Writing and building software for OTBN

OTBN is the OpenTitan Big Number accelerator and this document describes how to write and build software for it. The OTBN reference manual describes the hardware and associated ISA.

Since OTBN is designed for cryptographic offload, not general computation, it has no C compiler. However, it does have the usual binutils programs: an assembler, linker and disassembler. The core of OTBN’s instruction set is based on RV32I, the RISC-V base integer instruction set. As such, we implement the toolchain as a thin wrapper around RISC-V binutils.

Assembly format and examples

The OTBN ISA and programmer model are described in the OTBN reference manual. In particular, note that the register x1 has special stack-like behaviour. There are some example programs at sw/otbn/code-snippets. These range from simple examples of how to use pseudo-operations up to example cryptographic primitives.

For specific formatting and secure coding guidelines, see the OTBN style guide.

The tools


The OTBN assembler is called otbn_as.py and can be found at hw/ip/otbn/util/otbn_as.py. This has the same command line interface as riscv32-unknown-elf-as (indeed, it’s a wrapper around that program). The only difference in default flags is that otbn_as.py passes -mno-relax, telling the assembler not to request linker relaxation. This is needed because one of these relaxations generates GP-relative loads, which assume x3 is treated as a global pointer (not true for OTBN code).

To assemble some code in foo.s to an ELF object called foo.o, run:

hw/ip/otbn/util/otbn_as.py -o foo.o foo.s


The OTBN linker is called otbn_ld.py and can be found at hw/ip/otbn/util/otbn_ld.py. This is a thin wrapper around riscv32-unknown-elf-ld, but supplies a default linker script that matches the OTBN memory layout. This linker script creates .start, .text and .data output sections. The .start and .text sections go to IMEM, with .start coming first. The .data section goes to DMEM. Since OTBN has a strict Harvard architecture with IMEM and DMEM both starting at address zero, the .start and the .data sections will both start at VMA zero. The instruction and data segments have distinct LMAs (for addresses, see the IMEM and DMEM windows at hw/ip/otbn/data/otbn.hjson).

Since the entry point for OTBN is always address zero, the entry vector should be the one and only thing in the .start section. To achieve that, put your entry point (and nothing else) in the .text.start input section like this:

.section .text.start
  jal x0, main


This ensure that, even if there are multiple objects being linked together, the intended entry point will appear in the right place.

To link ELF object files to an OTBN ELF binary, run

hw/ip/otbn/util/otbn_ld.py -o foo foo0.o foo1.o foo2.o


To disassemble OTBN code, use otbn_objdump.py, which can be found at hw/ip/otbn/util/otbn_objdump.py. This wraps riscv32-unknown-elf-objdump, but correctly disassembles OTBN instructions when run with the -d flag.

To disassemble the ELF binary linked in the previous section, run

hw/ip/otbn/util/otbn_objdump.py -d foo