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. Notice that, it is possible to have an output port with the address F0H, and an input port with the same address F0H. In other words, we consider that it is having the input port number F0H. Thus, the chip responds when the 8085 sends out address as F0H, IO/M* as 1, and RD* as 0. Here as the port address is F0H so the bits ranging from A7 to A0 should have the bit patternĪ7 A6 A5 A4 A3 A2 A1 A0 = 1 1 1 1 0 0 0 0, with RD* = 0, and IO/M* = 1Īll these bits will pass through a NAND Gate to product the output logic 1 as Chip-Select (CS), and so the input port chip gets selected. A possible chip select circuit to connect an input port with an address as F0H is as shown in the followingFig. IN instruction is the only instruction using which read the input port content to the Accumulator. The result of execution of this instruction is shown below with an example. IN F0H is an example instruction of this type. First Byte specifies the opcode, and the next Byte provides the 8-bit input port address. Input port’s 8-bit address is indicated in the instruction as a8. In 8085 processor we come across from the IOR machine cycle just only for the execution process of IN a8 instruction.Įxample: In 8085 Instruction set, IN is a mnemonic that stands for INput the Byte from input port’s content to the Accumulator. Also the point to be noted that in an IOR machine cycle, only the data is received by the accumulator from the addressed input port. The value of 16 bit in the register pair WZ are sent out as the address in an Input Output Read machine cycle. The point to be noted that in an IOR machine cycle, Wand Z has identical has port address of 8 bit. The Waveforms for the IOR machine cycle are shown in the figure below. Summary − So this instruction NOP requires 1-Byte, 1-Machine Cycle (Opcode Fetch) and 4 T-States for execution as shown in the timing diagram.The three clock cycles at the last stages in the IN 35H instruction is an example of machine cycle for IOR. The timing diagram against this instruction NOP execution is as follows − Otherwise “Insert” and “Delete” keys are available on our microprocessor kit, which might help us for new code insertion and pre-existing code deletion. Thus, it is a good programming practice to have a few NOP instructions in the program at regular intervals od code, especially during program development. In future when extra code is to be inserted then we can replace NOP instructions and can do needful accordingly. NOP instruction is very useful when we require to keep some memory space void with in our program for future instruction insertions accordingly. So introduction of this NOP instruction can produce synchronization of the speed between these two. It is very useful for generating small-time delays of the order of a few microseconds.Ĩ085 Microprocessor works faster compared to the speed of its other peripheral devices. NOP instruction can be used to create small-time delay in the execution of the code. In spite of the fact that it does nothing, still it has got many different applications. Only it occupied 1-Byte of memory space and spends 4-Machine Cycles. This instruction does nothing during execution. NOP is a mnemonic that stands for “No Operation”. In 8085 Instruction set, NOP is an instruction which is falling under Machine Control Instruction category.
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