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Altera_Forum
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14 years ago

Using custom instruction from uCLinux user app on nios2mmu

Hello, Is it possible to use a custom instruction from an user application running on NIOS2MMU - uCLinux. I'm trying to run the CRC design example on uCLinux. When I try to compile the ...
Altera_Forum's avatar
Altera_Forum
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14 years ago

FWIW, if you are doing CRC16 (the usual one for hdlc comms) then a custom instruction for the following C can be used: 

static __inline__ uint32_t
crc_step(uint32_t crc, uint32_t byte_val)
{
     uint32_t t = crc ^ (byte_val & 0xff);
     t = (t ^ t << 4) & 0xff;
     return crc >> 8 ^ t << 8 ^ t << 3 ^ t >> 4;
}
The 4 levels of xor easily execute in a single clock.

My notes suggest that the above C compiles to 11 instructions, and a lookup table version to 7 (with the table base in a global register).