mov     tcdt,r0 / r0 has contents of data register
       add     $3,r0 / overshoot
       cmp     r0,2(r1)
       bgt     0b / if gt keep reading
       br      taper / else reverse

tape3: / I/O transfer
       bic     $30000,(r1) / clear bits 12 and 13 of I/O queue entry
       jsr     r0,poke / do the I/O
       bit     $4,active / still busy see if pick up r-ahead, w-behind
       bne     1f / yes
       movb    $1,(r2) / no, indicate too bad
1:
       jsr     r0,wakeup; runq; 31. / wait up
       br      4f / retisp

drum: / interrupt handler
       jsr     r0,setisp / save r1,r2,r3, and clockp on the stack
       jsr     r0,trapt; dcs; rfap; 1 / check for stray interrupt or
                                      / error
               br 3f / no, error
       br      2f / error

disk:
       jsr     r0,setisp / save r1,r2,r3, and clockp on the stack
       jmp     *$0f
0:
       jsr     r0,trapt; rkcs; rkap; 2
       	       br 3f / no, errors
       mov     $115,(r2) / drive reset, errbit was set
       mov     $1f,0b-2 / next time jmp *$0f is executed jmp will be
                        / to 1f
       br      4f
1:
       bit     $20000,rkcs
       beq     4f / wait for seek complete
       mov     $0b,0b-2
       mov     rkap,r1
2:
       bit     $3000,(r1) / are bits 9 or 10 set in the 1st word of
                          / the disk buffer
       bne     3f / no, branch ignore error if outstanding
       inc     r1
       asr     (r1)
       asr     (r1)
       asr     (r1) / reissue request
       dec     r1
3:
       bic     $30000,(r1) / clear bits 12 and 13 in 1st word of buffer
       mov     ac,-(sp)
       mov     mq,-(sp) / put these on the stack
       mov     sc,-(sp)
       jsr     r0,poke
       mov     (sp)+,sc
       mov     (sp)+,mq / pop them off stack
       mov     (sp)+,ac