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Whisper is a RISCV instruction set simulator (ISS) developed for the verification of the Swerv micro-controller. It allows the user to run RISCV code without RISCV hardware. It has an interactive mode where the user can single step the target RISCV code and inspect/modify the RISCV registers or the simulated system memory. It can also run in lock step with a Verilog simulator serving as a "golden model" against which an implementation is checked after each instruction of a test program.


To use Whisper, you would need to download its source code, compile it, prepare some target test program, compile the test program to RISCV binary code and then run the RISCV binary within the Whisper simulator. In particular you would need:

  1. A Linux machine to host the RISCV tool-chain and Whisper.

  2. The RISCV tool-chain which contains a cross-compiler to compile C/C++ code to RISCV binary. This can be installed on most Linux distributions using your distros pacakge manager (apt, dnf, pacman etc.). Otherwise it can be built from the upstream source code.


    $ sudo apt install gcc-riscv64-unknown-elf


    $ sudo pacman -Syu riscv64-elf-gcc
  3. The Whisper source code which can be downloaded from github.

  4. The g++ compiler version 7.2 or higher to compiler Whisper. The g++ compiler can be installed from a Linux distribution. Alternatively, the source code can be downloaded from

  5. The boost library version 1.67 of higher compiled with c++-14 or c++-17. Boost source can be downloaded from

Compiling Whisper

On a Unix system, in the whisper directory, do the following: make BOOST_DIR=x where x is the path to your boost library installation.

Preparing Target Programs

Standalone C/assembly programs not requiring operating system support (such programs cannot do any I/O) should be compiled as follows:

$ riscv32-unknown-elf-gcc -mabi=ilp32 -march=rv32imc -static -O3 -nostdlib -o test1 test1.c

The key switch in the above compilation command is "-nostdlib" which prevents the compiler from linking-in the standard C library.

Note that without the standard C library, there is no "_start" symbol. The linker will complain that the start symbol is missing and will use another symbol as the default start address of the program. The user can always override that start address (program counter at the beginning of the simulation) by using the --startpc command line option.

Also note that without an operating system, the simulator does not know when the program finishes. It will execute instructions indefinitely. Consider the following test program:

main(int argc, char* argv[])
  int x = 1;
  int y = 2;
  int z = x + y;
  return z;

The simulator will start execution at the ELF file entry point (address corresponding to main) and will return to address 0 (initial value of return address register) when the instruction corresponding to "return z" is executed. This will most likely cause an illegal instruction exception and given that no trap handlers are loaded into the memory, it will cause an infinite loop of illegal traps. To avoid this, simple stand-alone no-operating-system programs should define a global 32-bit integer named "tohost" and should write to that location at the end of the program. This signals the simulator to terminate the program.

Here's a modified version of the above program that stops once main is done:

#include <stdint.h>

volatile uint32_t tohost = 0;

main(int argc, char* argv[])
  int x = 1;
  int y = 2;
  int z = x + y;
  return z;

void _start()
  main(0, 0);
  tohost = 1;

And here's how to compile and run the above program

$ riscv32-unknown-elf-gcc -mabi=ilp32 -march=rv32imc -nostdlib -g -o test2 test2.c
$ whisper test2

If no global variable named "tohost" is written by the program, the simulator will stop on its own if a sequence of 64 consecutive illegal instructions is encountered.

For programs requiring minimal operating system support (e.g. brk, open, read and write) the user can compile with the newlib C library and use the simulator with the "--newlib" option.

Here's a sample program:

#include <stdio.h>

main(int argc, char* argv[])
   printf("hello world\n");
   return 0;

And here's how to compile and run it (assuming riscv32-unknown-elf-gcc was compiled with newlib):

$ riscv32-unknown-elf-gcc -mabi=ilp32 -march=rv32imc -static -O3 -o test3 test3.c
$ whisper --newlib test3

Note that in this case the simulator will intercept the exit system call invoked by the C library code and terminate the program accordingly. There is no need for the "tohost" mechanism.

Running Whisper

Running whisper with -h or --help will print a brief description of all the command line options. To run a RISCV program, prog, in whisper, one would issue the Linux command:

whisper prog

which will run the program until it writes to the "tohost" location.

A program compiled with the newlib C library need not have a "tohost" location. Such a program will run until it calls exit. Such a program would be run as follows:

whisper --newlib prog

Command Line Options

The following is a brief description of the command line options:

   Produce help message.

   Enable tracing to standard output of executed instructions.

--xlen len
   Specify register width (32 or 64), defaults to 32.

--isa string
   Select the RISCV options to enable. The currently supported options are
   a (atomic), c (compressed instructions), d (double precision fp), 
   f (single precision fp), i (base integer), m (multiply divide),
   s (supervisor mode), u (user mode), and v (vector). By default, only i, m and c
   are enabled. Note that option i cannot be turned off. Example: "--isa imcf".
   It is recommended to avoid this option if the configuration of the "misa" CSR is
   included in the JSON configuration file.

--target program
   Specify target program (ELF file) to load into simulated memory. In newlib
   emulations mode, program options may follow program name.

--hex file
   Hexadecimal file to load into simulator memory.

--logfile file
   Enable tracing to given file of executed instructions.

--consoleoutfile file
   Redirect console output to given file.

--commandlog file
   Enable logging of interactive/socket commands to the given file.

--startpc address
   Set program entry point to the given address (in hex notation with a 0x prefix).
   If not specified, use the ELF file entry point.

--endpc address
   Set stop program counter to the given address (in hex notation with a 0x 
   prefix). Simulator will stop once instruction at the stop program counter
   is executed. If not specified, use the ELF file _finish symbol.

--tohost address
   Memory address to which a write stops the simulator (in hex with 0x prefix).

--consoleio address
   Memory address corresponding to console io (in hex with 0x prefix).
   Reading/writing a byte (using lb/sb instruction) from given address
   reads/writes a byte from the console.

--maxinst limit
   Limit executed instruction count to given number.

   After loading any target file into memory, the simulator enters interactive

   Enable debug triggers (triggers are automatically enabled in interactive and
   server modes).

   Enable performance counters.

   Run in gdb mode enabling remote debugging from gdb.

--profileinst file
   Report executed instruction frequencies to the given file.

--setreg spec ...
   Initialize registers. Example --setreg x1=4 x2=0xff

--disass code ...
   Disassemble instruction code(s). Example --disass 0x93 0x33

--configfile file
   Configuration file (JSON file defining system features).

--snapshotdir path
   Directory prefix for saving snapshots: Snapshots (see --sanpshotpreid)
   are placed in sub-directories of the given path. Default: "snapshot".

--snapshotperiod n
   Snapshot period: Save a snapshot every n instructions putting data in
   directory specified by --snapshotdir.

--loadfrom path
   Snapshot directory from which to restore a previously saved (snapshot)

   Emulate limited emulation of newlib system calls. Done automatically 
   if newlib symbols are detected in the target ELF file.

   Emulate limited emulation of Linux system calls. Done automatically 
   if Linux symbols are detected in the target ELF file.

   Bare metal mode: Disable emulation of Linux/newlib system call emulation
   even if Linux/newlib symbols detected in the target ELF file.

--stdout path
   Redirect the standard output of the newlib/Linux target program to the
   file specified by the given path.

--stderr path
   Redirect the standard error of the newlib/Linux target program to the
   file specified by the given path.

--alarm period
   External interrupt period in micro-seconds: Convert period to an instruction
   count, n, assuming a 1ghz clock, and set to 1 the timer bit of the MIP
   CSR every n instructions. The timer bit of MIP is automatically cleared if
   the interrupt is actually taken (interrupts enabled in MSTATUS and timer
   bit set in MIE CSR). No-op if n is zero.

   Use ABI register names (e.g. sp instead of x2) in instruction disassembly.

   Produce additional messages.

   Print version.

Interactive Mode

Whisper is started in interactive mode using the "--interactive" command line option. Here's are some examples:

$ whisper --interactive
$ whisper --interactive test1

In the second example, the program test1 is first loaded into the simulated memory. In interactive mode the user can issue commands to control the execution of the target program and to set/examine the registers and memory location of the simulated system. The help command will produce a list of all available interactive commands. The "help x" command will produce information about command x.

Here's the output of the "help" command:

help [<command>]
  Print help for given command or for all commands if no command given.

  Run till interrupted.

until <address>
  Run until address or interrupted.

step [<n>]
  Execute n instructions (1 if n is missing).

peek <res> <addr>
  Print value of resource res (one of r, f, c, m) and address addr.
  For memory (m) up to 2 addresses may be provided to define a range
  of memory locations to be printed.
  examples: peek r x1   peek c mtval   peek m 0x4096

peek pc
  Print value of the program counter.

peek all
  Print value of all non-memory resources.

poke res addr value
  Set value of resource res (one of r, c or m) and address addr.
  Examples: poke r x1 0xff  poke c 0x4096 0xabcd

disass opcode <code> <code> ...
  Disassemble opcodes. Example: disass opcode 0x3b 0x8082

disass function <name>
  Disassemble function with given name. Example: disass func main

disass <addr1> <addr2>>
  Disassemble memory locations between addr1 and addr2.

elf file
  Load elf file into simulated memory.

hex file
  Load hex file into simulated memory.

replay_file file
  Open command file for replay.

replay n
  Execute the next n commands in the replay file or all the
  remaining commands if n is missing.

replay step n
  Execute consecutive commands from the replay file until n
  step commands are executed or the file is exhausted.

reset [<reset_pc>]
  Reset hart.  If reset_pc is given, then change the reset program
  counter to the given reset_pc before resetting the hart.

  Terminate the simulator.

Newlib Emulation

Whisper will emulate the newlib open, close, read, write, brk and exit system calls. This allows simple programs to run and use the newlib C-library functions such as printf, fopen, fread, fwrite, fclose, malloc, free and exit. Here an example of running a program with limited C-library support:

$ whisper --newlib test3

And here is an example of passing the command line arguments arg1 and arg2 to the to the target program test3:

$ whisper --newlib "test3 arg1 arg2"

And examples of passing command line switches to a target program that requires them:

$ whisper --newlib "test4 -opt1 val1 -opt2"
$ whisper --newlib --target "test4 -opt1 val1 -opt2"

Debugging RISCV Programs Using Gdb and Whisper

With the --gdb option, whisper will follow the gdb remote debugging protocol. This allows the user to debug a RISCV program using a cross-compiled gdb and whisper. For example, to debug a RISCV program named xyz on a Linux x86 machine, we would start the (cross-compiled) RISCV gdb as follows:

$ riscv-unknown-elf-gdb xyz

at the gdb prompt, we would connect to whisper by issuing a "target remote" gdb command as follows:

target remote | whisper --gdb xyz

Configuring Whisper

A JSON configuration file may be specified on the command line using the --configfile switch. Numeric parameters may be specified as integers or as strings. For example, a core count of 4 may be specified as: "cores" : 4 or "cores" : "4" If expressed as a string, a numeric value may be prefixed with 0x to specify headecimal notation (JSON does not support hexadecimal notation for integers).

The value of a boolean parameters may be specified as an integer with 0 indicating false and non-zero indicating true. Alternatively it may be specified using the strings "false", "False", "true", or "True".

Command line options override settings in the configuration file.

Here is a sample configuration file:

    "xlen" : 32,
    "enable_zfh" : "true",
    "enable_zba" : "true",
    "enable_zbb" : "true",
    "abi_names" : "true",

    "csr" : {
        "misa" : {
            "reset-comment" : "imabfv",
            "reset" : "0x40201123",
            "mask-comment" : "Misa is not writable by CSR instructions",
            "mask" : "0x0"
         "mstatus" : {
            "mstatus-comment" : "Hardwired to zero except for FS, VS, and SD.",
            "reset" : "0x80006600",
            "mask" : "0x0",
            "poke_mask" : "0x0"

Configuration parameters


Number of cores in simulated system.


Integer register size in bits.


Object defining memory organization. Fields of memap:

  • size: Field defining physical memory size
  • page_size: Field defining page size
  • inst: Array of entries defining areas of physical memory where instruction fetch is legal (if not used, then all of memory is valid for fetch). Each entry is an array of 2 integers defining the start and end address of a fetch region.
  • data: Array of entries defining areas of physical memory where data load/store is legal (if not used, then all of memory is valid for load/store). Each entry is an array of 2 integers defining the start and end address of a data region.


"memmap" : { "size" : "0x100000000", "page_size" : 4096,
             "inst" : [0, "0x20000000"] }
internal and external memory mapped registers

A memory region may be a memory-mapped register (e.g. PIC/PLIC/CLINT) of arbitrary size. In json file you can define multiple memory-mapped-register regions each has its own base address and size and an indication if the region is internal or external. In addition, there is a single map for all implemented registers in all regions, each register has name, count, size and mask. Unimplemented register has mask=0.

Access to memory-map-register region either for implemented or unimplemented register must be aligned to register size (default 4) and access size must be equals to register size


 "memory_mapped_registers" : {
    "default_mask" : 0,
    "address" : [
    "size" : [
    "internal" : "false",
   "registers" : {
       "meip" : {
          "count" : 31,
          "address" : "0x4001000",
          "mask" : "0x0"
       "meipl" : {
          "count" : 31,
          "address" : "0x4000004",
          "mask" : "0xf"
       "mtime" : {
          "count" : 1,
          "address" : "0x800bff8",
          "size" : 8,
          "mask" : "0xffffffffffffffff"


Number of implemented performance counters. If specified number is n, then CSRs (counters) mhpmcounter3 to mhpmcounter3+n-1 are implemented and the remaining counters are hardwired to zero. Same for the mhpmevent CSRs.


Whisper will count events associated with performance counters when this is set to true. Note that pipeline specific events (such as mispredicted branches) are not supported. Synchronous events (such as count retired load insructions) are supported.


If set to true then registers are identified by their ABI names in the log file (e.g. ra instead of x1).


The CSR configuration is a map wher each key is a CSR name and the corresponding value is an object with the following fields: "number", "reset", "mask", "poke_mask", "exists", and "shared". Set "exists" to "false" to mark a non implemented CSR (read/write instructions to such a CSR will trigger illegal instruciton exception). Set "mask" to the write mask of the CSR (zero bits correspond to bits that will be preserved by write instructions). Set "reset" to reset value of the CSR. Set "shared" to "true" for CSRs that are shared between harts. The "number" fields should be used to define the number (address) of a non-standard CSR. The poke_mask should be used for the rare cases where poke operation may modify some bits that are not modifiable by CSR write instructions.


The vector configuration is an object with the following fields:

  • bytes_per_vec: vector size in bytes
  • min_bytes_per_elem: narrowest suppoted element size in bytes (default 1).
  • max_bytes_per_elem: widest supported element size in bytes (no default).


"vector" : {
   "bytes_per_vec" : 16,
   "max_bytes_per_elem" : 8


Defines the PC value after reset


Defines the PC address after a non-maskable-interrupt.


Enable support for debug triggers.


The MISA register is read only. It is not possible to change XLEN at run time by writing to the MISA register.

The "round to nearest break tie to max magnitude" rounding mode is not implemented unless you compile with the softfloat library: "make SOFT_FLOAT=1" in which case simulation of floating point instructions slows down significantly.

Suppprted extensions: A, B, C, D, F, I, M, S, U, V, ZFH, ZBA, ZBB, and ZBS.


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