/ u4 -- unix

setisp:
       mov     r1,-(sp)
       mov     r2,-(sp)
       mov     r3,-(sp)
       mov     clockp,-(sp)
       mov     $s.syst+2,clockp
       jmp     (r0)

clock: / interrupt from 60 cycle clock
       mov     r0,-(sp) / save r0
       tst     *$lks / restart clock?
       mov     $s.time+2,r0 / increment the time of day
       inc     (r0)
       bne     1f
       inc     -(r0)
1:
       mov     clockp,r0 / increment appropriate time category
       inc     (r0)
       bne     1f
       inc     -(r0)
1:
       mov     $uquant,r0 / decrement user time quantum
       decb    (r0)
       bge     1f / if less than 0
       clrb    (r0) / make it 0
1: / decrement time out counts return now if priority was not 0
       cmp     4(sp),$200 / ps greater than or equal to 200
       bge     2f / yes, check time outs
       tstb    (r0) / no, user timed out?
       bne     1f / no
       cmpb    sysflg,$-1 / yes, are we outside the system?
       bne     1f / no, 1f
       mov     (sp)+,r0 / yes, put users r0 in r0
       sys     0 / sysrele
       rti
2: / priority is high so just decrement time out counts
       mov     $toutt,r0 / r0 points to beginning of time out table
2:
       tstb    (r0) / is the time out?
       beq     3f / yes, 3f (get next entry)
       decb    (r0) / no, decrement the time
       bne     3f / isit zero now?
       incb    (r0) / yes, increment the time
3:
       inc     r0 / next entry
       cmp     r0,$touts / end of toutt table?
       blo     2b / no, check this entry
       mov     (sp)+,r0 / yes, restore r0
       rti / return from interrupt
1: / decrement time out counts; if 0 call subroutine
       mov     (sp)+,r0 / restore r0
       mov     $240,*$ps / set processor priority to 5
       jsr     r0,setisp / save registers