Author: Tom Kerrigan
Date: 17:21:37 10/25/01
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On October 25, 2001 at 18:08:07, Robert Hyatt wrote: >On October 25, 2001 at 13:45:05, Tom Kerrigan wrote: > >>On October 25, 2001 at 11:53:09, Robert Hyatt wrote: >> >>>For 64 bit development since mid-60's, the driving force has been a push for >>>more precision in FP (64 bits) and _faster_ execution (because all 32 bit >>>computers from the 60's had double-precision (64 bit FP) but it was too slow.) >> >>As I said in another post, FP has very little to do with the bitiness of a chip. >>Everybody agrees that x86s are 32-bit, but the P4 has 128-bit wide SIMD >>registers and double precision FP ALUs. > >That doesn't matter. _how_ do you gate the FP values around _inside_ the >cpu? On 64 bit datapaths or multiplexed on 32 bit datapaths? 64 bit busses, obviously. If you have a 64 bit reg file (well, 128 in SSE2's case) and an FP ALU, cache interface, and main memory interface that are just as wide or wider, why in the world would you go to the extra work of muxing 32 bit values across the busses in between them? Like I said, FP is separate from int, enough that they were usually put on different chips until recently, and there's no reason why the busses on the FP side of things have to be as narrow as on the int side. A chip with all this 64 bit stuff can still be 32 bit because the int unit still drives the chip--does all the branching, addressing, blah blah blah. -Tom
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