27 Mar Design and develop a simple ALU as a part of a CPU

About this Assignment

In this course, you began by learning the abstract concepts of Boolean logic and binary number system and ended by using these concepts in order to implement advanced computer architectures such as CPUs, memory systems, and parallel computers.

 

Upon completion of this project, you will be able to:

 

Design and develop a simple ALU as a part of a CPU

Decide on the number of operands for the ALU and design its instruction set

Design and construct the control unit of the CPU

Write assembly language for the constructed CPU

Prompt

In this project, you will design and build a simple CPU on Logisim and write programs that can run on it. If you haven’t yet, you can download Logisim by following this link: http://www.cburch.com/logisim/download.html

 

Your design will go through four phases. In the first phase, you will design and build the ALU using Logisim. In the second phase, you will design the instruction set that implements the instructions you designed in phase one. In the third phase, you will design and implement a control unit for this ALU using Logisim. By connecting the CU to the ALU, you will get a functional CPU. In phase four of the project, you will write assembly language programs for the CPU you built.

 

Phase One

Start by building an 8-bit ALU using Logisim. This ALU can implement 16 instructions on 8-bit operands. We would suggest the following minimum list of instructions:

 

Arithmetic addition

Increment

Decrement

Comparison (with 3 outputs: one for equals, one for less than and one for greater than)

Logic bitwise Not

Logic bitwise And

Logic bitwise Or

Register right logic shift

Register left logic shift

In addition to these nine instructions, please suggest five more instructions that the ALU can implement for a total of 14 instructions (we are reserving 2 instructions for branching). Justify the importance of the five instructions you added in a Word doc to submit as part of this assignment. Label these instructions as ‘Phase One.’

 

After you’ve suggested and justified your five suggested instructions, please build at least the nine above-mentioned operations as blocks in Logisim.

 

Phase Two

In phase two of the project, you are required to design the instruction set of the ALU/CPU as follows:

 

Create the opcode table for the ALU by giving a binary code and a name for each instruction you built in Logisim in phase one.

Decide how many operands you want your instructions to handle and justify your choice. We suggest either one operand with accumulator or two operands with the result stored in one of the input registers.

In Logisim, add a multiplexer to the circuit you built in phase one that chooses one of the available operations. The simplest way to create this part of the CPU is to connect the outputs of the multiplexer to the inputs of AND arrays connected to the output of the operation blocks.

Please record your answer to phase two in the same Word doc and label it ‘Phase Two.’

 

Phase Three

In phase three, you are required to use Logisim to implement the control unit for at least the following three operations:

 

addition

logic bitwise AND

right logic shift

In order to finish this phase, you need to add operand registers according to the decision you took for the number of operands in phase two and, if needed, a flag register.

 

Please record your answer to phase three in the same Word doc and label it ‘Phase Three.’

 

Phase Four

In order to be able to write assembly language for the CPU we need to add two instructions (without implementation):

 

branch to an address (name it JMP)

conditional branch to an address (name it CJMP and suppose that the jump takes place if the comparison operation result is ‘equals’)

Now, write the following programs using the assembly language you designed in the previous phases of the project as well as these two branching additional instructions:

 

Write a program that adds two operands.

Write a program that adds operands until the new value to be added is 0. You do not need to implement the input operations to modify the contents of the registers. Just assume that by the end of each iteration, the register content is modified.

Write a program that increments by 2 the content of a register 10 times.

Write a program that shifts the content of a register until the least significant bit is 0. Think of a way to stop shifting if the content of the register is 11111111 and add it to your program.

Please record your programs in the same Word doc and label them under the section ‘Phase Four.’

 

Related Chapters and Lessons

This assignment covers material presented in the chapters Digital Circuit Theory: Combinational Logic Circuits and Instruction Set Architecture. The following lessons, in particular, should help with the assignment:

 

Creating an Assembly Language Using an Instruction Set

Practical Application for Computer Architecture: Instruction Set Architecture

In addition to lessons in these chapters, you may refer to external sources as well.

 

Sources

Any sources you use for this project, whether external or on Study.com, should be cited on a reference page that follows APA format. If you are unsure about how to use the APA format to cite your sources, please see the following lessons:

 

What is APA Formatting

How to Format APA Citations

Grading Rubric

Your project will be graded based on the following rubric:

 

Category Unacceptable (0-1) Needs Improvement (2-3) Acceptable (4-5) Total Possible Points

Addition of 5 instructions to the ALU (x1) No additional instructions are suggested and/or no choice justification Less than 5 additional instructions are suggested and/or the justification is not clear or not correct The project contains 5 additional instructions with clear and correct choice justification 5

Build the 9 original instructions in Logisim (x1) Not all 9 instructions are implemented in Logisim and/or there are major functional issues All 9 instructions are implemented in Logisim with some minor functional issues All 9 instructions are implemented in Logisim and functioning correctly 5

Create the opcode table for the ALU (x1) All or most of the instructions are missing from the opcode table Some instructions are missing from the opcode table The project contains a complete opcode table 5

Build the instruction selection unit using a multiplexer (x1) The instruction selection unit is missing or not functioning correctly The instruction selection unit is built in Logisim with minor functional issues the instruction selection unit is correctly built in Logisim with no functional issues 5

Add the operand registers and if needed the flag register (x1) Registers are not added and/or not connected to the circuit N/A Registers are added and correctly connected to the circuit 5

Design and implement the control unit in Logisim (x1) The control unit is not implemented in Logisim and/or has major issues The control unit is implemented in Logisim with minor issues The control unit is correctly implemented in Logisim 5

Write a program that adds two operands (x1) The program is missing or contains major errors The program is written but contains minor errors The program is correctly written with no errors 5

Write a program that adds operands until the new value to be added is 0 (x1) The program is missing or contains major errors The program is written but contains minor errors The program is correctly written with no errors 5

Write a program that increments by 2 the content of a register 10 times (x1) The program is missing or contains major errors The program is written but contains minor errors The program is correctly written with no errors 5

Write a program that shifts the content of a register until the least significant bit is 0, with a way to stop shifting if the content of the register is 11111111 (x1) The program is missing or contains major errors The program is written but contains minor errors The program is correctly written with no errors 5

Total Points 50