UNIVERSITY OF WEST ATTICA
SCHOOL OF ENGINEERING
DEPARTMENT OF COMPUTER ENGINEERING AND INFORMATICS

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Digital Circuit Design

Simple Circle of a MIPS Processor

Vasileios Evangelos Athanasiou
Student ID: 19390005

GitHub · LinkedIn

Supervisor: Ioannis Vogiatzis, Professor

UNIWA Profile · LinkedIn

Co-supervisor: Panagiotis Karkazis, Associate Professor

UNIWA Profile · LinkedIn

Athens, September 2023

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Project Overview

The processor is designed to execute each instruction in a single clock cycle.
It implements the core components of the MIPS architecture, including:

• Arithmetic Logic Unit (ALU)
• Register File
• Instruction Memory
• Data Memory
• Control Unit

The design demonstrates how control signals and data paths cooperate to complete instruction execution within one cycle.

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Table of Contents

Section	Folder / File	Description
1	assign/	Assignment material
1.1	assign/DSD_ASSIGNMENT_MIPS_2023.pdf	MIPS processor assignment description (English)
1.2	assign/ΣΨΣ_ΑΣΚΗΣΗ_MIPS_2023.pdf	MIPS processor assignment description (Greek)
2	docs/	Theoretical documentation
2.1	docs/Simple-Circle-MIPS-Porcessor.pdf	Single-cycle MIPS processor architecture (English)
2.2	docs/Απλός-Κύκλος-MIPS-Επεξεργαστή.pdf	Single-cycle MIPS processor architecture (Greek)
3	src/	VHDL source code implementations
3.1	src/19390005_ATHANASIOU_02_MIPS.vhd	Top-level single-cycle MIPS processor implementation
3.2	src/19390005_ATHANASIOU_03_testbench.vhd	Top-level MIPS processor testbench
3.3	src/control_unit.vhd	Main control unit
3.4	src/control_unit_tb.vhd	Control unit testbench
3.5	src/alu.vhd	Arithmetic Logic Unit (ALU)
3.6	src/alu_tb.vhd	ALU testbench
3.7	src/alu_control.vhd	ALU control logic
3.8	src/alu_control_tb.vhd	ALU control testbench
3.9	src/register_file.vhd	Register file implementation
3.10	src/register_file_tb.vhd	Register file testbench
3.11	src/instructions_memory.vhd	Instruction memory
3.12	src/instructions_memory_tb.vhd	Instruction memory testbench
3.13	src/data_memory.vhd	Data memory
3.14	src/data_memory_tb.vhd	Data memory testbench
3.15	src/program_counter.vhd	Program counter
3.16	src/program_counter_tb.vhd	Program counter testbench
3.17	src/pc_adder.vhd	Program counter adder
3.18	src/pc_adder_tb.vhd	Program counter adder testbench
3.19	src/sign_extend_16to32.vhd	Sign extension unit
3.20	src/sign_extend_16to32_tb.vhd	Sign extension unit testbench
3.21	src/shifter_2left.vhd	Shift-left-by-2 unit
3.22	src/shifter_2left_tb.vhd	Shift-left-by-2 unit testbench
3.23	src/mux_32x_2to1.vhd	32-bit 2-to-1 multiplexer
3.24	src/mux_32x_2to1_tb.vhd	32-bit 2-to-1 multiplexer testbench
3.25	src/mux_5x_2to1.vhd	5-bit 2-to-1 multiplexer
3.26	src/mux_5x_2to1_tb.vhd	5-bit 2-to-1 multiplexer testbench
3.27	src/fullAdder_32bit.vhd	32-bit full adder
3.28	src/fullAdder_32bit_tb.vhd	32-bit full adder testbench
3.29	src/and2_gate.vhd	2-input AND gate
3.30	src/and2_gate_tb.vhd	2-input AND gate testbench
4	README.md	Repository overview and usage instructions

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Architecture Components

1. Arithmetic Logic Unit (ALU)

The ALU performs arithmetic and logical operations, including:

• Addition and subtraction
• Logical operations: AND, OR, XOR

The operation is selected via a 4-bit control signal (operationSig).

Zero Signal

• Outputs 1 when the result of a subtraction is zero.
• Used for branching decisions (e.g., BEQ/BNE).

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2. Register File

A storage unit containing 32-bit registers.

• Read Operations: Supports two simultaneous reads
• Write Operation: Supports one write, enabled only when RegWrite = 1

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3. Memory Units

• Instruction Memory

  • Stores program instructions
  • Supplies the instruction pointed to by the Program Counter (PC)

• Data Memory

  • Stores runtime data
  • Controlled by MemRead and MemWrite signals

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4. Control Units

• Main Control Unit

  • Generates control signals such as RegDst, Branch, MemRead, MemWrite, MemtoReg, etc.
  • Operates based on the instruction opcode

• ALU Control Unit

  • Decodes instruction type
  • Selects the exact ALU operation to perform

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5. Supporting Logic

• Program Counter (PC):
  A 32-bit register holding the address of the next instruction

• Sign Extension:
  Expands 16-bit immediate values to 32-bit values

• Multiplexers:
  2-in-1 multiplexers select:

  • Destination registers
  • ALU input sources
  • Write-back data (ALU result vs memory output)

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Instruction Execution Flow

Each instruction follows a six-step execution pipeline, completed in a single cycle:

1. Fetch – Read instruction from Instruction Memory using PC
2. Decode – Decode instruction and read registers
3. Prepare – Route operands to the ALU
4. Execute – Perform arithmetic or logical operation
5. Memory Access – Read/write Data Memory if required
6. Update PC – Compute and store the next instruction address

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Implementation Details

• Hardware Description Language: VHDL
• Primary Entity: alu
• Testbench: alu_tb
  • Verifies operations such as addition, subtraction, and branch-related logic (e.g., BNE)

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Installation & Setup Guide

This repository contains a Single-Cycle MIPS Processor implemented in VHDL, along with detailed testbenches for each major component.
The project is intended for academic use in Digital Systems / Computer Architecture courses and focuses on simulation and verification, not FPGA deployment.

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Prerequisites

1. Operating System

Supported platforms:

• Windows (recommended – best compatibility with ModelSim)
• Linux
• macOS (may require alternative simulators or legacy support)

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2. VHDL Simulation Software

Recommended Simulator

• ModelSim Altera Starter Edition
  (or ModelSim Intel FPGA Edition / Questa Intel FPGA Starter)

Required features:

• VHDL-2008 support
• Hierarchical simulation
• Waveform visualization
• Testbench execution

Note: This project was verified using ModelSim. Other simulators (e.g., GHDL) may require adjustments.

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3. Development Tools (Optional but Recommended)

• Visual Studio Code + VHDL extension
• Notepad++
• Vim / Emacs
• ModelSim built-in editor

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4. Knowledge Prerequisites

To fully understand and modify the project, familiarity with:

• Digital logic design
• VHDL syntax and structural design
• MIPS single-cycle architecture
• Datapath & control signal design
• Testbench-based verification

is strongly recommended.

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Installation

1. Clone the Repository

Using Git:

git clone https://github.com/Digital-Circuit-Design/MIPS.git

Alternative (Without Git)

• Open the repository URL in your browser
• Click Code → Download ZIP
• Extract the ZIP file to a local directory

2. Install ModelSim

1. Download ModelSim Altera Starter Edition (or Intel FPGA equivalent)
2. Complete installation following vendor instructions
3. Verify installation by launching ModelSim successfully

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Project Setup in ModelSim

1. Create a New Project

1. Open ModelSim
2. Select File → New → Project
3. Set:
   • Project Name
   • Project Location (your cloned repository path)
4. Choose Create Project

2. Add VHDL Source Files

1. In the Add Items to Project window:
   • Select Add Existing File
2. Navigate to the src/ directory
3. Add:
   • All *.vhd files (designs and testbenches)
4. Finish project creation

3. Compile the Project

1. In the Project tab:
   • Select Compile → Compile All
2. Ensure:
   • No syntax errors
   • Successful compilation messages in the transcript

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Simulation & Verification

1. Simulating Individual Components

Each module has its own testbench (*_tb.vhd).

Example (ALU):

1. Select alu_tb as the top-level entity
2. Click Simulate → Start Simulation
3. Choose work.alu_tb
4. Click OK

Run simulation:

run -all

Verify:

• Arithmetic operations
• Logical operations
• Zero flag behavior

2. Simulating the Full MIPS Processor

To simulate the complete single-cycle processor:

1. Select:
   • 19390005_ATHANASIOU_03_testbench.vhd
2. Start simulation:
   • Simulate → Start Simulation
3. Run:

run -all

Observe:

• Instruction fetch
• Control signal generation
• Register file reads/writes
• ALU execution
• Memory access
• Program Counter updates

3. Waveform Analysis

1. Add internal signals to the waveform viewer
2. Inspect:
   • Control signals (RegWrite, Branch, MemRead, etc.)
   • ALU inputs/outputs
   • PC evolution
   • Register values

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Open the Documentation

1. Navigate to the docs/ directory
2. Open the report corresponding to your preferred language:
   • English: Simple-Circle-MIPS-Porcessor.pdf
   • Greek: Απλός-Κύκλος-MIPS-Επεξεργαστή.pdf