Build a 2-bit CPU

Objectives


Learn the basic CPU structure and Organization
Learn Registers, Tri-state Buffers and Decoders
Learn a 1-bit CPU 
Build a 2-bit CPU

The best way to learn about microprocessors is to build one. In this lab, you are going to learn a one-bit CPU and build a 2-bit CPU in Logisim. Through the building process, you will learn the CPU organization. It also prepares you for the next two labs.

Description of the CPU structure

Here is another diagram for your reference.

In the above diagram you notice that there is no address buffer or data buffer, no flags coming out of the ALU and are ignoring many of the registers. These simplifications make the problem manageable while still illustrating many principles. In addition, an external input has been added to allow us to input values to the CPU bus for testing purposes. With the CPU built we will execute micro-operations to make the circuit execute instructions. Each register stores one bit of data and requires two control signals. Register-in loads the register with the contents of the bus. Register-out enables the tri-state buffer in the register which lets the register's output onto the bus. The ALU performs two operations, XOR and AND. The operation to be performed is to be determined by the code in the IR. A 0 in the IR indicates a XOR operation and a 1 indicates an AND operation.

The following control signals will be required for our one bit CPU in this lab:


        ACCout, ACCin, TMPout, TMPin, ALUout,

        IRin, Bout, Bin, Cout, Cin, EXTout.

A control unit would normally generate these control signals but in our case input pins will be used for these control signals.

Registers

When you build a CPU, you need registers to hold the instruction, operands, etc. as shown in the CPU structure diagram.
Here is a demonstration of how to use 1-bit, 2-bit, 8-bit registers.

TRI-STATE BUFFER

In order to be able to build a CPU, we will have to look at how several registers/devices are connected to a common bus without causing short circuits and blown chips.
Imagine if one device is outputting a one and the other a 0 then what will be the output on bus? To avoid this problem we use a device called a tri-state buffer, which besides the states 0 and 1 has a floating third state.
These can be used to connect several devices to a bus and open one at a time. The tri-state buffers (or controlled buffers) are held in the floating state
except when that device wants to take control of the bus.
If there are more than one devices that try to take control of the bus at the same time there will be a conflict problem.

Here is a demonstration of using 1-bit, 2-bit, and 8-bit tri-state buffer:

Multiplexers (MUX-4)

A multiplexer is a combinational logic circuit designed to select one of several input lines to a single common output line accordig to the selection control signals. For example, 2-1 Multiplexer has one control signal, 4-1 Mutiplexer has two control signals, 8-1 Multipleaxer has three control signals and so on.

Here is a demonstration of 4-1 Multilexer with four input lines D0, D1, D2, D3 for your reference. This device can be used as the control unit of your 2-bit or 8-bit CPU circuit.

	Truth Table of a 4-1 Multiplexer (MUX 4)
	========================================
	Input	Selection Output
	----------------------------------------
	  	S1  S0      Q
	D0	0   0       D0
	D1	0   1       D1
	D2	1   0       D2
	D3	1   1       D3
	-----------------------------------------

Decoders

A docoder could serve as the control unit for your CPUs. It is worthwhile to investigate the the decoders.

Build a 1-bit CPU

 
Many devices may be connected to a common bus by using tri-state buffers. 
You may build a 1-bit CPU as described above to get a feel about it.

Here is a picture of the 1-bit CPU circuit for your reference.

In the above 1-bit CPU , What are these control signals EXTin, EXTout and Cin for?
Design the bove 1-bit CPU descriped in the section one of the lab notes and implement the circuit.
Here are Some Relevant Details:

The ALU performs two operations, XOR and AND. The operation to be performed is to be determined by the code in the IR. The value 0 in the IR indicates an XOR operation and the value 1 indicates an AND operation.

The following control signals will be required for our 1-bit CPU in this lab:
```
        ACCout, ACCin, TMPout, TMPin, ALUout,

        IRin, Bout, Bin, Cout, Cin, EXTout, EXTin.
```
To implement the operation AND, let's assume that it uses the operands in registers B and C, and then stores the result in C,
The following instructions would have to be executed.
(assume the external input pin is set to the value '1' - the pre-condition for the AND operation):
And of course, register B and C should be pre-loaded with desired values.
```
        At the beginning, set all the "out" pins to '0' so that
        the bus will be available for use without conflicts.

        1. EXTout, IRin.

        2. Bout, ACCin.

        3. Cout, TMPin.

        4. ALUout, Cin.
```
Practice these microinstructions with your newly built 1-bit CPU.

Do the following experienment:


        Write out the steps necessary to implement the operation XOR
        that uses the operands in the accumulator and register C, and then
        stores the result in register B.

Lab Assignment

Copyright: Department of Computer Science, University of Regina.