RISC-V is an open-source instruction set architecture (ISA) that has gained popularity due to its simplicity and flexibility. In this article, we’ll explore the fundamentals of RISC-V assembly language by building an assembler in C#. Our goal is to read RISC-V assembly code, identify the instruction type, and convert it into machine code. We’ll use Visual Studio as our development environment for this project.

Setting Up the Project

Let’s begin by creating a new project in Visual Studio. We’ll build our RISC-V assembler step by step. We mention that we will be building the assembler incrementally, and the first task is to set up a loop to read a file containing RISC-V assembly code and iterate over each line. This loop will serve as the foundation for processing the assembly code.

string[] lines = File.ReadAllLines(filePath);

foreach (string line in lines)
{
  // Process the 'line' here, e.g., identify instruction type, parse, and convert to machine code
}

Identifying Instruction Types

RISC-V instructions are categorized into different types: R, U, I, B, S, and J. To determine the type of instruction, we’ll use a lookup table for opcodes, func2, and func7. You can find the lookup table in this file.

Here’s an example of how to identify the instruction type:

switch (opCode)
{
    case (OpCode)0b0110011:
        return InstructionType.R;
    case (OpCode)0b0010111:
        return InstructionType.U;
    case (OpCode)0b0110111:
        return InstructionType.U;
    case (OpCode)0b0010011:
        return InstructionType.I;
    case (OpCode)0b1100011:
        return InstructionType.B;
    case (OpCode)0b0000011:
        return InstructionType.I;
    case (OpCode)0b0100011:
        return InstructionType.S;
    case (OpCode)0b1101111:
        return InstructionType.J;
    default:
        return InstructionType.Unknown;
}

You can find the implementation of this function in the RiscVAssembler.cs file.

Parsing Instructions

Now that we can identify the instruction type, let’s parse each instruction based on its type. We’ll start with the R-type instructions, which have the syntax: op rd, rs1, rs2.

For example, the instruction add x10, x1, x2 can be parsed as follows:

Regex rTypeRegex = new Regex(@"^(\w+)\s+(\w+),\s+(\w+),\s+(\w+)$");
Match rTypeMatch = rTypeRegex.Match(instruction);
if (rTypeMatch.Success)
{
    return new RiscVInstruction
    {
        Instruction = instruction,
        Opcode = rTypeMatch.Groups[1].Value,
        Rd = rTypeMatch.Groups[2].Value,
        Rs1 = rTypeMatch.Groups[3].Value,
        Rs2 = rTypeMatch.Groups[4].Value,
        Immediate = null,
        InstructionType = InstructionType.R
    };
}

You can find the complete implementation of the R-type instruction parser in the R_Parser.cs file.

Converting to Machine Code

Once we’ve parsed an instruction, we can convert it into machine code. Each instruction type has its own format. For R-type instructions, the format is as follows:

R type: .insn r opcode6, func3, func7, rd, rs1, rs2
+-------+-----+-----+-------+----+---------+
| func7 | rs2 | rs1 | func3 | rd | opcode6 |
+-------+-----+-----+-------+----+---------+
31      25    20    15      12   7        0

For example, the instruction add x10, x1, x2 is translated into 00000000001000001000010100110011, where:

• Opcode 6: 0110011
• Rd = 01010
• Func 3 = 000
• Rs1 = 00001
• Rs2 = 00010
• Func7 = 0000000

Here’s an example of how to convert the parsed instruction into machine code:

string opcode = ((int)instruction.OpcodeBin).ToBinary(7);
string rdBinary = Convert.ToString(int.Parse(instruction.Rd.Substring(1)), 2).PadLeft(5, '0');
string func3 = ((int)instruction.Funct3).ToBinary(3);
string rs1Binary = Convert.ToString(int.Parse(instruction.Rs1.Substring(1)), 2).PadLeft(5, '0');
string rs2Binary = Convert.ToString(int.Parse(instruction.Rs2.Substring(1)), 2).PadLeft(5, '0');
string func7 = ((int)instruction.Funct7).ToBinary(7);

return new MachineCode($"{func7}{rs2Binary}{rs1Binary}{func3}{rdBinary}{opcode}", instruction.Instruction);

You can find the complete implementation of machine code generation in the R_MachineCode.cs file.

Conclusion

In this article, we’ve embarked on a journey to learn RISC-V assembly language by building an assembler in C#. We’ve covered the basics of reading RISC-V assembly code, identifying instruction types, parsing instructions, and converting them into machine code. This project serves as a valuable learning experience for understanding the inner workings of RISC-V assembly language and its translation into machine code. To delve deeper into the RISC-V architecture, refer to the RISC-V Specification.

Here’s the GitHub repository link for the project where you can find the code for building a RISC-V assembler in C#: SharpRISCV GitHub Repository.

If you find the project helpful and informative, don’t forget to give it a star on GitHub to show your support.

In the next part of our RISC-V assembly language learning series, we will explore addressing modes, labels, and offsets, which are essential concepts for understanding and writing more complex assembly programs. Stay tuned for the next installment!