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- Newsgroups: comp.lang.c++,comp.programming,comp.arch
- Path: uu4news.netcom.com!friend!news
- From: rich@kastle.com (Richard Krehbiel)
- Subject: Re: Why are 32 bit better than 16 bit pgms?
- Message-ID: <1996Feb6.135808.12257@friend.kastle.com>
- Sender: news@friend.kastle.com (News)
- Reply-To: rich@kastle.com
- Organization: Kastle Development Associates
- X-Newsreader: Forte Free Agent 1.0.82
- References: <4er4m4$78q@news1.ucsd.edu> <1996Feb5.163838.24531@amc.com>
- Date: Tue, 6 Feb 1996 13:57:21 GMT
-
- curtis@amc.com (Curtis Green) wrote:
-
- >The "bits" quoted for the processor (16 bit, 32 bit, 64 bit, etc) is the
- >size of the data path (data bus).
-
- [rest cut - I just want to gripe on this one point. :-) ]
-
- Why did Intel and IBM call the 8088 a 16 bit CPU? It has an 8 bit
- bus.
-
- Why call the 386SX a 16 bit CPU when it can run any 386DX (32 bit)
- code?
-
- Why don't they call the Pentium a 64 bit CPU since it has a 64 bit
- data bus? Why doesn't DEC call the Alpha AXP a 128 bit CPU (or is it
- 256 - I'm not sure).
-
- What in the hell could you call the Motorola 68020/030 when for each
- bus transaction it may choose 8, 16, or 32 bits as requested by the
- peripheral being addressed?
-
- My humble opinion:
-
- "Bitness" has become a marketing term, i.e. meaningless (rather like
- the term "RISC"). There are a dozen different pieces inside a CPU
- these days each with it's own bit width. Outside the chip there's the
- data bus and address bus width. Inside the chip you have the general
- register width, special register widths (i.e. Segment register = 16
- bits, FP register = 80 bits), ALU width (multiplied by the number of
- ALUs if superscalar), internal-cache-to-register path width(s), etc.
- Then there's the architectural virtual address size versus actual
- virtual address size versus the physical address size versus the
- implemented MMUs capabilities. (Intel advertised that the 386 could
- address 64 Terabytes - which is baloney. They added up 13 bits of
- segment plus 32 bits of offset. They somehow forgot that this "45
- bit" virtual address is then funnelled into a 32 bit paged MMU.) You
- probably also have to consider what mode the OS software's running
- too.
-
- What really matters is how fast it'll run what software. You might as
- well forget making any generalizations based on bit size.
-
- --
- Richard Krehbiel, Kastle Systems, Arlington VA USA
- rich@kastle.com (work) or richk@mnsinc.com (personal)
-
-