CS251 - Computer Organization and Design - Spring 2008

Lecture 26 - Multi-cycle Control

Practical Details

1. Assignment 6

Multi-cycle Control

The Basic Idea

1. You have already put in the
   • datapaths
   • MUXes
   • control signals
2. Work out the control signal values for each state.
   • Note that different instruction types mean that there is more than one state per step (cycle).
3. Make a finite-state machine

This is done in detail in both the notes and in the text-book. Sometimes the details are a little sketchy, but you should be able to fill in the blanks by this point. This is what you have to do in order to master the material.

Assignment 6, which asks you to give datapaths and control cycles for a much simpler will give you some indication how well you are doing.

Control Signals

One-bit

Two-bit

Implementation

Either

1. Combinational logic: fast, small, inflexible
   • input: last state x opcode
   • output: this state x control signals
2. Programmable logic array (PLA): fast, big, more flexible
3. Read-only memory (could be flash): slower, very big, yet more flexible
   • last-state x opcode can be 20 or 30 bits
   • an address this big makes a very big ROM
4. Micro-code: faster or slower, small, the most flexible

Micro-code

Anything we can make with a finite state machine we can make with a program.

This has re-entered CS hardware with a new name

• V(ery)L(ong)I(nstruction)W(ord)

Exceptions

What do we do when

1. an external event needs to be accepted by the processor
2. something screwy happens, like an address fault

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