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Minerva

A 32-bit RISC-V soft processor

Minerva is a CPU core that currently implements the RISC-V RV32IM instruction set. Its microarchitecture is described in plain Python code using the nMigen toolbox.

Quick start

Minerva requires Python 3.6+ and nMigen. Installation instructions for nMigen can be found here.

python setup.py install
python cli.py generate > minerva.v

To use Minerva in its minimal configuration, you need to wire the following ports to minerva_cpu:

  • clk
  • rst
  • ibus__*
  • dbus__*
  • external_interrupt
  • timer_interrupt
  • software_interrupt

Features

The microarchitecture of Minerva is largely inspired by the LatticeMico32 processor.

Minerva is pipelined on 6 stages:

  1. Address The address of the next instruction is calculated and sent to the instruction cache.
  2. Fetch The instruction is read from memory.
  3. Decode The instruction is decoded, and operands are either fetched from the register file or bypassed from the pipeline. Branches are predicted by the static branch predictor.
  4. Execute Simple instructions such as arithmetic and logical operations are completed at this stage.
  5. Memory More complicated instructions such as loads, stores and shifts require a second execution stage.
  6. Writeback Results produced by the instructions are written back to the register file.

Pipeline Diagram Image

The L1 data cache is coupled to a write buffer. Store transactions are in this case done to the write buffer instead of the data bus. This enables stores to proceed in one clock cycle if the buffer isn't full, without having to wait for the bus transaction to complete. Store transactions are then completed in the background as the write buffer gets emptied to the data bus.

Configuration

The following parameters can be used to configure the Minerva core.

Parameter Default value Description
reset_address 0x00000000 Reset vector address
with_icache False Enable the instruction cache
icache_nways 1 Number of ways in the instruction cache
icache_nlines 128 Number of lines in the instruction cache
icache_nwords 4 Number of words in a line of the instruction cache
icache_base 0x00000000 Base of the instruction cache address space
icache_limit 0x80000000 Limit of the instruction cache address space
with_dcache False Enable the data cache
dcache_nways 1 Number of ways in the data cache
dcache_nlines 128 Number of lines in the data cache
dcache_nwords 4 Number of words in a line of the data cache
dcache_base 0x00000000 Base of the data cache address space
dcache_limit 0x80000000 Limit of the data cache address space
with_muldiv False Enable RV32M support
with_debug False Enable the Debug Module
with_trigger False Enable the Trigger Module
nb_triggers 8 Number of triggers
with_rvfi False Enable the riscv-formal interface

Testing

A riscv-formal testbench for Minerva is available here.

Possible improvements

In no particular order:

  • RV64I
  • Floating Point Unit
  • Stateful branch prediction
  • MMU
  • ...

If you are interested in sponsoring new features or improvements, get in touch at contact [at] lambdaconcept.com .

License

Minerva is released under the permissive two-clause BSD license. See LICENSE file for full copyright and license information.