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Ibex Simple System

Simple System gives you an Ibex based system simulated by Verilator that can run stand-alone binaries. It contains:

  • An Ibex Core
  • A single memory for instructions and data
  • A basic peripheral to write ASCII output to a file and halt simulation from software
  • A basic timer peripheral capable of generating interrupts based on the RISC-V Machine Timer Registers (see RISC-V Privileged Specification, version 1.11, Section 3.1.10)
  • A software framework to build programs for it

Prerequisites

  • Verilator Note Linux package managers may have Verilator but often a very old version that is not suitable. It is recommended Verilator is built from source.
  • The Python dependencies of this repository. Install them with pip3 install -U -r python-requirements.txt from the repository root.
  • RISC-V Compiler Toolchain - lowRISC provides a pre-built GCC based toolchain https://github.com/lowRISC/lowrisc-toolchains/releases
  • libelf and its development libraries. On Debian/Ubuntu, install it by running apt-get install libelf-dev.

Building Simulation

The Simple System simulator binary can be built via FuseSoC. From the Ibex repository root run:

fusesoc --cores-root=. run --target=sim --setup --build lowrisc:ibex:ibex_simple_system --RV32E=0 --RV32M=ibex_pkg::RV32MFast

Building Software

Simple System related software can be found in examples/sw/simple_system.

To build the hello world example, from the Ibex reposistory root run:

make -C examples/sw/simple_system/hello_test

The compiled program is available at examples/sw/simple_system/hello_test/hello_test.elf. The same directory also contains a Verilog memory file (vmem file) to be used with some simulators.

To build new software make a copy of the hello_test directory named as desired. Look inside the Makefile for further instructions.

If using a toolchain other than the lowRISC pre-built one examples/sw/simple_system/common/common.mk may need altering to point to the correct compiler binaries.

Running the Simulator

Having built the simulator and software, from the Ibex repository root run:

./build/lowrisc_ibex_ibex_simple_system_0/sim-verilator/Vibex_simple_system [-t] --meminit=ram,<sw_elf_file>

<sw_elf_file> should be a path to an ELF file (or alternatively a vmem file) built as described above. Use ./examples/sw/simple_system/hello_test/hello_test.elf to run the hello_test binary.

Pass -t to get an FST trace of execution that can be viewed with GTKWave. If using the hello_test binary the simulator will halt itself, outputting some simulation statistics:

Simulation statistics
=====================
Executed cycles:  633
Wallclock time:   0.013 s
Simulation speed: 48692.3 cycles/s (48.6923 kHz)

Performance Counters
====================
Cycles:                     483
NONE:                       0
Instructions Retired:       266
LSU Busy:                   59
Fetch Wait:                 16
Loads:                      21
Stores:                     38
Jumps:                      46
Conditional Branches:       53
Taken Conditional Branches: 48
Compressed Instructions:    182

The simulator produces several output files

  • ibex_simple_system.log - The ASCII output written via the output peripheral
  • ibex_simple_system_pcount.csv - A CSV of the performance counters
  • trace_core_00000000.log - An instruction trace of execution

Simulating with Synopsys VCS

Similar to the Verilator flow the Simple System simulator binary can be built using:

fusesoc --cores-root=. run --target=sim --tool=vcs --setup --build lowrisc:ibex:ibex_simple_system --RV32E=0 --RV32M=ibex_pkg::RV32MFast --SRAMInitFile=`<sw_vmem_file>`

<sw_vmem_file> should be a path to a vmem file built as described above, use ./examples/sw/simple_system/hello_test/hello_test.vmem to run the hello_test binary.

To run the simulator:

./build/lowrisc_ibex_ibex_simple_system_0/sim-vcs/lowrisc_ibex_ibex_simple_system_0

Pass -gui to use the DVE GUI.

Simulating with Riviera-PRO

To build and run Simple System run:

fusesoc --cores-root=. run --target=sim --tool=rivierapro lowrisc:ibex:ibex_simple_system --RV32E=0 --RV32M=ibex_pkg::RV32MFast --SRAMInitFile=\"$(readlink -f <sw_vmem_file>)\"

<sw_vmem_file> should be a path to a vmem file built as described above, use ./examples/sw/simple_system/hello_test/hello_test.vmem to run the hello_test binary.

System Memory Map

Address Description
0x20000 ASCII Out, write ASCII characters here that will get output to the log file
0x20008 Simulator Halt, write 1 here to halt the simulation
0x30000 RISC-V timer mtime register
0x30004 RISC-V timer mtimeh register
0x30008 RISC-V timer mtimecmp register
0x3000C RISC-V timer mtimecmph register
0x100000 – 0x1FFFFF 1 MB memory for instruction and data. Execution starts at 0x100080, exception handler base is 0x100000