Configuring a RISC-V CPU

Among the architectures supported in Renode, RISC-V is one of the most prominent. Renode supports both 32 and 64-bit versions of RISC-V, Privileged Architecture in various versions and a broad range of extensions.

The most common base ISA sets supported by Renode are:

  • RV32/64I Base (I)

  • Integer Multiplication and Division (M)

  • Atomic Instructions (A)

  • Single-Precision Floating-Point (F)

  • Double-Precision Floating-Point (D)

  • Compressed Instructions (C)

  • Control and Status Register Instructions (Zicsr)

  • Instruction-Fetch Fence (Zifencei)

All of the above constitute the G set.

Renode also supports many of the ISA extensions like:

  • Vector Operations (V)

  • Bit-manipulation

    • Address generation instructions (Zba)

    • Basic bit-manipulation (Zbb)

    • Carry-less multiplication (Zbc)

    • Single-bit instructions (Zbs)

  • Half-Precision Floating-Point (Zfh)

Lastly, Renode also supports custom, non standard instruction sets. Some of them can be selected like other instruction sets(e.g. Xandes), others are defined directly in their respective core classes (e.g. CV32E40P).

To define a RISC-V core in your simulation, you must edit the Renode Platform file (.repl) and add a node for the CPU.

Picking specific ISA variants

Start by adding CPU.RiscV32 or CPU.RiscV64 to the .repl file:

cpu: CPU.RiscV32 @ sysbus

then use the cpuType argument to specify the ISA and the required extensions.

Start with “rv32” or “rv64”, depending on the architecture width, followed by a list of enabled ISA sets. Extensions with names longer than one character have to be separated by an underscore.

For example:

cpu: CPU.RiscV32 @ sysbus
  cpuType: "rv32imaf_zicsr_zifencei"

which denotes a 32-bit RISC-V with the base instruction set (I), integer multiplication and division (M), atomic instructions (A), single-precision floating-point (F), CSR instructions (Zicsr), and instruction-fetch fence (Zifencei).

Customizing the CPU

There are additional parameters you can pass to the CPU while creating it in the .repl file. All of these are optional.

  • timeProvider - sets the peripheral to be used as the time provider for the CPU, which is used to populate the time CSR. Typically you would provide your instance of the clint interrupt controller

  • privilegeArchitecture - selects which version of the Privileged Architecture the CPU should follow. The default is 1.11. Available values are:

    • PrivilegeArchitecture.Priv1_09

    • PrivilegeArchitecture.Priv1_10

    • PrivilegeArchitecture.Priv1_11

  • endianness - specifies the endianness of the CPU, defaulting to little endian

  • nmiVectorAddress and nmiVectorLength - allow for customizing the non-maskable interrupt vector, if supported by the CPU

  • allowUnalignedAccesses - defines if an exception should be raised whenever the software performs an unaligned access on memory. Defaults to false

  • interruptMode - Allows you to enforce interrupt handling mode. Defaults to auto. Available modes:

    • Auto (0) - Checks mtvec’s LSB to detect the mode

    • Direct (1) - All exceptions set PC to mtvec’s BASE value

    • Vectored (2) - Asynchronous interrupts set PC to mtvec’s BASE + 4 * cause

Adding a custom RISC-V instruction

One of the most important features of RISC-V is its customizability, also with non-standard instructions.

There are several ways in which a custom instruction can be added to a RISC-V CPU in Renode.

They all revolve around two arguments

  • Pattern - A bit pattern which specifies which instructions to match to execute your custom handler. Characters 1 and 0 denote which bit has to be set in that position, while any other character means “any value”. The pattern has to have a length of 64, 32, or 16 characters.

  • Handler - The code to execute when the pattern matches


You can use a Python script for handling custom instructions by using the InstallCustomInstructionHandlerFromString or InstallCustomInstructionHandlerFromFile methods present on RISC-V CPUs.

For example:

sysbus.cpu InstallCustomInstructionHandlerFromString "10110011100011110000111110000010" "cpu.DebugLog('custom instruction executed!')"

The Python script has the instruction variable available, which contains the opcode of the instruction that was called.


You can use a C# function for handling custom instruction by using the InstallCustomInstruction method. It takes the same pattern argument, but the handler argument is Action<UInt64>

public class MyCustomRiscV : RiscV32
    public MyCustomRiscV(IMachine machine, IRiscVTimeProvider timeProvider = null, uint hartId = 0,
                    PrivilegeArchitecture privilegeArchitecture = PrivilegeArchitecture.Priv1_11,
                    Endianess endianness = Endianess.LittleEndian, string cpuType = "rv32imfc_zicsr_zifencei")
      : base(machine, cpuType, timeProvider, hartId, privilegeArchitecture, endianness, allowUnalignedAccesses : true)
      // As mentioned, the pattern arguments takes a 16, 32, or 64 character long string.
      // Characters 0 and 1 specify the bits that should be set,
      // while all other characters mean that given bit can be either 1 or 0.
      // In this case we use F for the Imm field, B for rs1, and D for rD to make the pattern more readable
      //      |        |       +-- [ 7:11] - rD
      //      |        +---------- [15:19] - rs1
      //      +------------------- [20:31] - Imm
      // We could've used "-----------------100-----0001011" as the pattern and it'd mean the same thing,
      // but suddenly it's not so clear where the different fields are inside the opcode we're matching.
      InstallCustomInstruction(pattern: "FFFFFFFFFFFFBBBBB100DDDDD0001011", handler: opcode =>
        this.Log(LogLevel.Noisy, "(p.lbu rD, Imm(rs1!)) at PC={0:X}", PC.RawValue);
        // rD = Zext(Mem8(rs1))
        var rD = (int)BitHelper.GetValue(opcode, 7, 5); // Extract value from opcode starting at bit 7 and length of 5
        var rs1 = (int)BitHelper.GetValue(opcode, 15, 5); // Extract value from opcode starting at bit 15 and length of 5
        var rs1Value = (long)GetRegisterUnsafe(rs1).RawValue;
        SetRegisterUnsafe(rD, ReadByteFromBus((ulong)rs1Value));

        // rs1 += Imm[11:0]
        var imm = (int)BitHelper.SignExtend((uint)BitHelper.GetValue(opcode, 20, 12), 12); // Extract value from opcode starting at bit 20 and length of 12
                                                                                           // and sign-extend it to full int
        SetRegisterUnsafe(rs1, (ulong)(rs1Value + imm));

Verilated Custom Function Units

You can connect up to 4 CFUs to every RISC-V core that’s simulated in Renode.

After you’ve compiled your CFU with the Verilator Integration Library (see: Example CFU project) you can attach the CFU to your CPU.

To do so, add this line to your .repl:

cfu0: Verilated.CFUVerilatedPeripheral @ cpu 0

and this line to your .resc:

cpu.cfu0 SimulationFilePathLinux @<PATH_TO_COMPILED_CFU_BINARY>

If you’re on Windows as opposed to Linux you must use SimulationFilePathWindows, and respectively SimulationFilePathMacOS on macOS. Instructions referencing your CFU will then be forwarded to the Verilated CFU.

All CFU instructions follow this pattern: FFFFFFFAAAAABBBBBIIICCCCCNN01011

  • N - CFU number to forward the instruction call to

  • C - Register in which the resulting value will be placed

  • I, F - Function ID. The final ID that’s passed to the CFU is FFFFFFFIII

  • B - Source register 1. Its value will be read and passed into the CFU

  • A - Source register 2. Same as the above

Adding a custom RISC-V Control and Status Register

In a similar fashion to custom instructions, you can also define custom CSRs.


To use a Python script to handle your custom CSR you can use RegisterCSRHandlerFromString and RegisterCSRHandlerFromFile. The first argument is the CSR ID, while the second refers to the handler script.

The script gets passed a request variable, which contains fields isRead, isWrite, and value.

if request.isRead: # If the CPU tries to read your CSR, isRead will be True
    cpu.DebugLog('CSR read!')
elif request.isWrite: # Otherwise isWrite will be True and value will contain the value being written
    cpu.DebugLog('CSR written: {}!'.format(hex(request.value)))


To use C# functions to handle custom CSRs you can use the RegisterCSR method. Similarly it takes the ID as the first argument, but then takes the read handler and the write handler separately, in that order.

Let’s take the previously defined custom RISC-V CPU and add a custom CSR to it which will return a random value every time it’s read.

public class MyCustomRiscV : RiscV32
    public MyCustomRiscV(IMachine machine, IRiscVTimeProvider timeProvider = null, uint hartId = 0,
                    PrivilegeArchitecture privilegeArchitecture = PrivilegeArchitecture.Priv1_11,
                    Endianess endianness = Endianess.LittleEndian, string cpuType = "rv32imfc_zicsr_zifencei")
      : base(machine, cpuType, timeProvider, hartId, privilegeArchitecture, endianness, allowUnalignedAccesses : true)
      this.random = EmulationManager.Instance.CurrentEmulation.RandomGenerator;

      InstallCustomInstruction(pattern: "FFFFFFFFFFFFBBBBB100DDDDD0001011", handler: opcode =>
        // Custom Instruction implementation from before

        () => (ulong)random.Next(),           // Read handler
        value => { /* Ignore all writes */ }, // Write handler
        name: "RND"                           // Optionally add a name of your CSR

    private readonly PseudorandomNumberGenerator random;

    // Creating an enum isn't required, but it's cleaner
    // and allows to name otherwise arbitrary CSR IDs
    private enum CustomCSR : ulong
      Rnd = 0xfc0,

Last update: 2024-04-19