Figure 1 illustrates a simplified RISC-V microarchitecture, highlighting only the major functional units. These units play a crucial role in executing instructions efficiently. The key components can be categorized as follows:

  1. Program Counter (PC) – Keeps track of the instruction sequence by incrementing to the next program instruction on every clock cycle.
  2. Instruction Memory – Stores the program instructions that are fetched and executed sequentially.
  3. Register File – Receives instructions from the instruction memory, decodes them, and provides the appropriate inputs to the ALU.
  4. Arithmetic Logic Unit (ALU) – Performs mathematical and logical operations on the provided inputs.
  5. Data Memory – Stores ALU results or acts as a temporary storage unit for data required in future computations.
Figure 1: Abstract Microarchitecture Overview

Figure 1: Abstract Microarchitecture Overview

Regardless of the specific instruction being executed, every instruction cycle in a RISC-V processor follows these two fundamental steps:

  1. Instruction Fetch – The program counter (PC) fetches the instruction from the instruction memory.
  2. Register Read – Based on the instruction fields, one or two registers are selected and read from the register file.

Program Flow

The program counter (PC) increments to fetch the next instruction from the instruction memory. In the RISC-V architecture, instructions are stored as 32-bit values, each representing a specific operation.

Once fetched, this 32-bit instruction is sent to the register file, where the appropriate registers are accessed. For example, from the Figure 1, the registers rs1, rs2, and rd are represented by 5-bit fields A1, A2, and A3, allowing for a total of 32 addressable registers, as specified in the RISC-V architecture manual.

Based on the instruction type:

  • The register file outputs two 32-bit values, RD1 and RD2, which serve as inputs to the Arithmetic Logic Unit (ALU) for computation.
  • The ALU processes these values, performing arithmetic or logical operations.
  • The result is either stored back in the register file, in register rd via the WD3 pin or written to memory for later use.

This describes a basic instruction execution flow. A more detailed breakdown of each step will be explored in upcoming sections.