README.md

NEORV32 Core Verification using RISCOF

1. Prerequisites
2. Setup Configuration
3. Device-Under-Test (DUT)

This repository is a port of the "RISCOF RISC-V Architectural Test Framework" to test the NEORV32 RISC-V Processor for compatibility to the RISC-V user and privileged ISA specifications. Sail RISC-V is used as reference model. Currently, the following tests are supported:

• rv32i_m\B - bit-manipulation (Zba + Zbb + Zbc + Zbs)
• rv32i_m\C - compressed instructions
• rv32i_m\I - base integer ISA
• rv32i_m\M - hardware integer multiplication and division
• rv32i_m\privilege - privileged machine-mode architecture
• rv32i_m\Zicond - conditional operations
• rv32i_m\Zifencei - instruction stream synchronization

💡 The general structure of this repository was setup according to the RISCOF installation guide.

Prerequisites

Several tools and submodules are required to run this port of the architecture test framework. The repository's GitHub Actions workflow takes care of installing all the required packages.

• neorv32 submodule - the device under test (DUT)
• riscv-arch-test submodule - architecture test cases
• RISC-V GCC toolchain - for compiling native rv32 code
• Sail RISC-V - the reference model (a pre-built binary can be found in the bin folder)
• RISCOF - the architecture test framework (including riscv-isac and riscv-config)
• GHDL - the awesome VHDL simulator for simulating the DUT

The framework (running all tests) is invoked via a single shell script run.sh that returns 0 if all tests were executed successfully or 1 if there were any errors. The exit code of this script is used to determine the overall success of the GitHub Actions workflow.

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Setup Configuration

The RISCOF config.ini file is used to configure the plugins to be used: the device-under-test ("DUT") and the reference model ("REF"). The ISA, debug and platform specifications, which define target-specific configurations like available ISA extensions, ISA spec. versions and platform modules (like MTIME), are defined by YAML files in the according plugin folders.

• DUT: neorv32 in plugin-neorv32
• REF: sail_cSim in plugin-sail_cSim

Each plugin folder also provides low-level environment files like linker scripts (to generate an executable image matching the target's memory layout) as well as platform-specific code (for example to initialize the target and to dump the final test signatures/results).

The official RISC-V architecture tests repository provides the individual test cases for all (ratified) RISC-V ISA extensions (user and privilege ISA) that are currently supported by the DUT. Each test case checks a single instruction or core feature and is compiled into a plugin-specific executable using a prebuilt RISC-V GCC toolchain.

The "golden reference" data is generated by the Sail RISC-V Model. This data is compared against the results of the DUT. The final test report is made available as CSS-flavored HTML file via the GitHib actions artifact.

💡 Prebuilt sail-riscv binaries for 64-bit x86 Linux are available in the bin folder.

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Device-Under-Test (DUT)

The sim folder provides a plain-VHDL testbench and shell scripts to simulate the NEORV32 processor using GHDL. The testbench provides generics to configure the DUT's RISC-V ISA extensions and also to pass a plain ASCII HEX file, which represents the memory image containing the actual executable. This file generated from a test-case-specific ELF file. The makefile in the sim folder takes care of compilation and will also convert the final memory image into a plain HEX file. Note that this makefile uses the default software framework from the NEORV32 submodule.

The testbench implements a CPU-external memory module that get initialized with the actual memory image generated by the test framework. This memory is attached to the processor via its external Wishbone bus interface and is mapped to the core's reset address at 0x00000000. The memory is implemented as 4 individual byte-wide memories each providing 1/4 of the total memory size. This "splitting" is required as GHDL has problems handling large objects (see ghdl/ghdl#1592).

The final test result data (= test signature) is written to a file (DUT-neorv32.signature) by the testbench via consecutive write accesses to address 0xF0000004. The testbench also provides several memory-mapped "triggers" (at address 0xF0000000) to quit the current simulation using VHDL08's finish statement or to trigger CPU-external RISC-V machine-level interrupts (MTI: machine timer interrupt; MEI: machine external interrupt; MSI: machine software interrupt).

The simulation scripts and the makefile for generating the memory initialization file are invoked and orchestrated from a DUT-specific Python script in the DUT's plugin folder (-> plugin-neorv32/riscof_neorv32.py). This Python script makes extensive use of shell commands to move and execute files and scripts.

⚠️ The Python scripts of both plugins override the default SET_REL_TVAL_MSK macro from riscv-arch-test/riscv-test-suite/env/arch_test.h to remove the BREAK exception cause from the relocation list as the NEORV32 sets mtval to zero for this type of exception. This is explicitly permitted by the RISC-V priv. spec.

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