FREESS: An Educational Simulator of a RISC-V-Inspired Superscalar Processor Based on Tomasulo's Algorithm

TL;DR

FREESS is a free, interactive simulator designed for educational purposes to demonstrate instruction-level parallelism and out-of-order execution in a RISC-V-inspired superscalar processor based on Tomasulo's algorithm.

Contribution

It introduces an open-source, configurable simulator that visually demonstrates key microarchitectural features and instruction pipeline stages for learning advanced computer architecture.

Findings

Enables exploration of dynamic instruction execution

Visualizes parallel instruction issuing and execution

Supports configurable architectural parameters

Abstract

FREESS is a free, interactive simulator that illustrates instruction-level parallelism in a RISC-V-inspired superscalar processor. Based on an extended version of Tomasulo's algorithm, FREESS is intended as a hands-on educational tool for Advanced Computer Architecture courses. It enables students to explore dynamic, out-of-order instruction execution, emphasizing how instructions are issued as soon as their operands become available. The simulator models key microarchitectural components, including the Instruction Window (IW), Reorder Buffer (ROB), Register Map (RM), Free Pool (FP), and Load/Store Queues. FREESS allows users to dynamically configure runtime parameters, such as the superscalar issue width, functional unit types and latencies, and the sizes of architectural buffers and queues. To simplify learning, the simulator uses a minimal instruction set inspired by RISC-V (ADD, ADDI, BEQ, BNE, LW, MUL, SW), which is sufficient to demonstrate key pipeline stages: fetch, register renaming, out-of-order dispatch, execution, completion, commit, speculative branching, and memory access. FREESS includes three step-by-step, illustrated examples that visually demonstrate how multiple instructions can be issued and executed in parallel within a single cycle. Being open source, FREESS encourages students and educators to experiment freely by writing and analyzing their own instruction-level programs and superscalar architectures.