Question

When 8085 executes the IN instruction, in the third machine cycle, it places the _____ on the low order bus and the _____ as well as asserts the _____.
1. input port address, high order bus, \(\overline {RD} \)
2. contents of accumulator, high order bus, \(\overline{WR}\)
3. contents of accumulator, contents of register, input port
4. contents of register, accumulator, output port

Answer 1

Correct Answer - Option 1 : input port address, high order bus, \(\overline {RD} \)

Concept:-

IN instruction: It is a 2-byte instruction, having 1-byte opcode and 1 byte for input port address.

Example: IN 55_H

It means to read from an input device having an 8-bit port address as 55_H.

This means the content of the accumulator will be updated with the content at the input address.

In the first two machine cycles, the address of the accumulator will be assessed, and in the third machine cycle, it places the input address on the lower order bus and the higher-order bus as well as asserts \(\overline {RD} \). 

As it has to read from the input device \(\overline {RD} \) has to be low.

So, we can say option 1 is the correct one.