Microprocessor Design/Multi Cycle Processors

From Wikibooks, open books for an open world
Jump to: navigation, search

Single-cycle processors suffer from poor speed performance. Control and data signals must propagate completely through the processor in a single cycle, which means that cycle times need to be long, and many parts of the hardware tend to be dormant for much of the cycle.

Multi-Cycle Stages[edit]

Multi-cycle processors break up the instruction into its fundamental parts, and executes each part of the instruction in a different clock cycle. Since signals have less distance to travel in a single cycle, the cycle times can be sped up considerably.

Typically, an instruction is executed over at least 5 cycles, which are named as such:

Fetch the instruction from memory
Decode the instruction, and generate the necessary control signals
Feed the necessary control signals into the ALU and produce a result
Read from memory, if specified
Write the result back to the register file or to memory.

This is just a textbook example, and modern processes tend to use many more steps than this to execute an instruction.

Example: MicroChip PIC16 Microcontroller

The PIC Microcontroller, manufactured by MicroChip Technology Inc, is a family of embedded microcontrollers. The PIC units vary, but execute an instruction every 2-4 clock cycles. All instructions typically execute in the same number of cycles, except for branch instructions.

Hardware Reuse[edit]

The primary benefit to a multicycle design is to be able to share hardware elements, specifically the ALU, among various tasks. In a multicycle processor, a single ALU can be used to update the instruction pointer (in the IF cycle), perform the operation (in the EX cycle), and calculate a necessary memory address (in the MEM cycle). Multicycle processors also allow computers that have a single memory unit, instead of the two separate instruction and data memory units of the traditional harvard machine. This is because the instructions are loaded on one cycle, and the data memory is interfaced on another cycle.

Multi-cycle processors are typically used in applications where resources are at a premium, and speed is not as important.