problem with custom istruction: clk_en never changes

Probably worth double-checking that you have added the custom instruction to the cpu, and that the code you have is actually trying to execute the instruction.

I've only used combinatorial custom instructions. A simple one of those might be a good 'first test' since there is less to go wrong.

FWIW I've not actually verified that 'readra' and 'readrb' have any effect (ie whether setting them zero disables the pipeline stall against the source registers). writerc will have an effect (enables the register file update).

The custom instruction itself can use any of the instruction fields and the values of the two source registers for any purpose, I've used the 'B' field as extra opcode bits.