Author: Eugene Nalimov
Date: 09:43:41 10/26/01
Go up one level in this thread
Bob, here you are wrong. System 360 used separate sets of integer and FP registers. And I believe that before PCs came in it was "mainstream" by any definition. On more modern architectures -- MC68k, PPC, MIPS, Alpha, ARM, etc. etc. all use 2 different set of registers -- integer and FP (of course low-end models can have no FP unit, in this case they have not only FP registers, but also FP instructions as well). Eugene On October 25, 2001 at 23:15:10, Robert Hyatt wrote: >On October 25, 2001 at 20:21:37, Tom Kerrigan wrote: > >>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? > >Simple. Many architectures don't have special FP registers at all. The general >registers do either, depending on the opcode chosen. In fact, I don't know >of any mainstream machine that does it the way Intel does, although I don't >claim to know how they all work... > > >> >>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. >> > >Actually this is only true for Intel. IBM big iron, Vaxes, Sparcs, etc all >use general-purpose registers that can hold ints or FP values. The opcodes >control what is done to the data... > >Different size busses do cause problems. In the case of Intel, the CPU >transfers 64 bits to/from memory. FP operations can starve scalar pipe >operations for data, and vice-versa... > > >>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 > >Maybe. Or maybe there is no separation between FP and int instructions at >all. That is really an intel-approach. Until the PC, machines didn't really >have any sort of FP processor. All the instructions passed thru one pipe >using one set of registers that contained both int and fp (and address for >that matter) data...
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