OUT a8 instruction in 8085 Microprocessor

In 8085 Instruction set, OUT is a mnemonic that stands for OUTput Accumulator contents to an output port whose8-bit address is indicated in the instruction as a8. It occupies 2 Bytes in the memory. First Byte specifies the opcode, and the next Byte provides the 8-bit port address. 

OUT F0H is an example instruction of this type. The result of execution of this instruction is shown below with an example.

OUT instruction is the only instruction using whichAccumulator contents can be sent out to an output port. A possible chip select circuit to connect an output port with an address as F0His as shown in the following Fig.

Chip select circuit for output port F0H

Here as the port address is F0H so the bits ranging from A7 to A0 should have the bit pattern

A7 A6 A5 A4 A3 A2 A1 A0 =1 1 1 1  0 0 0 0, with WR* = 0, and IO/M* = 1

All these bits will pass through a NAND Gate to product the output logic 1 as Chip-Select (CS), and so the output port chip gets selected. Thus, the chip responds when the 8085 sends out the address as F0H, IO/M* as 1, and WR* as 0. In other words, we consider that it is having the output port number F0H.

Notice that it is possible to have an input port with the address F0H, and an output port with the same address F0H. When the 8085 sends out the address as EFH and IO/M* as 1, only one of them is selected based on the RD* and WR* signal status values. Thus, it is possible to have a total of 256 input ports and a total of 256output ports.

The timing diagram against this instruction OUT F0H execution is as follows –

Summary − So this instruction OUT requires 2-Bytes, 3-Machine Cycles (Opcode Fetch, Memory Read, I/O write) and 10 T-States for execution as shown in the timing diagram.