4:
       jmp     retisp / u4-3

trapt:                                   / r2 points to the
       mov     (r0)+,r2 / device control register
       mov     *(r0)+,r1 / transaction pointer points to buffer
       tst     (sp)+
       tstb    (r2) / is ready bit of dcs set?
       bge     4b / device still active so branch
       bit     (r0),active / was device busy?
       beq     4b / no, stray interrupt
       bic     (r0)+,active / yes, set active to zero
       tst     (r2) / test the err(bit is) of dcs
       bge     2f / if no error jump to 2f
       tst     (r0)+ / skip on error
 2:
       jmp     (r0)
