oppt: / open paper tape for reading or writing
        mov    $100,*$prs / set reader interrupt enable bit
        tstb   pptiflg / is file already open
        bne    2f / yes, branch
1:
        mov    $240,*$ps / no, set processor priority to 5
        jsr    r0,getc; 2 / remove all entries in clist
               br .+4 / for paper tape input and place in free list
        br     1b
        movb   $2,pptiflg / set pptiflg to indicate file just open
        movb   $10.,toutt+1 / place 10 in paper tape input tout entry
        br     sret
2:
        jmp    error / file already open

iclose: / close file whose i-number is in r1
        tst    r1 / test i-number
        blt    2f / if neg., branch
        cmp    r1,$40. / is it a special file
        bgt    3b / no, return
        mov    r1,-(sp) / yes, save r1 on stack
        asl    r1
        jmp    *1f-2(r1) / compute jump address and transfer
1:
        ctty   / tty
        cppt   / ppt
        sret   / mem
        sret   / rfo
        sret   / rk0
        sret   / tap0
        sret   / tapl
        sret   / tap2
        sret   / tap3
        sret   / tap4
        sret   / tap5
        sret   / tap6
        sret   / tap7
        ccvt   / tty0
        ccvt   / ttyl
        ccvt   / tty2
        ccvt   / tty3
        ccvt   / tty4
        ccvt   / tty5
        ccvt   / tty6
        ccvt   / tty7
        error / crd

2: / negative i-number
        neg    r1 / make it positive
        cmp    r1,$40. / is it a special file
        bgt    3b / no. return
        mov    r1,-(sp)
        asl    r1 / yes. compute jump address and transfer
        jmp    *1f-2(r1)
1: