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 https://docs.opentitan.org/hw/ip/otbn/doc/index.html for documentation on the current version of OTBN; documentation matching the code in this directory can be found in the doc directory.
OTBN is currently in early development. Please ask questions and report issues through the GitHub issue tracker.
Note that OTBN is still in the early development stages so this simulation does not support the full ISA
A standalone environment to run OTBN alone in verilator is included. Build it with 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. The start address is hard coded to 0. Modify the ImemStartAddr parameter in ./dv/verilator/otbn_top_sim.sv to change this. The simulation is run as follows:
./build/lowrisc_ip_otbn_top_sim_0.1/sim-verilator/Votbn_top_sim -t \ --meminit=imem,imem_contents.vmem --meminit=dmem,dmem_contents.vmem
This will initialise the IMem with imem_contents.vmem and the DMem with dmem_contents.vmem. The -t argument enables tracing. The simulation automatically halts on an ecall instruction.
To build the RTL implementation of OTBN in an earlgrey simulation, run fusesoc without passing the OTBN_MODEL flag. For example, the Verilator simulation can be compiled as follows.
fusesoc --cores-root=. run --target=sim --setup --build lowrisc:systems:top_earlgrey_verilator
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 sanity checks on the data. Current tooling that uses this information:
util/yaml_to_doc.py: Generates a Markdown snippet which is included in the OTBN specification.