Difference between Machine Cycle, Bus Cycle and Execution Cycle
I am unable to understand the difference between Bus Cycle, Instruction Cycle and Machine Cycle. Please help me out. Thanks
Answer
First off, computers use a clock. The frequency of this clock indicates how many (Giga/Mega/Kilo) cyles per second that the clock wave changes. This is the basis of any cycle for the computer.
The bus cycle is the cycle or time required to make a single read or write transaction between the cpu and an external device such as external memory.
The machine cycle is the amount of cycles needed to do either a fetch, read or write operation. more here. The read or write may be more than a single bus cycle if the transaction between the CPU and memory is longer than the data width fetched or written. For example, on an 8080 machine, the data width is 8 bits. If the CPU needs to fetch or write 16 bits of data, that will require two bus cycles.
The instruction cycle is how many of these machine cycles are needed to complete an instruction. This varies depending on the instruction. For instance, some instructions after fetching them from memory need to fetch more data to complete the instruction, some need to write data at the end of the instruction cycle, some instructions don't do much at all, like the NOP, which basically fetches the instruction and does nothing for one machine cycle.
I hope this helps a bit. If not, maybe microprocessor timing diagrams will help clear things up a bit more.