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Subject: quick bitboard question

Author: Robert Hyatt

Date: 12:04:55 08/26/05


I'm fixing to change the way I number bits to make bit 0 the LSB (as the BSF/BSR
instructions count on the X86) rather than the MSB as the leading zero
instruction counts on the Cray-1 architecture.

I have four choices for which square bit 0 represents (four logical choices,
anyway, I guess one could number bits randomly should they choose).

A1 = 0
H1 = 0
A8 = 0
H8 = 0

For those of you using bitboards, which did you use?  Since it doesn't matter to
me at all, since I am having to rewrite a bunch of stuff to make this work, I
thought I would try to match the most common layout which will make the programs
more compatible if we share any ideas.

I am leaning toward H1 = 0, H2 = 8, ..., H8 = 56 and A8 = 63, but I've not made
any decision yet...  The reason for this is that it is easier to visualize if
you think of the chess board as being composed of the 1 rank and the rightmost 8
bits of the bitboard, the 2nd rank is the next 8 bits, etc.

The alternative is H8=0, etc, so that the last rank is the rightmost bits, the
seventh rank is the next 8 bits, etc.  Anything else requires "mentally
mirroring" so that if A1 or A8 is bit zero, the bits are backward, since A8 is
the left end of a rank, and bit 0 is the right end of a set of 8 bits...






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