Help in CRC custom instructions please.

I don't use the IDE.... but the ALT_CI_CRC_INST_N constant is defined with the value qsys/sopc assigned to the custom instruction when it was added to the cpu definition.

So if you# include the correct header, and the custom instruction opcode is generated with that constant, and you've downloaded the correct fpga image it will work.

I'm not sure how you'll get any help with RSA though.

The custom instructions can't access memory (not strictly true since their logic could contain an Avalon master! - but you can't interwork with the data cache).

The only inputs (from the cpu) are the 32bit opcode word, and the values of two 32bit registers indexed by the 'A' and 'B' fields.

A combinatorial custom instruction could use the value of other vhdl registers (etc) but it can't have any side effects because it executes every clock (or rather the logic is fed the relevant inputs on every clock, and the output value is fed into a big mux and used if the opcode actually selects the relevant custom instruction).

Possibly you could use a clocked, multi-cycle (single-cycle) custom instruction to latch values into some logic - use the 'C' field (with readrc unset) to determine where to write (you can write both the A and B register values).

With 'readrc' set you return a value, use the 6 bit 'A' and 'B' register numbers to determine what to read (or use a 32bit value and the 'B' rg number).

Using a separate combinatorial custom instruction for the reads would save the 2 clock 'late result' penalty on the result - making coding easier.