fact rec

        BRA main
main    INP R0
        MOV R1, #1
        PSH {R0, R1}
        MOV R2, PC
        ADD R2, #2
        JMS fact
        OUT R1
        HLT
fact    POP {R0, R1}
        CMP R0, #0
        BEQ doneFact
        MUL R1, R0
        SUB R0, #1
        PSH {R0, R1}
        JMS fact
doneFact MOV PC, R2