG, 3, IS_SET  ; If a byte came in (indicated by bit 3 of the status register),
                    LDA  ACIA_DATA_REG             ; get it (this clears the flag in the status register too),
                    CMP  #$FF                      ; and see if it's $FF.
                    IF_EQ                          ; If it is,
                        INC  RX_STATE              ; move on to watch for a valid (non-$FF) first byte.
                    END_IF                         ; The $FF gets discarded.
                END_IF                             ; If no byte came in, just exit.
            END_OF


            CASE_OF  1                             ; To get here, we've received the $FF marker and we're looking for a valid 1st byte.
                IF_BIT   ACIA_STAT_REG, 3, IS_SET  ; If a byte came in,
                    LDA  ACIA_DATA_REG             ; get it,
                    IF_PLUS                        ; see if it's valid as a high byte, ie, that the high bit is clear, and if so,
                        STA  READING_BUF           ; store it as valid in the buffer area meant to prevent wrong readings between bytes,
                        INC  RX_STATE              ; and move on to watch for the 2nd byte which could be anything.
                    END_IF                         ; If it's not a valid high byte, just discard it, and leave RX_STATE as is.
                END_IF                             ; If no byte came in, just exit.
            END_OF

                                                        ; In the case of RX_STATE being 2, we've gotten the high byte of READING,
            CASE_OF  2                                  ; and we're waiting for the second byte which could be anything.
                IF_BIT  ACIA_STAT_REG, 3, IS_SET        ; See if a byte came in.  If one did,
                    COPY  VIA_SR, TO, READING           ; just transfer it
                    COPY  READING_BUF, TO, READING+1    ; (including the high byte now, which we had saved to prevent glitches
                    STZ   RX_STATE                      ; in the two-byte READING value), and go back to state 0.
                END_IF                                  ; If no byte came in, just exit.
            END_OF


            STZ   RX_STATE                         ; If there's any chance for state to go invalid, just reset it and start over.
        END_CASE
        RTS
 ;---------------------------------------