Risc-V: Minimum CSR requirements for simple RV32I implementation capable of leveraging GCC

Question

What would be the bare minimum CSR requirements for a RV32I capable of running machine code generated with GCC?

I'm thinking of a simple fpga-based (embedded) implementation. No virtual memory or linux support is required.

Also, what GCC flags should I use in order to prevent it from using unimplemented CSR related instructions?

I'm still quite confused after scanning through the RISCV Privileged ISA Specification.

Thanks!

Answer 1

Have a look at the RARS simulator as an example of a simple RISC V implementation.  It implements sufficient CSRs (e.g. the exception cause, processor status, exception pc, vector table address, etc..) that you can program an interrupt handler.

You'll need:

• utvec — sets the exception handler address
• ustatus — to enable/disable interrupts,
• uscratch — needed by software exception handler,
• ucause — tells the reason for exception
• uepc — tells the address of uncompleted instruction at exception

And some others.  In RARS, you can see the registers implemented in the register display, Control and Status tab.

I believe RARS supports the timer, so has some CSRs for that.  It also provides a floating point unit, so some CSRs for exceptions for that as well as rounding configuration.  For handling memory access exceptions, it has utval.  And then it offers some counters.  See also table 2.2 in Document Version 20190608-Priv-MSU-Ratified

I would think that your usage of CSRs would be restricted to standalone application configuration, e.g. initial bootup, and interrupt handling, both of which would be written in assembly.

Hard to imagine that compiled C code (object files, .o's) would touch the CSRs in any way.  If you have an example of that, please share it.

In some environments, the C implementation allows for standalone (e.g. unhosted) programs.  It is possible that such a program created by some compiler includes startup configuration and an exception handler though more likely that these would be user supplied.  See, for example, http://cs107e.github.io/guides/gcc/