Microprocessor Design/Single Cycle Processors

Single-cycle processors are what we have been studying so far: an instruction is fetched from memory, it is executed, and the results are stored all in a single clock cycle.

The benefits of single-cycle processors is that they tend to be the most simple in terms of hardware requirements, and they are easy to design. Unfortunately, they tend to have poor data throughput, and require long clock cycles (slow clock rate) in order to perform all the necessary computations in time.

Cycle Times
The length of the cycle must be long enough to accommodate the longest possible propagation delay in the processor. This means that some instructions (typically the arithmetic instructions) will complete quickly, and time will be wasted each cycle. Other instructions (typically memory read or write instructions) will have a much longer propagation delay.



As this image shows, an instruction is not over until all 5 components have acted. This means that the length of the cycle must be the length of the longest instruction. The longest path from one end of the processor to the other is called the critical path and is used to determine the cycle time.

Redundant Hardware
Single cycle processors typically require a number of ALUs (or a single master ALU, and smaller ALUs) to handle the increment operations on the instruction pointer, and the memory address calculations for the data memory. When resources are at a premium, having multiple ALU units in your design can be costly and pointless. It requires nearly as many resources to construct an adder that adds a constant value as it does to construct a more general purpose adder unit.

Single Cycle Designs
It is very rare, if not completely unheard of, for a modern processor unit to have a single-cycle design. The reasons for this are the long cycle times, the wasted resources, and the large amount of wasted time in each cycle. What the single-cycle lacks in timing and efficiency, it makes up for in simplicity and elegance. It is for this reason that single-cycle processors work as a good teaching tool, but are not often employed in actual designs.