OpenTitan Big Number Accelerator (OTBN)
This directory contains the implementation of the OpenTitan Big Number Accelerator (OTBN). OTBN is a coprocessor for asymmetric cryptographic operations like RSA or Elliptic Curve Cryptography (ECC).
See here for documentation on
the current version of OTBN; documentation matching the code in this directory
can be found in the
OTBN is under active development. Please ask questions and report issues through the GitHub issue tracker.
Build OTBN software
An assembler, linker and disassembler for OTBN can be found in
hw/ip/otbn/util. These are wrappers around a RISC-V GCC and binutils toolchain
so one must be available (see the OpenTitan documentation on obtaining a
For more details about the toolchain, see the user
otbn_ld.py can be used to build .elf files for use with
simulations. They work work similarly to binutils programs they wrap.
hw/ip/otbn/util/otbn_as.py -o prog_bin/prog.o prog.s hw/ip/otbn/util/otbn_ld.py -o prog_bin/prog.elf prog_bin/prog.o
Will assemble and link
prog.s resulting in
prog_bin/prog.elf that can be run
directly on the ISS or the standalone RTL simulation.
Work with the ISA
The instruction set is described in machine readable form in
data/insns.yml. This is parsed by Python code in
util/insn_yaml.py, which runs various basic checks on the data. The
binutils-based toolchain described above uses this information. Other
util/yaml_to_doc.py: Generates a Markdown snippet which is included in the OTBN specification.
dv/rig/otbn-rig: A random instruction generator for OTBN. See dv/rig/README.md for further information.
Run the Python simulator
The quickest way to run an OTBN-only program is to use the Python simulator.
First, generate a
.elf. file either using the usual build process or by
otbn_ld.py as shown above. Then, from
$ hw/ip/otbn/dv/otbnsim/standalone.py -t path/to/prog.elf
Run the standalone RTL simulation
A standalone environment to run OTBN alone in Verilator is included. Build it
fusesoc as follows:
fusesoc --cores-root=. run --target=sim --setup --build lowrisc:ip:otbn_top_sim
It includes functionality to set the initial Dmem and Imem contents from a .elf
file. The start address is hard coded to 0. Modify the
./dv/verilator/otbn_top_sim.sv to change this. A .elf (see above for build
instructions) can be loaded and run as follows:
./build/lowrisc_ip_otbn_top_sim_0.1/sim-verilator/Votbn_top_sim \ --load-elf=prog_bin/prog.elf
The simulation automatically halts on an
ecall instruction and prints the
final register values. The ISS is run in parallel and final register and memory
state will be cross-checked.
Tracing functionality is available in the
Votbn_top_sim binary. To obtain a
full .fst wave trace pass the
-t flag. To get an instruction level trace pass
--otbn-trace-file=trace.log argument. The instruction trace format is
To run several auto-generated binaries against the Verilated RTL, use
the script at
dv/verilator/run-some.py. For example,
hw/ip/otbn/dv/verilator/run-some.py --size=1500 --count=50 X
will generate and run 50 binaries, each of which will execute up to
1500 instructions when run. The generated binaries, a Verilated model
and the output from running them can all be found in the directory
Run the smoke test
A smoke test which exercises some functionality of OTBN can be found, together
with its expected outputs (in the form of final register values), in
./hw/ip/otbn/dv/smoke. The test can be run using a script.
This will build the standalone simulation, build the smoke test binary, run it and check the results are as expected.
Run the ISS on its own
There are currently two versions of the ISS and they can be found in
dv/otbnsim. The easiest to use is
takes an OTBN binary as an ELF file (as produced by the standard
linker script for
otbn_ld.py) and can dump the resulting DMEM if given
--dmem-dump argument. To see an instruction trace, pass the
There is also
dv/otbnsim/otbnsim.py. This takes flat binary files
with the contents of IMEM and DMEM and, when finished, generates a
cycle count and dumps DMEM contents. This is used to implement the
model inside of simulation, but is probably not very convenient for
command-line use otherwise.
Test the ISS
The ISS has a simple test suite, which runs various instructions and
makes sure they behave as expected. You can find the tests in
dv/otbnsim/test and can run them with
make -C dv/otbnsim test.