Specifications

Priorities permitting, the processor relinquishes the bus to that

device.

When the device has control of the bus, it sends the processor an

interrupt command with the address of the words in memory con-

taining the address and status of the appropriate device service

routine.

The processor then “pushes”-

first, the current central processor

status (PS) and then, the current program counter (PC) onto the

processor stack.

The new. PC and PS (the “interrupt vector”) are taken from the loca-

tion specified by the.device and the next location. and the device

Figure 2-2 Nested device Servicing