Part- 1: Step-by-Step Guide to Building a RISC-V Processor from Scratch

🌟 Introduction to RISC-V Architecture

Building your own processor might sound challenging, but with the right guidance, it's not only possible—it’s fun! 😃 In this guide, we’ll walk you through the steps to build a simple RISC-V processor from scratch.

RISC-V, an open-source instruction set architecture (ISA), is gaining popularity among engineers and hobbyists because of its flexibility and transparency. Unlike proprietary architectures, RISC-V allows complete freedom to explore, modify, and build without licensing restrictions. Whether you're new to computer architecture or an enthusiast, this guide will help you step by step.

By the end, you’ll have a working RISC-V processor and a solid understanding of how processors function at a fundamental level. Ready? Let's get started! 🔧💡

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🧠 What is RISC? - Reduced Instruction Set Computing

RISC stands for Reduced Instruction Set Computing. The concept is simple: keep the instruction set small and efficient so the processor works faster. Fewer instructions mean simpler design and easier understanding. Let’s break it down!

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🛠️ Step 1: Choosing the Instruction Set Architecture (ISA)

We’ll use the RV32I instruction set, which is the core set of instructions in RISC-V. It's designed to be simple and beginner-friendly, making it perfect for building a processor from scratch.

Key Features of RV32I:

• Works with 32-bit data and addresses.
• Includes essential instructions like arithmetic, logic, memory access, and control flow.
• Open-source, so you can modify and expand as needed. 🎨

This basic instruction set keeps things manageable, helping you focus on learning without overwhelming complexity.

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🔍 Step 2: Understanding RISC-V Instructions

In RISC-V, there are six main types of instructions, each with a specific role in processing data and controlling the execution flow:

Instruction Control Signals Table 📝

Instruction	Opcode	RegWrite	RegDst	ALUSrc	Branch	MemWrite	MemtoReg	ALUOp	Jump
R-type	110011	1	1	0	0	0	0	10	0
LW (Load)	11	1	0	1	0	0	1	0	0
SW (Store)	100011	0	X	1	0	1	X	0	0
BEQ	1100011	0	X	0	1	0	X	1	0
ADDI	10011	1	0	1	0	0	0	0	0
JAL	1101111	1	X	X	0	0	X	XX	1
JALR	1100111	1	X	1	0	0	X	XX	1

1️⃣ R-type Instructions (Register-to-Register Operations)

• Instructions: ADD, SUB, AND, OR, XOR, SLL, SRL, SRA, SLT, SLTU

• Operation: The ALU performs operations on two register operands (rs1 and rs2) and stores the result in a register (rd).

• Opcode: 0110011

2️⃣ I-type Instructions (Immediate Operations)

• Instructions: ADDI, ANDI, ORI, XORI, SLTI, SLTIU, SLLI, SRLI, SRAI
• Operation: The ALU performs operations similar to R-type but uses an immediate value instead of a second register.
• Opcode: 0010011

💡 Note: Load instructions like LW involve memory operations and are not handled by the ALU.

3️⃣ S-type Instructions (Store Operations)

• Instructions: SW, SH, SB
• Operation: The ALU calculates the memory address using a base register and an immediate value (rs1 + imm).
• Opcode: 0100011

4️⃣ B-type Instructions (Branch Operations)

• Instructions: BEQ, BNE, BLT, BGE, BLTU, BGEU
• Operation: The ALU compares two register values (rs1 and rs2) to check branch conditions.
• Opcode: 1100011

5️⃣ U-type Instructions (Upper Immediate Instructions)

• Instructions: LUI (Load Upper Immediate), AUIPC (Add Upper Immediate to PC)
• Operation: These instructions deal with loading or adding upper immediate values to registers or the program counter.
• Opcode: 0110111 (LUI), 0010111 (AUIPC)

6️⃣ J-type Instructions (Jump Instructions)

• Instructions: JAL (Jump and Link), JALR (Jump and Link Register)
• Operation: These instructions update the program counter.
• Opcode: 1101111 (JAL), 1100111 (JALR)