PORT
    .BY $00,$00,$00,$00

INIT

    LDA #$80
    STA $DD03 // CIA2 PortB Bit7 as OUT
    LDA $DD01 // force Clock-Stretching (SuperCPU)
    STA $DD01 // and release Port
    RTS

READ

    LDA $DC01 // read Port1
    AND #$1F
    STA PORT+$00

    LDA $DC00 // read Port2
    AND #$1F
    STA PORT+$01

    LDA $DD01 // CIA2 PortB Bit7 = 1
    ORA #$80
    STA $DD01

    LDA $DD01 // read Port3
    AND #$1F
    STA PORT+$02

    LDA $DD01 // CIA2 PortB Bit7 = 0
    AND #$7F
    STA $DD01

    LDA $DD01 // read Port4
    PHA // Attention: FIRE for Port4 on Bit5, NOT 4!
    AND #$0F
    STA PORT+$03
    PLA
    AND #$20
    LSR
    ORA PORT+$03
    STA PORT+$03
    RTS