Bus access request - HOLD,HLDA in minimum mode

Question 


In minimum mode of microprocessor 8088, HOLD and HLDA pin are used to control the bus system access by both CPU and I/O devices. Draw simplified data/wave form for HOLD, HLDA and bus access pin/device.

Answer

Bus access request - RQ/GT' maximum mode

Question

Explain how external chip/IO devices request and control the system bus in maximum mode. Draw the corresponding timing diagram.

Answer

In maximum mode, the RQ'/GT' (request and grant) pin will play the important role. Whenever a device want access to the system bus, the device will lower RQ'/GT' for one cycle to request the access. After receiving the request, 8086/8088 will grant the access to system bus by also lowering the RQ'/GT' pin for one cycle.

Note that even though it is the same pin that being lowered at both occasion, the purpose is different

Request --> RQ' = 0' = 1= requesting access
Grant --> GT' = 0' = 1 = granting access