IO/M* PIN in 8085 Microprocessor

In Intel 8085 microprocessor I/O and memory operation are differentiated by IO/m` status signal.

IO/M` stands for ‘input-output/memory`’. When IO/M` is logic 0, it means that the address sent out by the processor is for addressing a memory location. When IO/M` is logic 1, it means that the address sent out by the processor is for addressing an I/O port.

Fig: Pin diagram of 8085

Let us consider this following example and also the voltage level at IO/M* pin. If we consider instruction MVI E, ABH then it means that ABH will be moved or copied to the register E. And as a result the previous value of E will get over written.

Address	Hex Codes	Mnemonic	Comment
2000	1E	MVIE, ABH	E ← ABH
2001	AB		ABH as operand

This instruction will have seven T-states as shown below.

Summary − So this instruction MVI E, ABH requires 2-Bytes, 2-Machine Cycles Opcode Fetch and Memory Read) and 7 T-States for execution as shown in the timing diagram.

Arjun Thakur

Published on 03-Jan-2019 12:36:41

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