Help in CRC custom instructions please.

There is no point quoting the entire previous message!

__builtin_custom_ini() is just a way of getting a 'custom' opcode into your program's object code. There is nothing special about it, it doesn't generate anything different/special by being 'builtin' - there is no requirement to use it to generate a 'custom' instruction, you can use a gcc asm statement of your own with exactly the same effect.

Knowing what a particular custom instruction actually does is something that you (as a system designer) need to know. If you build with the altera IDE scripts (etc) then the relevant constants are propogated through the system for you.

Otherwise you have to make sure that the values match.

For instance you might request the hardware engineer set custom instruction 0 to calculate a crc and instruction 1 to 'frob' (whatever that might be) two values. This is not really any different from defining the register layout for an avalon slave (etc).

FWIW, if you actually want to calculate CRC16 (used on most hdlc) there is a simple piece of combinatorial logic to add in a byte. There might we a similar reduction for CRC32 (ethernet).