Time required to execute and fetch an entire instruction is called instruction cycle. It consists:
- Fetch cycle – The next instruction is fetched by the address stored in program counter (PC) and then stored in the instruction register.
- Decode instruction – Decoder interprets the encoded instruction from instruction register.
- Reading effective address – The address given in instruction is read from main memory and required data is fetched. The effective address depends on direct addressing mode or indirect addressing mode.
- Execution cycle – consists memory read (MR), memory write (MW), input output read (IOR) and input output write (IOW)
The time required by the microprocessor to complete an operation of accessing memory or input/output devices is called machine cycle. One time period of frequency of microprocessor is called t-state. A t-state is measured from the falling edge of one clock pulse to the falling edge of the next clock pulse.
Fetch cycle takes four t-states and execution cycle takes three t-states.
Timing diagram for fetch cycle or opcode fetch:
Above diagram represents:
- 05 – lower bit of address where opcode is stored. Multiplexed address and data bus AD0-AD7 are used.
- 20 – higher bit of address where opcode is stored. Multiplexed address and data bus AD8-AD15 are used.
- ALE – Provides signal for multiplexed address and data bus. If signal is high or 1, multiplexed address and data bus will be used as address bus. To fetch lower bit of address, signal is 1 so that multiplexed bus can act as address bus. If signal is low or 0, multiplexed bus will be used as data bus. When lower bit of address is fetched then it will act as data bus as the signal is low.
- RD (low active) – If signal is high or 1, no data is read by microprocessor. If signal is low or 0, data is read by microprocessor.
- WR (low active) – If signal is high or 1, no data is written by microprocessor. If signal is low or 0, data is written by microprocessor.
- IO/M (low active) and S1, S0 – If signal is high or 1, operation is performing on input output. If signal is low or 0, operation is performing on memory.
- 8255 microprocessor operating modes
- 8085 program for Linear search | Set 2
- 8085 program to convert a BCD number to binary
- 8085 program to find the element that appears once
- 8085 program to find maximum and minimum of 10 numbers
- 8085 program to search a number in an array of n numbers
- 8085 program to find maximum of two 8 bit numbers
- Computer Organization | Different Instruction Cycles
- 8085 program to add two 8 bit numbers
- Addressing modes in 8085 microprocessor
- 8085 program to multiply two 8 bit numbers
- Registers of 8085 microprocessor
- Flag register in 8085 microprocessor
- 8085 program to convert an 8 bit number into Grey number
- 8085 program to find the factorial of a number
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to email@example.com. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.