This repository host an RISC-V implementation written in SpinalHDL. There is some specs :

• RV32IM instruction set
• Interrupts and exception handling with the Machine mode from the riscv-privileged-v1.9.1 specification.
• Pipelined on 5 stages (Fetch, Decode, Execute, Memory, WriteBack)
• 1.17 DMIPS/Mhz with all extension
• Optimized for FPGA
• Optional MUL/DIV/REM extension
• Optional instruction and data caches
• Optional MMU
• Two implementation of shift instructions, Single cycle / shiftNumber cycle
• Each stage could have bypass or interlock hazard logic
• FreeRTOS port https://github.com/Dolu1990/FreeRTOS-RISCV

The hardware description of this CPU is done by using an very software oriented approach (without any overhead in the generated hardware). There is a list of software concepts used :

• There is very few fixed things. Nearly everything is plugin based. The PC manager is a plugin, the register file is a plugin, the hazard controller is a plugin ...
• There is an automatic a tool which allow plugins to insert data in the pipeline at a given stage, and allow other plugins to read it in another stages through automatic pipelining.
• There is an service system which provide a very dynamic framework. As instance, a plugin could provide an exception service which could then be used by others plugins to emit exceptions from the pipeline.

Dependencies

On Ubuntu 14 :

# JAVA JDK 7 or 8
sudo apt-get install openjdk-7-jdk

# SBT
echo "deb https://dl.bintray.com/sbt/debian /" | sudo tee -a /etc/apt/sources.list.d/sbt.list
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 2EE0EA64E40A89B84B2DF73499E82A75642AC823
sudo apt-get update
sudo apt-get install sbt

# Verilator (for sim only)
sudo apt-get install git make autoconf g++ flex bison
git clone http://git.veripool.org/git/verilator   # Only first time
unsetenv VERILATOR_ROOT  # For csh; ignore error if on bash
unset VERILATOR_ROOT  # For bash
cd verilator
git pull        # Make sure we're up-to-date
git tag         # See what versions exist
autoconf        # Create ./configure script
./configure
make
sudo make install

CPU generation

You can find two example of CPU instantiation in :

• src/main/scala/VexRiscv/GenFull.scala
• src/main/scala/VexRiscv/GenSmallest.scala

To generate the corresponding RTL as a VexRiscv.v file, run (it could take time the first time you run it):

sbt "run-main VexRiscv.GenFull"

# or
sbt "run-main VexRiscv.GenSmallest"

Tests

To run tests (need the verilator simulator), go in the src/test/cpp/regression folder and run :

# To test the GenFull CPU
make clean run

# To test the GenSmallest CPU
make clean run IBUS=IBUS_SIMPLE DBUS=DBUS_SIMPLE CSR=no MMU=no DEBUG_PLUGIN=no MUL=no DIV=no

Interactive debug of the simulated CPU via GDB/OpenOCD in Verilator

It's as described to run tests, but you just have to add DEBUG_PLUGIN_EXTERNAL=yes in the make arguments. Work for the GenFull, but not for the GenSmallest as this configuration has no debug module.

Then you can use the https://github.com/SpinalHDL/openocd_riscv tool to create a GDB server connected to the target (the simulated CPU)

src/openocd -c "set VEXRISCV_YAML PATH_TO_THE_GENERATED_CPU0_YAML_FILE" -f tcl/target/vexriscv_sim.cfg

Cpu plugin structure

There is an example of an pseudo ALU plugin :

//Define an signal name/type which could be used in the pipeline
object ALU_ENABLE extends Stageable(Bool)
object ALU_OP     extends Stageable(Bits(2  bits))  // ADD, SUB, AND, OR
object ALU_SRC1   extends Stageable(UInt(32 bits))
object ALU_SRC2   extends Stageable(UInt(32 bits))
object ALU_RESULT extends Stageable(UInt(32 bits))

class AluPlugin() extends Plugin[VexRiscv]{

  //Callback to setup the plugin and ask for different services
  override def setup(pipeline: VexRiscv): Unit = {
    import pipeline.config._
    //Do some setups as for example specifying some instruction decoding by using the Decoding service
    val decoderService = pipeline.service(classOf[DecoderService])

    decoderService.addDefault(ALU_ENABLE,False)
    decodingService.add(List(
        M"0100----------" -> List(ALU_ENABLE -> True, ALU_OP -> B"01"),
        M"0110---11-----" -> List(ALU_ENABLE -> True, ...)
    ))
  }


  //Callback to build the hardware logic
  override def build(pipeline: VexRiscv): Unit = {
    import pipeline._

    execute plug new Area {
      import execute._
      //Add some logic in the execute stage
      insert(ALU_RESULT) := input(ALU_OP).mux(
        B"00" -> input(ALU_SRC1) + input(ALU_SRC2),
        B"01" -> input(ALU_SRC1) - input(ALU_SRC2),
        B"10" -> input(ALU_SRC1) & input(ALU_SRC2),
        B"11" -> input(ALU_SRC1) | input(ALU_SRC2),
      )
    }

    writeBack plug new Area {
      import writeBack._
      //Add some logic in the execute stage
      when(input(ALU_ENABLE)){
        input(REGFILE_WRITE_DATA) := input(ALU_RESULT)
      }
    }
  }
}