Objectives

Learn the basic CPU structure and Organization
Learn Memory, Registers, Tri-state Buffers and Decoders
Build an 8-bit CPU with a memory

Describing the CPU structure

        
In the above design/diagram, the following components have been added:
 <1.>PC - Program counter, use an 8-bit register for it
 <2.>PC+1 can be implemented by using an 8-bit ADDER to add a constant 00000001 to the PC.
 <3.>MAR - memory address register
 <4.>MEM - 8-bit memory


By now you have enough knowledge to build an 8-bit CPU without a memory.

We are going to introduce the memory below so that you will have enough knowledge to implement the above design.

Introducing 8-bit memory in Logisim

	When you build a CPU, you need a memory to hold the instructions, 
	operands, etc. as shown in the CPU structure diagram.

	Here is a link to the RAM in the Logisim Memory Library for your reference.

	Please read the document carefully and test the above given circuit for the RAM.

	This document can also be found in the Memory library 
	in the Library reference in the Help menu in the Logisim.

	Please build the circuit and try to operate on it with the following steps:


	Select 0x02(i.e. b00000010) from ETXin, Open EXTout,

	double click on MARin; close EXTout

	Select 0x08(i.e. b00001000) from EXTin, Open EXTout,

	Set MEMio to 1 to store value to the memory,

	Set MEMenable to 1 to be able to access the memory,

	double click on MEMclk; close EXTout 

	Set MEMio to 0 to read the value.

	You just stored the number 0x08 in the memory location 0x02.
	And at the end,  you read the value in memory location 0x02.

Review 4-bit Parallel Adder

	Look at the following circuit.  It helps you to make an 8-bit PC+1. 

	

	It demonstrates the following: 

	1.	How to use the 4-bit inputs and outputs

	2.	How to use the 4 bit adder to obtain the sum 
		The initial carry in Cin is 1-bit. 
		The final carry out Cout is 1-bit.

Lab Assignment

Materials from last lab for your reference

We are going to keep the following lab materials for your reference.

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.

        ACCout, ACCin, TEMPout, TEMPin, ALUout,

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

Registers

TRI-STATE BUFFER

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

Multiplexers (MUX-4)

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

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.

1. In the above 1-bit CPU , What are control signals EXTout and Cin for?

2. 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.
   
   

3. 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. EXTin, EXTout, IRin.   // Load '1' in register IR for AND operation.
   
      2. EXTin, EXTout, Bin.    // Load '0' in register B.
   
      3. EXTin, EXTout, Cin.    // Load '1' in register C.
   
      4. Bout, ACCin.
   
      5. Cout, TMPin.
   
      6. ALUout, Cin.
   
   

   Practice these microinstructions with your newly built 1-bit CPU.

4. 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