Accelerating a C Function with User Logic

A custom instruction only has two 32bit inputs (+ the opcode word) and can generate one 32bit result (although it can access other fpga resources), typically they will be synchronous - but you could arrange to do things asynchronously.

A custom component would have to be accessed via the avalon bus, so getting data to/from nios registers is slower (the nios cpu always stalls for the duration of an Avalon cycle). It is probably more appropriate if you need to access other fgpa resources - especially large memory blocks.

I've only used combinatorial custom instructions - mainly to speed up CRC16 and byteswapping.