Author: Dann Corbit
Date: 01:27:16 01/22/03
Go up one level in this thread
On January 22, 2003 at 03:29:05, Matt Taylor wrote:
>On January 21, 2003 at 17:03:33, Dann Corbit wrote:
>
>>On January 21, 2003 at 15:48:57, Sander de Zoete wrote:
>>
>>>The following instruction I found for the new architecture of Intel Chips
>>>
>>>BSWAP—Byte Swap
>>>
>>>Description
>>>Reverses the byte order of a 32-bit (destination) register:
>>>
>>>Operation
>>>TEMP ¬ DEST
>>>DEST(7..0) ¬ TEMP(31..24)
>>>DEST(15..8) ¬ TEMP(23..16)
>>>DEST(23..16) ¬ TEMP(15..8)
>>>DEST(31..24) ¬ TEMP(7..0)
>>>Flags Affected
>>>None.
>>>Opcode Instruction Description
>>>0F C8+ rd BSWAP r32 Reverses the byte order of a 32-bit register.
>>>
>>>It is only valid for 486 architecture.
>>>
>>>If this instruction can be used, it should be very easy to reverse Reverse
>>>BitBoards into Forward Boards again. Saving a lot of updating in make and unmake
>>>move and also be much faster using the bitboards for generating attack
>>>board for evaluation purposes, incheck checks etc.
>>>
>>>The thing left to do is to reverse the bits per byte.
>>>
>>>I use the following trick, but it doesn't seem to work voor the value 9. Let me
>>>show you what I mean: (and ofcourse, can you help me?)
>>>
>>>In C code, it looks like this:
>>>
>>>typedef unsigned __int64 BITBOARD;
>>>
>>>void ReverseBitsPerByte(BITBOARD bitboard)
>>>{
>>>// 1. Load the constant, k = 5555555555555555h
>>>BITBOARD k = 0x5555555555555555;
>>>
>>>// 2. x = [(x shl 1) and k] or [(x and k) shr 1] result is: EFCDAB8967452301
>>>bitboard = ((bitboard<<1) & k) | ((bitboard & k)>>1 );
>>>}
>>>
>>>Initial bitboard:
>>>11111110
>>>11011100
>>>10111010
>>>10011000
>>>01110110
>>>01010100
>>>00110010
>>>00010000
>>>
>>>Result of ReverseBitsPerByte(bitboard):
>>>01111111
>>>00111011
>>>01011101
>>>00011000<--what the ^*&* goes wrong here? Should be 00011001.
>>>01101110
>>>00101010
>>>01001100
>>>00001000
>>>
>>>Thanks for any help or suggestions.
>>
><snip code>
>
>These are all basically the same algorithm. It's going to take a while to
>rearrange everything when you do it bit-by-bit. Sander has the right idea by
>first swapping bytes and then trying to reverse the bits within the bytes (8*2
>ops + the swap as opposed to 64*2 ops).
The algorithms don't perform identically.
They don't require 64 ops.
Once you get the "Sander" algorithm completed, compare the two (and also the
COBRA algorithm).
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