GF-RV16

An experimental 16-bit RISC-V ISA which I hope to build hardware to support one day

The tools, documents, and source code here are to validate that the ISA is reasonable - i.e. that it is both useful (easy enough to write code with) and implementable (i.e. instructions can be encoded reasonably in a 16-bit encoding space, and can execute on a relatively simple back end core)

More information: https://github.com/gfoot/gf-rv16/wiki

Try it in your web browser

Web-based Simulator - this is a live embedding of the latest code, running in-browser using Brython

Python command-line tools

Assembling some code, running the simulator

$ python tools/assem.py src/tests/testcode.s

$ python tools/sim.py src/tests/testcode.s --trace

Library tests

$ python tools/sim.py src/tests/randtest.s

Outputs a lot of random bits that can be fed into a randomness checker.

$ python tools/sim.py src/tests/malloctest.s

Tests basic allocation and freeing behaviour, including coallescing adjacent free regions

$ python tools/sim.py src/tests/malloctest2.s

Stress-tests allocation and freeing randomly, small and large blocks, to try to fragment the heap and make life hard for the allocator

Analysing some code

$ python tools/stats.py src/tests/malloctest2.s

This gathers statistics about instruction usage and immediate ranges used by the assembled code.

ISA metrics

$ python tools/isatest.py

Prints information about how much of the 16-bit encoding space is used for each instruction, so you can experiment a bit with immediate constant bit widths and see if everything is likely to fit.

Note that tools/isaprops.py has - along with its own definitions about how immediate constants are encoded - some code to determine the actual immediate values that each instruction supports, so that it can check the assembled code didn't violate them, as well as explain what the restrictions are. Maybe it should be combined somehow with isatest.py.

Source code

src contains various test programs, many of which are runnable through the simulator.

pseudotest.s is a bit different - originally intended to check generation of real instructions to support pseudoinstructions, it evolved into a more general test suite for the assembler. So it is not intended to be executed - but when it is assembled, the assembler will check that the generated code matches the expectations listed in comments in pseudotest, and report errors for any differences.

src/lib contains library code used by many of the programs, including basic I/O, string handling, and malloc/free, using approximately the same interfaces as C's standard library does.

Documentation

See the wiki on github for more information: https://github.com/gfoot/gf-rv16/wiki