RudolV by bobbl
RISC-V processor for real-time systems. Project goal is to preserve a predictable and tight timing model while increasing the performance. Therefore speculative components like caches, branch prediction and out-of-order execution are avoided or replaced by predictable alternatives.
- Multiple target FPGAs
- Predictable timing model
- CSR extension interface
- Simulation with Icarus Verilog or Verilator
- RV32IMC Zicsr Zifence
- Privileged machine-level ISA 1.11
- Formal verification with riscv-formal
- Unit tests
- riscv-compliance (predecessor of riscv-arch-test)
- riscv-arch-test (modified version that fits in 64 KiByte)
- Zephyr 2.2.0 support
- Dhrystone: Dhrystone MIPS/MHz: 0.736 ... 1.815 (depending on Dhrystone implementation)
- CoreMark: 1.354 per MHz
|Current state||Single issue in-order pipeline with 5 stages, scratchpad memory|
|Medium-term goal||Minimal hardware support for a software emulated MMU to run Linux|
|Long-term ideas||Dual issue|
|Decoupled but predictable pre-fetching for 0-cycle jumps|
Structure of Repository
|scripts/||Scripts for synthesis and simulation|
|scripts/sim/||Simulation with Icarus Verilog or Verilator|
|scripts/icestorm/||Synthesis with ICEStorm for Lattice FPGAs|
|scripts/libero/||Synthesis with Libero for Microsemi FPGAs|
|scripts/quartus/||Synthesis with Quartus for Intel FPGAs|
|scripts/vivado/||Synthesis with Vivado for Xilinx FPGAs|
|src/||CSR extensions and FPGA-specific verilog code|
|sw/||Software that can be executed on RudolV|
|sw/compliance/||RISC-V compliance tests|
|sw/coremark/||EEMBC CoreMark benchmark|
|sw/dhrystone/||Multiple implementations of the Dhrystone benchmark|
|sw/uart/||UART driver and bootloader|
|verify/||Formal verification with riscv-formal|
|config.sh||Configure paths to external tools|
|FPGA||Cyclone IV||iCE40 UP5K||IGLOO2 M2GL025||Kinex-7|
|Board||DE2-115||UltraPlus MDP||Future Creative||Genesys-2|
- 64 KiByte RAM (Microsemi only 56 KiByte)
- UART with 115200 Bit/s
- 32 bit timer and interrupt
- Bootloader waits for an executable image via UART
For FPGA support change to the manufacturer-specific directory and run the make script
cd scripts/TOOLNAME ./make.sh
to display the make targets. They differ from FPGA to FPGA, but all have in common, that the first argument must be the board name. To do the synthesis, use
./make.sh BOARD synth
On some boards, a separate place and route pass is necessary:
./make.sh BOARD pnr
Programming the device is done with
./make.sh BOARD prog
Programming (and UART communication) requires some access rights that can be
granted by udev rules. The udev rules for all supported boards can be found in
Simply copy this file to
After programming or pushing the reset button, RudolV waits for the binary application image to be transfered via the UART. The format is simple: first send the length of the image (in bytes) as decimal ASCII string. Followed by a blank (0x20) and then the binary data. The following script can be used:
sw/bootloader/send_elf.sh /dev/ttyUSB0 IMAGE.elf
Use a terminal emulator like picocom at 115200 baud to communicate with the program:
picocom -b 115200 --imap lfcrlf /dev/ttyUSB0
The architecture avoids speculative components to provide a predictable timing as required by hard real-time systems.
Data hazards are avoided by operand forwarding, therefore most instructions are executed in one cycle. Since the memory is single ported, memory accesses take two cycles.
The jump target is computed in the execute stage, resulting in a two cycle latency. There is no dynamic branch prediction but subsequent instructions are only killed in the case of a taken branch. This behaviour can be considered as a static always not taken prediction. If the branch target is not aligned to a 32-bit boundary, a taken branch or unconditional jump needs an additional cycle.
|RV32M||MUL, MULH, MULHSU, MULHU, DIV, DIVU, REM, REMU||35|
|unconditional jump||JAL, JALR||3/4|
|taken branch||BEQ, BNE, BLT, BGE, BLTU, BGEU||3/4|
|CSR access||CSRRW, CSRRS, CSRRC||2|
|memory access||LB, LH, LW, LBU, LHU, SB, SH, SW||2|
|not taken branch||BEQ, BNE, BLT, BGE, BLTU, BGEU||1|
|barrel shifter||SLL, SRL, SRA||1|
|arithmetic||ADD, SUB, SLT, SLTU, XOR, OR, AND, LUI, AUIPC||1|
CSR Extension Interface
Extra CSRs can be added with the follwing interface:
input clk input read input write input [31:0] wdata input [11:0] addr output [31:0] rdata output valid
addr is valid one cycle earlier that the other signals and stable for 5
cycles. Thus, address decoding is separated from the actual read or write
addr holds an CSR address that is supported by the extension, the
valid must be set in the following cycle. The latter is used to detect illegal
In the second cycle,
read is set if the CSR value should be read. If a valid
register is selected by
read is high,
rdata should be set to the
value of the register in the following cycle. It is only asserted for one cycle,
otherwise it must be cleared to 0.
If the CSR should be modified,
write is set in the sixth cycle and
holds the new value for the CSR. The bit set/clear operation is done inside the
pipeline and integrated in
This interface can also be used to connect external peripherals. Its advantage over a memory mapped interface is less impact on the critcal path and thus a potentially higher clock rate.
Examples of CSR extensions and waveforms can be found in src/csr.v:
|verilog module||no||CSRs / description|
|CsrUartBitbang||07C0||Minimal UART interface for software bitbanging|
|CsrUartChar||0BC0||Read and write bytes via UART|
|CsrPinsIn||0FC1||Read external pins (e.g. buttons or switches)|
|CsrPinsOut||0BC1||Write external pins (e.g. LEDs)|
|CsrTimerAdd||0BC2||Simple timer that asserts the timer interupt|
|CsrScratch||0BC3||Register that holds 32 bits (mainly for testing)|
For simulation use the make script in scripts/sim/:
cd scripts/sim ./make.sh
will display the usage of the script.
Icarus Verilog and Verilator are supported and any .elf file can be executed with optional debug output and .vcd generation.
The RISC-V Compliance Tests were renamed to RISC-V Architecture Tests and have undergone major revisons. There are much more tests and they are auto-generated, but unfortunately the old compliance tests were removed and the integration of riscv-tests was canceled.
Therefore local copies of the old tests are maintained in this repository. To build the compliance tests run:
make -C sw/compliance/
The riscv-tests are extended by some RudolV-specific tests and can be build by
make -C sw/tests/
Run all tests with Icarus:
cd scripts/sim ./make.sh icarus tests
Simulate only a single test with debug output:
cd scripts/sim ./make.sh icarus debug ../../sw/tests/build/rudolv_irq.elf
There is another, more serious problem with the new RISC-V Architecture Tests: the binaries are extremely large and require up to 1.7 MiBytes of memory. And there are no plans to reduce the memory footprint.
A closer look at the source code reveals, that the reason for the memory demand is simple: When branches or jumps are tested, the memory between instruction and target is filled with nops. Only 3 jumps are responsible for the 1.7 MiByte binary. In addition, the branch tests can be optimised by overlapping branch instructions and targets. With these two modifications, all riscv-arch-tests fit into 64 KiBytes.
The modified tests are in a separate repository. To test RudolV with them use
cd sw/testsuites/ ./make.sh riscv-arch-test
RudolV supports Zephyr with an out-of-tree board description. This means that a plain, unpatched installation of Zephyr can be used to build an .elf file for RudolV. To install Zephyr follow the instructions on https://docs.zephyrproject.org/latest/getting_started/index.html, particularly topics 2 (dependencies) and 5 (SDK). Topics 3 (source code) and 4 (python dependencies) are integrated in RudolV's make script:
cd sw/zephyr ./make.sh zephyr
Technically, a complete Zephyr port consists of board, DeviceTree, SoC and driver definitions. However, as of Zephyr 2.2.0, out-of-tree SoC definitions and drivers do not yet work properly. Therefore, all RudolV customisations are packed into the board definition at sw/zephyr/board/.
For example, tu build the hello_word sample, type
cd sw/zephyr west build -b rudolv ./zephyrproject/zephyr/samples/hello_world -- -DBOARD_ROOT=.
or use the make script:
./make.sh elf ./zephyrproject/zephyr/samples/hello_world
Dhrystone is widely used, but it depends to such an extend on the compiler settings and libc implementation that it is hardly comparable. Here are 3 different implementations from repositories of other RISC-V cores. Build them with
cd sw/dhrystone ./make.sh all
And send the images in
sw/dhrystone/build/ to the bootloader. Or run it with
cd scripts/icarus ./make.sh run ../../sw/dhrystone/build/riscv.elf ./make.sh run ../../sw/dhrystone/build/picorv32.elf ./make.sh run ../../sw/dhrystone/build/scr1.elf
|Dhrystone source||reference core||Fmax||cycles/iteration||RudolV c/i||DMIPS/MHz||RudolV D/M|
EEMBC CoreMark Benchmark
Run one iteration of EEMBC CoreMark with Icarus Verilog:
cd sw/coremark/ ./make.sh icarus
The results can be found in
sw/coremark/coremark/run1.log. Since this is a
simulation, the error message can be ignored and the computed iterations/sec
corresponds to CoreMark/MHz.
To get the CoreMark of an FPGA implementation, build an ELF image with UART output that executes 1000 iterations and send it to the bootloader:
cd sw/coremark/ ./make.sh elf 1000 ../bootloader/send_elf.sh /dev/ttyUSB1 build/coremark_iter_1000.elf
CoreMark requires the benchmark to run for at least 10 seconds. If RudolV is too fast, build an ELF file image with more iterations and send this one to the bootloader.
The CoreMark/MHz of RudolV is 1.295 and 0.892 without bitwise multiplier and divider.
sudo apt install iverilog verilator
Open source synthesis for Lattice FPGAs is possible with Project Icestorm. Installation guidelines are on the website. Synthesis for the other FPGAs is only possible with vendor-specific toolchains (Libero, Quartus, Vivado). A guide to install Libero can be found here.
To build the software, a RISC-V assembler and a C compiler with libgcc for
RV32IM is required. Install the GNU toolchain to
git clone --recursive https://github.com/riscv/riscv-gnu-toolchain cd riscv-gnu-toolchain mkdir build cd build ../configure --prefix=$DESTDIR --with-arch=rv32im make
By default, the build scripts of RudolV expect that the external toolchains
are in the search path (see config_default.sh. To set
specific path for every tool, copy
edit the paths there.
Licensed under the ISC licence (similar to the MIT/Expat license).
Copyright (c) Jörg Mische <email@example.com> Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted, provided that the above copyright notice and this permission notice appear in all copies. THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.