As of Release 1.0, RARS supports most of RV32IMFN (base 32 bit instruction set + multiplication, floating point, and user-level interrupts). The guiding reference for implementing the instruction set has been version 2.2 of the official specification.
The RARS IDE provides program editing and assembling but its real strength is its support for interactive debugging. The programmer can easily set and remove execution breakpoints or step through execution forward or backward (undo) while viewing and directly editing register and memory contents.
For bug reporting, please create an issue at https://github.com/TheThirdOne/rars/issues for general questions or comments email the lowRISC mailing list info@lowrisc.org.