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SERV is an award-winning bit-serial RISC-V core


Create a directory to keep all the different parts of the project together. We will refer to this directory as $SERV from now on

Download the main serv repo

cd $SERV && git clone

Install FuseSoC

pip install fusesoc

Initialize the FuseSoC standard libraries

fusesoc init

Create a workspace directory for FuseSoC

mkdir $SERV/workspace

Register the serv repo as a core library

cd $SERV/workspace && fusesoc library add serv ../serv

Check that the CPU passes the linter

cd $SERV/workspace && fusesoc run --target=lint serv

Running test software

Build and run the single threaded zephyr hello world example with verilator

cd $SERV/workspace
fusesoc run	--target=verilator_tb serv --uart_baudrate=57600 --firmware=../serv/sw/zephyr_hello.hex

..or... the multithreaded version

fusesoc run	--target=verilator_tb serv --uart_baudrate=57600 --firmware=../serv/sw/zephyr_hello_mt.hex --memsize=16384

...or... the philosophers example

fusesoc run	--target=verilator_tb serv --uart_baudrate=57600 --firmware=../serv/sw/zephyr_phil.hex --memsize=32768

...or... the synchronization example

fusesoc run	--target=verilator_tb serv --uart_baudrate=57600 --firmware=../serv/sw/zephyr_sync.hex --memsize=16384

Other applications can be tested by compiling and converting to bin and then hex e.g. with found in $SERV/serv/riscv-target/serv

Run the compliance tests

Build the verilator model (if not already done)

cd $SERV/workspace && fusesoc run --target=verilator_tb --setup --build serv

Download the tests repo

cd $SERV && git clone

Run the compliance tests

cd $SERV/riscv-compliance && make TARGETDIR=$SERV/serv/riscv-target RISCV_TARGET=serv RISCV_DECICE=rv32i RISCV_ISA=rv32i TARGET_SIM=$SERV/workspace/build/serv_0/verilator_tb-verilator/Vserv_wrapper

Run on hardware

Only supported so far is a single threaded Zephyr hello world example on the icebreaker and tinyFPGA BX boards


Pin A6 is used for UART output with 115200 baud rate.

cd $SERV/workspace
fusesoc run --target=tinyfpga_bx serv
tinyprog --program build/serv_0/tinyfpga_bx-icestorm/serv_0.bin


Pin 9 is used for UART output with 57600 baud rate.

cd $SERV/workspace
fusesoc run --target=icebreaker serv

Run with --firmware=../serv/sw/blinky.hex as the last argument to run the LED blink example instead

Good to know

Don't feed serv any illegal instructions after midnight. Many logic expressions are hand-optimized using the old-fashioned method with Karnaugh maps on paper, and shamelessly take advantage of the fact that some opcodes aren't supposed to appear. As serv was written with 4-input LUT FPGAs as target, and opcodes are 5 bits, this can save quite a bit of resources in the decoder.

The bus interface is kind of Wishbone, but with most signals removed. There's an important difference though. Don't send acks on the instruction or data buses unless serv explicitly asks for something by raising its cyc signal. Otherwise serv becomes very confused.

Don't go changing the clock frequency on a whim when running Zephyr. Or well, it's ok I guess, but since the UART is bitbanged, this will change the baud rate as well. As of writing, the UART is running at 115200 baud rate when the CPU is 32 MHz. There are two NOPs in the driver to slow it down a bit, so if those are removed I think it could achieve baud rate 115200 on a 24MHz clock.. in case someone wants to try


  • Applications have to be preloaded to RAM at compile-time
  • Store bootloader and register file together in a RAM
  • Make it faster and smaller