Author: Robert Hyatt
Date: 20:11:38 10/24/01
Go up one level in this thread
On October 24, 2001 at 21:52:29, Tom Kerrigan wrote: >On October 24, 2001 at 17:48:26, Tony Werten wrote: > >>Could be. I'm quite sure though that's what they said about 32bits when the >>got >>16 bits, and about 64 when they got 32. >> >>People don't know what they want until there has been a big advertisement >>campagne explaining why they really need it. After that it goes fast. > >There was a super-big need for 32 bit addressing. > >Very few <= 16 bit chips have address spaces as wide as their datapaths. >Examples: the 8 bit Z80 had a 16 bit address space and the 16 bit 8086 had a 20 >bit address space. These are pretty hacky architectures because they need >separate "sub-datapaths" to do addressing. Programming these chips is also hacky >because you need segments and near/far pointers and crap like that. > >Most 32 bit chips are 32 bits all the way through and people get along with them >just fine, with no hacks or awkward programming tricks to increase address >space. > >Basically, there was a big, obvious need for ~32 bit addressing, but never a >big, obvious need for > 32 bit addressing. > >This is backed up by the fact that 64 bit CPUs are readily available and few >people use them. > >There are definitely cases where you want > 32 bit addressing, but not many. >Remeber, 64 bits gives you 4 _billion_ times as much address space as 32 bits. > >I don't expect to see 128-bit CPUs (or at least a need for them) within the >next, say, 30 years. > >-Tom Note that 64 bit processors don't necessarily have 64 bit address spaces. The Cray family is one example with 64 bit values, but a 32 bit address space. The MIPS guys have not yet implemented a full 64 bit address space for obvious reasons... In fact, I don't know of any 64 bit processors that really have a full 64 bit address space, due to the size of it and the unnecessary waste involved in gating addresses that are impossible to generate. I don't remember what the MIPS guys (for example) have done recently, but early chips had something like a 43 bit address space or some such oddball number...
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