Design and Implementation

of a Very Simple CPU

Sections 6.1 and 6.2

Stanton Lee

6.1 Specifying a CPU

• Determine its applications

• Choose an instruction set

• Design a state diagram

Overview

General CPU Cycle

Fetch Cycle: Fetch an instruction from memory then go to the decode cycle

Decode Cycle: Determine which instruction was fetched then go to the execute cycle

Execute Cycle: Execute the instruction then go back to the fetch cycle

Generic CPU State Diagram

6.2.1 Specifications

• The CPU can access 64 bytes of memory each byte being 8 bits wide

• The CPU will output a 6-bit address through pins A[5..0] and read an 8-bit value through pins D[7..0]

• There will only be one programmer usable 8-bit accumulator register labeled AC

Instruction Set

Internal Registers

AR: A 6-bit address register that supplies an address to memory via A[5..0]

PC: A 6-bit program counter that contains the address of the next instruction to be executed

DR: An 8-bit data register that receives instructions and data from memory via D[7..0]

IR: A 2-bit instruction register that stores an opcode

6.2.2 Fetching Instructions from Memory

First State of the Fetch Cycle

• Copy the address in PC to AR

• AR will then send the address to memory for reading

• FETCH1: AR <-- PC

Second State of the Fetch Cycle

• The memory will output the requested data to D[7..0]

• The CPU will read the data and store it in DR

• Increment PC

• FETCH2: DR <-- M, PC <-- PC + 1

Third State of the Fetch Cycle

• Copy the two high-order bits of DR to IR (opcode)

• Copy the six low-order bits of DR to AR

• These six bits in AR can be used for ADD and AND or ignored for JMP and INC

• FETCH3: IR <-- DR[7..6], AR <-- DR[5..0]

6.2.3 Decoding Instructions

• Determine which instruction was fetched

• Invoke the correct execution routine

• For this very simple CPU there are four routines

Fetch and Decode Cycles

6.2.4 Executing Instructions

The ADD Instruction

ADD1: DR <-- MFetch an operand from memory. Recall that AR alreadycontains an address from FETCH3.

ADD2: AC <-- AC + DRPerform the addition and store result in AC

Begin the Fetch Cycle again

The AND Instruction

This is similar to the ADD instruction

AND1: DR <-- MObtain an operand

AND2: AC <-- AC ^ DRPerform a logical AND and store

Begin the Fetch Cycle again

The JMP Instruction

JMP1: PC <-- DR[5..0]Simply copy the jump address to PC. The nextFetch Cycle will use that address.

Note that because the address was also copied to ARduring FETCH3 we can also perform PC <-- AR

Begin the Fetch Cycle again

The INC Instruction

INC1: AC <-- AC + 1Just add 1 to the accumulator’s value and store theresult back into the accumulator

Begin the Fetch Cycle again

Complete State Diagram

6.2.5 Establishing Data Paths

We now design the internal data paths of the CPU tosupport all possible data transfers

Two Approaches

• Direct paths between each pair of components that transfer data

• A common bus used by all components that need to transfer data

Data Transfers

AR: AR <-- PC; AR <-- DR[5..0]

DR: DR <-- M

IR: IR <-- DR[7..6]

These components only need to perform data loading

Data Transfers (continued)

PC: PC <-- PC + 1; PC <-- DR[5..0]

AC: AC <-- AC + DR; AC <-- AC ^ DR; AC <-- AC + 1

These components not only load data but they alsoperform increments

PreliminaryRegisterSection

Note the tri-statebuffers

Modify the Design

• AR transfers data to memory not to the bus

• IR does not supply data to the other registers

• AC does not supply data either (unrealistic for a real CPU)

• The bus is 8 bits wide but not all transfers use 8 bits. Use bits 5..0 of the bus for AR and PC. Use 7..6 for IR.

• The CPU must be able to compute AC + DR and AC ^ DR. Therefore we need an ALU.

FinalRegisterSection

Note the ALU

6.2.6 Design of an ALU

We now consider the implementation of a simple ALU

• The two inputs are DR via the bus and AC (direct)

• The output goes to AC

Functions

1. Add the two inputs

2. Logically AND the two inputs

A Very Simple ALU

Note the 2-1 MUX and the Control Signal

6.2.7 Designing the Control Unit

Components

• Counter: Contains the current state

• Decoder: Generates individual signals for each state

• Logic Block: Generates control signals for each component

Generic Control Unit

CPU States

Recall that this CPU has a total of 9 states:

FETCH1, FETCH2, FETCH3ADD1, ADD2AND1, AND2JMP1INC1

We will require a 4-bit counter and 4-16 decoder

Some Guidelines

• Assign FETCH1 to counter value 0 and use the CLR input of the counter to reach this state

• Assign sequential states to sequential counter values and use INC on the counter to traverse these states

• Assign the first state of each execute routine based on the opcodes and maximum number of states

The Mapping Function

Some of the execute routines have two states. Thereforewe must ensure that their first states are at least two apart.

A mapping function that satisfies this is as follows:

1IR[1..0]0

Diagram for Control Unit

Control Signals

We create control signals for the registers

PCLOAD = JMP1PCINC = FETCH2DRLOAD = FETCH2 OR ADD1 OR AND1ACLOAD = ADD2 OR AND2ACINC = INC1IRLOAD = FETCH3

Control Signals (continued)

We create control signals for the ALU, the buffers,and M.

MEMBUS = FETCH2 OR ADD1 OR AND1PCBUS = FETCH1

READ = FETCH2 OR ADD1 OR AND1

The ALU has a control input ALUSEL of either 0 or 1to select the proper function.

Control Signal Diagram

End of Presentation