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

Read cycle timing diagram in minimum mode

Question

Draw a simplified timing diagram for a read cycle of the 8088 microprocessor (minimum mode). The diagram should include at least the following
i.    Address pin
ii.    Address/Data pin
iii.    WR' pin
iv.    CLK timing
v.    ALE pin
 
 Answer

Read/Write bus cycle difference in minimum and maximum mode

Question
Given in the diagram below is the 8088 chip and 8288 chip connection in maximum mode. Explain the read / write difference between maximum and minimum mode operation in 8088 chip.

 Diagram: Maximum mode connection

Table: 8288 Command Table

Answer
 In minimum mode, the read and write command signal are generated by 8088 using the WR' and RD' pin.

In maximum mode, the control signal to read and write are generated by external chip which is 8288. The corresponding command include MRDC' (for read) and MWTC (for write).

The following two diagram depic the difference for Read bus cycle in both minimum and maximum mode.

Difference between minimum & maximum mode from control signal perspective

Question
What is the difference between minimum and maximum mode from the control signals perspective?

Answer
In minimum mode, all control signals are self generated by 8086/8088 microprocessor such as RD' and WR'.

In maximum mode, the control signals are generated by external chip such as MRDC' (memory read) and MWTC' (memory write) by 8288 chip.