TIMING DIAGRAM for various machine cycles of INTEL 8085

  •    The sequence of operations that a processor has to carry out while executing the instruction is called Instruction Cycle.

  •     Each instruction cycle of a processor in turn consists of a number of machine cycles. The machine cycles are the basic operations performed by the processor.

  •     To execute an instruction, the processor executes one or more machine cycles in a particular order. The machine cycles of a processor are also called Processor Cycles.

  •     The manufacturer of microprocessors defines the timings and status of various signals during the processor cycles.

    In general, the instruction cycle of an instruction can be divided into two as Fetch and Execute. The fetch cycle is executed to fetch the opcode from memory and the execute cycle is executed to decode the instruction and to perform the work instructed by the instruction.


The 8085 microprocessor has 7 (seven ) basic machine cycles. They are

1. Opcode fetch cycle ( 4 Tor 6 T )
2. Memory read cycle (3 T )
3. Memory write cycle (3 T )
4. I/O read cycle (3 T)
5. I/O write cycle (3 T)
6. Interrupt acknowledge cycle (6 Tor 12 T )
7. Bus idle cycle. ( 2 Tor 3 T )

Each instruction of the 8085 processor consists of one to five machine cycles, i.e., when the 8085 processor executes an instruction, it will execute some of the machine cycles in a specific order.

The processor takes a definite time to execute the machine cycles. The time taken by the processor to execute a machine cycle is expressed in T -states. One T -state is equal to the time period of the internal clock signal of the processor. The T -state starts at the falling edge of a clock.


The timing diagram provides information about the various condition (high state or low state or high impedance state) of the signals while a machine cycle is executed. The timing diagrams are supplied by the manufacturer of the microprocessor. The timing diagrams are essential for a system designer. Only from the knowledge of timing diagrams, the matched peripheral devices like memories, ports, etc. can be selected to form a system with microprocessor as CPU .

The machine cycles are the basic operations performed by the processor, while instructions are executed. The time taken for performing each machine cycle is expressed in terms of  T-states. One T-state is the time period of one clock cycle of the microprocessor.

The various machine cycles are

1.  Opcode fetch ……………..    -  4 / 6 T
2.  Memory Read …………….    -  3 T
3.  Memory Write …………….    -  3 T
4.  I/O Read …………………..    -  3 T
5.  I/O Write ………………….    -  3 T
6.  Interrupt Acknowledge ……    -  6 / 12 T
7.  Bus Idle ……………………    -  2 / 3 T

The T -states required by the 8085 processor to execute each machine cycle are mentioned within brackets in the list of machine cycles, given above.

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