951 B
Known differences between SimH, 11/70, and w11a
SimH: trap and interrupt service order has J11 behavior
The 11/70 (and the 11/45) differ from all other PDP-11 models in the order in which interrupts and traps are honored after the successful completion of an instruction. On the 11/70, interrupts have precedence over T-bit trace traps, on all other models interrupts have the lowest priority.
As consequence, RTI can be used on an 11/70 to exit from an interrupt driver,
and exactly one trace trap will happen when an interrupt is honored after
a traced instruction. On all other models, RTT should be used to exit from
an interrupt driver to prevent a double trace trap, one before the interrupt
and one after the hander exit.
SimH uses the J11 service order with interrupts having the lowest priority for all PDP-11 models.
The w11 implements the proper 11/70 service order.
See also traced WAIT.