Analyzing RV32/RV64 Trade-offs for FreeRTOS Latency on 8-Stage RISC-V Soft Processors
TL;DR
The design and evaluation of FreeRTOS on custom 8-stage RV32/RV64 RISC-V soft processors with machine-mode CSRs, a CLINT timer, trap and exception control logic, and the required context-switch path are presented.
Abstract
Although many commercial RISC-V platforms provide real-time operating system support, practical examples that explain how to enable a preemptive RTOS on a custom bare-metal RISC-V soft processor remain limited, leaving the interaction between processor microarchitecture, interrupt handling, and RTOS context switching difficult to understand from simple hardware implementation examples. This paper presents the design and evaluation of FreeRTOS on custom 8-stage RV32/RV64 RISC-V soft processors with machine-mode CSRs, a CLINT timer, trap and exception control logic, and the required context-switch path. Using this platform, we compare RV32 and RV64 under the same microarchitectural organization and firmware structure using two Rhealstone-derived latency microbenchmarks, task switching and task preemption, measured with the mcycle counter. RV64 requires 36.6% and 17.7% more cycles for task switching and preemption, respectively. Instruction-level analysis attributes this overhead to the doubled RV64 trap frame, wider pointer-based kernel data structures, and 64-bit scheduler priority handling. The RTL, firmware, and benchmark code are released as open source.