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Dec 17, 2021

Spike RISC-V ISA Simulator


Spike, the RISC-V ISA Simulator, implements a functional model of one or more RISC-V harts. It is named after the golden spike used to celebrate the completion of the US transcontinental railway.

Spike supports the following RISC-V ISA features:

  • RV32I and RV64I base ISAs, v2.1
  • RV32E and RV64E base ISAs, v1.9
  • Zifencei extension, v2.0
  • Zicsr extension, v2.0
  • M extension, v2.0
  • A extension, v2.1
  • F extension, v2.2
  • D extension, v2.2
  • Q extension, v2.2
  • C extension, v2.0
  • Zbkb, Zbkc, Zbkx, Zknd, Zkne, Zknh, Zksed, Zksh scalar cryptography extensions (Zk, Zkn, and Zks groups), v1.0
  • Zkr virtual entropy source emulation, v1.0
  • V extension, v1.0 (requires a 64-bit host)
  • P extension, v0.9.2
  • Zba extension, v1.0
  • Zbb extension, v1.0
  • Zbc extension, v1.0
  • Zbs extension, v1.0
  • Zfh and Zfhmin half-precision floating-point extensions, v1.0
  • Zmmul integer multiplication extension, v1.0
  • Zicbom, Zicbop, Zicboz cache-block maintenance extensions, v1.0
  • Conformance to both RVWMO and RVTSO (Spike is sequentially consistent)
  • Machine, Supervisor, and User modes, v1.11
  • Hypervisor extension, v1.0
  • Svnapot extension, v1.0
  • Svpbmt extension, v1.0
  • Svinval extension, v1.0
  • CMO extension, v1.0
  • Debug v0.14
  • Smepmp extension v1.0

As a Spike extension, the remainder of the proposed Bit-Manipulation Extensions is provided under the Spike-custom extension name Xbitmanip. These instructions (and, of course, the extension name) are not RISC-V standards.

These proposed bit-manipulation extensions can be split into further groups: Zbp, Zbs, Zbe, Zbf, Zbc, Zbm, Zbr, Zbt. Note that Zbc is ratified, but the original proposal contained some extra instructions (64-bit carryless multiplies) which are captured here.

To enable these extensions individually, use the Spike-custom extension names XZbp, XZbs, XZbc, and so on.

Versioning and APIs

Projects are versioned primarily to indicate when the API has been extended or rendered incompatible. In that spirit, Spike aims to follow the SemVer versioning scheme, in which major version numbers are incremented when backwards-incompatible API changes are made; minor version numbers are incremented when new APIs are added; and patch version numbers are incremented when bugs are fixed in a backwards-compatible manner.

Spike's principal public API is the RISC-V ISA. The C++ interface to Spike's internals is not considered a public API at this time, and backwards-incompatible changes to this interface will be made without incrementing the major version number.

Build Steps

We assume that the RISCV environment variable is set to the RISC-V tools install path.

$ apt-get install device-tree-compiler
$ mkdir build
$ cd build
$ ../configure --prefix=$RISCV
$ make
$ [sudo] make install

If your system uses the yum package manager, you can substitute yum install dtc for the first step.

Build Steps on OpenBSD

Install bash, gmake, dtc, and use clang.

$ pkg_add bash gmake dtc
$ exec bash
$ export CC=cc; export CXX=c++
$ mkdir build
$ cd build
$ ../configure --prefix=$RISCV
$ gmake
$ [doas] make install

Compiling and Running a Simple C Program

Install spike (see Build Steps), riscv-gnu-toolchain, and riscv-pk.

Write a short C program and name it hello.c. Then, compile it into a RISC-V ELF binary named hello:

$ riscv64-unknown-elf-gcc -o hello hello.c

Now you can simulate the program atop the proxy kernel:

$ spike pk hello

Simulating a New Instruction

Adding an instruction to the simulator requires two steps:

  1. Describe the instruction's functional behavior in the file riscv/insns/<new_instruction_name>.h. Examine other instructions in that directory as a starting point.

  2. Add the opcode and opcode mask to riscv/opcodes.h. Alternatively, add it to the riscv-opcodes package, and it will do so for you:

     $ cd ../riscv-opcodes
     $ vi opcodes       // add a line for the new instruction
     $ make install
  3. Rebuild the simulator.

Interactive Debug Mode

To invoke interactive debug mode, launch spike with -d:

$ spike -d pk hello

To see the contents of an integer register (0 is for core 0):

: reg 0 a0

To see the contents of a floating point register:

: fregs 0 ft0


: fregd 0 ft0

depending upon whether you wish to print the register as single- or double-precision.

To see the contents of a memory location (physical address in hex):

: mem 2020

To see the contents of memory with a virtual address (0 for core 0):

: mem 0 2020

You can advance by one instruction by pressing the enter key. You can also execute until a desired equality is reached:

: until pc 0 2020                   (stop when pc=2020)
: until reg 0 mie a                 (stop when register mie=0xa)
: until mem 2020 50a9907311096993   (stop when mem[2020]=50a9907311096993)

Alternatively, you can execute as long as an equality is true:

: while mem 2020 50a9907311096993

You can continue execution indefinitely by:

: r

At any point during execution (even without -d), you can enter the interactive debug mode with <control>-<c>.

To end the simulation from the debug prompt, press <control>-<c> or:

: q

Debugging With Gdb

An alternative to interactive debug mode is to attach using gdb. Because spike tries to be like real hardware, you also need OpenOCD to do that. OpenOCD doesn't currently know about address translation, so it's not possible to easily debug programs that are run under pk. We'll use the following test program:

$ cat rot13.c 
char text[] = "Vafgehpgvba frgf jnag gb or serr!";

// Don't use the stack, because sp isn't set up.
volatile int wait = 1;

int main()
    while (wait)

    // Doesn't actually go on the stack, because there are lots of GPRs.
    int i = 0;
    while (text[i]) {
        char lower = text[i] | 32;
        if (lower >= 'a' && lower <= 'm')
            text[i] += 13;
        else if (lower > 'm' && lower <= 'z')
            text[i] -= 13;

    while (!wait)
$ cat 
OUTPUT_ARCH( "riscv" )

  . = 0x10010000;
  .text : { *(.text) }
  .data : { *(.data) }
$ riscv64-unknown-elf-gcc -g -Og -o rot13-64.o -c rot13.c
$ riscv64-unknown-elf-gcc -g -Og -T -nostartfiles -o rot13-64 rot13-64.o

To debug this program, first run spike telling it to listen for OpenOCD:

$ spike --rbb-port=9824 -m0x10000000:0x20000 rot13-64
Listening for remote bitbang connection on port 9824.

In a separate shell run OpenOCD with the appropriate configuration file:

$ cat spike.cfg 
interface remote_bitbang
remote_bitbang_host localhost
remote_bitbang_port 9824

set _CHIPNAME riscv
jtag newtap $_CHIPNAME cpu -irlen 5 -expected-id 0x10e31913

target create $_TARGETNAME riscv -chain-position $_TARGETNAME

gdb_report_data_abort enable

$ openocd -f spike.cfg
Open On-Chip Debugger 0.10.0-dev-00002-gc3b344d (2017-06-08-12:14)
riscv.cpu: target state: halted

In yet another shell, start your gdb debug session:

[email protected]:~/SiFive/spike-test$ riscv64-unknown-elf-gdb rot13-64
GNU gdb (GDB)
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "--host=x86_64-pc-linux-gnu --target=riscv64-unknown-elf".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
Find the GDB manual and other documentation resources online at:
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from rot13-64...done.
(gdb) target remote localhost:3333
Remote debugging using localhost:3333
0x0000000010010004 in main () at rot13.c:8
8	    while (wait)
(gdb) print wait
$1 = 1
(gdb) print wait=0
$2 = 0
(gdb) print text
$3 = "Vafgehpgvba frgf jnag gb or serr!"
(gdb) b done 
Breakpoint 1 at 0x10010064: file rot13.c, line 22.
(gdb) c
Disabling abstract command writes to CSRs.

Breakpoint 1, main () at rot13.c:23
23	    while (!wait)
(gdb) print wait
$4 = 0
(gdb) print text