cmp     r1,(r0)+ / block # greater than or equal to maximum
       	                / block number allowed
       bhis    error10 / yes, error
       inc     *u.fofp / no, *u.fofp has next block number
       jsr     r0,preread / read in the block whose number is in r1
       bis     $40000,(r5) / set bit 14 of the 1st word of the I/O
       	                   / buffer
1:
       bit     $22000,(r5) / are 10th and 13th bits set (read bits)
       beq     1f / no
       cmp     cdev,$1 / disk or drum?
       ble     2f / yes
       tstb    uquant / is the time quantum = 0?
       bne     2f / no, 2f
       mov     r5,-(sp) / yes, save r5 (buffer address)
       jsr     r0,sleep; 31. / put process to sleep in channel 31 (tape)
       mov     (sp)+,r5 / restore r5
       br      1b / go back
2: / drum or disk
       jsr     r0,idle; s.wait+2 / wait
       br      1b
1: / 10th and 13th bits not set
       bic     $40000,(r5) / clear bit 14
       jsr     r0,tstdeve / test device for error (tape)
       add     $8,r5 / r5 points to data in I/O buffer
       jsr     r0,dioreg / do bookkeeping on u.count etc.
1: / r5 points to beginning of data in I/O buffer, r2 points to beginning
   / of users data
       movb    (r5)+,(r2)+ / move data from the I/O buffer
       dec     r3 / to the user's area in core starting at u.base
       bne     1b
       tst     u.count / done
       beq     1f / yes, return
       tst     -(r0) / no, point r0 to the argument again
       br      bread / read some more
1:
       mov     (sp)+,r0 / jump to routine that called readi
       jmp     ret

bwrite: / write on block structured device
       jsr     r0,tstdeve / test the device for an error
       mov     *u.fofp,r1 / put the block number in r1
       cmp     r1,(r0)+ / does block number exceed maximum allowable #
       bhis    error10 / yes, error
       inc     *u.fofp / no, increment block number
       jsr     r0,wslot / get an I/O buffer to write into
       jsr     r0,dioreg / do the necessary bookkeeping
1: / r2 points to the users data; r5 points to the I/O buffers data area
       movb    (r2)+,(r5)+ / ; r3, has the byte count
       dec     r3 / area to the I/O buffer
       bne     1b
       jsr     r0,dskwr / write it out on the device
       tst     u.count / done
       beq     1f / yes, 1f
       tst     -(r0) / no, point r0 to the argument of the call
       br      bwrite / go back and write next block
1: