Author: Ralf Elvsén
Date: 03:41:14 01/17/01
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On January 16, 2001 at 18:49:06, Robert Hyatt wrote: >On January 15, 2001 at 23:29:42, Larry Griffiths wrote: > >>On January 15, 2001 at 20:15:17, Ralf Elvsén wrote: >> >>>I looked ay the instruction set for the Itanium processor. >>>As far as I could see there was no instruction to get the first/last >>>bit set. >>> >>>Can someone with insight in processor design describe the >>>considerations done by the designers when they decide to >>>include or to not include such an instruction? (Assuming they >>>are not chess programmers :) >>> >>>Is it possible to quantify how the "trouble" to include >>>it scales with the number of bits? I.e. is it even less likely >>>to be found in an 128-bit processor? >>> >>>Ralf >> >>I asked this question a while back. >> >>I think IA32 instructions might still be supported and maybe the BSF BSR >>instructions are still available. >> >>Maybe you can verify this. >> >>Larry. > > >I actually think that they are not supported if I recall Eugene's comments >correctly. It is possible to find which byte is non-zero in a 64 bit word, >but I think it then requires a 256-entry table look-up to find the set bit. There is an instruction which tells you which byte (or which 16-bit field if you prefer that) is the first/last to have only zeros. This doesn't sound as useful as the one you describe. If the one you describe exists I missed it. Ralf
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