Re: iSCSI CRC: A CRC-checking example

[Steve Blightman <steve@alacritech.com>]

Just a small point -
If c is an unsigned 32 bit value and d is an unsigned 8 bit value, on
most machines for the non-reflected lookup I don't think the "&" is
necessary, is it?  Shifting right 24 places ensures the first 24 bits
are 0.

i.e For the non-reflected table lookup you have:
 CRC32(c,d) (c=(c<<8)^crc32tab[(c>>24)^(d)])

and the same number of instructions as for the reflected case.

As for all these issues about the reflection and the iversion, I suspect
we have all spent too much time worrying about this already.  These must
be very minor issues - the important thing is that the specification
clearly state which way it is to be done.  For those of us preparing to
implement the CRC in hardware, the arguments are very unsettling.  Must
I now add even more gates to make the reflection and the inversion
programmable, in case the committee changes its mind?

Steve


Douglas Otis wrote:

> Follow on:
>
> Slight correction:
>
>  For the non-reflected table lookup you have:
>  CRC32(c,d) (c=(c<<8)^crc32tab[((c>>24)^(d))&0xFF])
>
>  For the reflected table lookup you have:
>  CRC32(c,d) (c=(c>>8)^crc32tab[(c^(d))&0xFF])
>
> Doug