mov     $rkda+2,r3 /
       mov     ac,-(sp) / put remainder from divide on stack; gives
                        / sector number
       mov     $4,lsh / shift quotient 4 bits, to align with cyl and surf
                      / bits in rkda
       bis     mq,(sp) / or mq with sector; gives total disk address
       br      3f
prf: / drum
       bit     $1,active / test drum busy bit
       bne     2f / branch if bit is set
       bis     $1,active / set drum busy bit
       mov     r1,rfap / rfap points to current I/O queue entry for drum
       mov     $dae+2,r3
       clr     -(sp)
       movb    2(r1),1(sp) / move low byte of physical block number into
                           / high byte of stack
       clr     -(sp) / word
       movb    3(r1),(sp) / move high byte of physical block number into
                          / low byte of stack
       mov     (sp)+,-(r3) / load dae with high byte of physical block
                           / number
3:
       mov     (sp)+,-(r3) / load rkda register; load dar register
       mov     6(r1),-(r3) / load bus address register
       mov     4(r1),-(r3) / load word count register
       mov     $103,-(sp) / 103 indicates write operation when loaded
                          / in csr
       bit     $2000,(r1) / if bit 10 of word 1 of I/O queue entry is
                          / a one
       beq     3f / then read operation is indicated
       mov     $105,(sp) / 105 indicates read operation
3:
       mov     (sp)+,-(r3) / load csr with interrupt enabled, command, go
       br      seta
ptc: / tape I/O
       bit     $4,active
       bne     2f
       mov     tccm,r3
       swab    r3
       bic     $!7,r3
       add     $2,r3
       cmpb    r3,(r1)
       beq     3f
       movb    $1,tccm / stop transport if not same unit
3:
       bis     $4,active
       mov     r1,tcap
       mov     $20.,tcerrc
       mov     $tape1,tcstate
       movb    (r1),r3 / device
       sub     $2,r3 / now unit
       swab    r3
       bis     $103,r3 / now rbn,for,unit,ie
       mov     r3,tccm
 seta: / I/O queue bookkeeping; set read/write waiting bits.
       mov     (r1),r3 / move word 1 of I/O queue entry into r3