Path: utzoo!mnetor!uunet!mcvax!ukc!stc!datlog!slxsys!jpp From: jpp@slxsys.specialix.co.uk (John Pettitt) Newsgroups: comp.unix.xenix Subject: Re: 16-bit versus 32-bit memory performance Message-ID: <109@slxsys.specialix.co.uk> Date: 10 Dec 87 14:17:13 GMT References: <388@ddsw1.UUCP> <620@omen.UUCP> <435@spdcc.COM> Reply-To: jpp@slxsys.UUCP (John Pettitt) Organization: Specialix International, London, UK. Lines: 29 Summary: 32 bit cpu on 16 bit ram is a waste of money This should perhaps belong in comp.arch It would appear that most 8088,8086,186 and 286 systems are limited by the number of cycles taken to execute instructions (I.E the clock speed). However the 80386 (at 16 and esp at 20 Mhz) is limited by its memory bus bandwidth. That is the memory subsystem on most 286 boxes is fast enough have little or no real effect on performance compared to a change in clock speed. An 80386 however is largly limited by the rate that it can be 'fed' data and instructions. 16 Bit memory subsystems have a devestating effect on the 80386 for 2 reasons. Firstly 2 memory accesses are required rather than one thus doubling the access time. Secondly most 16 bit memory cards are designed for 8 or 10 Mhz operation not 16 Mhz so a significant number of wait states are needed when used with a 386. It would appear that a 'cache miss' on the Intel Inboard(tm) generates beteween 10 and 12 wait states thus making access to 16 bit ram slower than from the original 286. In conclustion - if you want a 32 bit CPU use 32 bit ram. If you just want the instruction set use the P9 (80388) - if it ever appears. (This posting written on a Dell 386 with 6 MB of 0 wait static 32 bit ram) -- John Pettitt - 144.5 MHz: G6KCQ, CIX: jpettitt, Voice: +44 1 398 9422 UUCP: ...uunet!mcvax!ukc!pyrltd!slxsys!jpp (jpp@slxsys.specialix.co.uk) Disclaimer: I don't even own a cat to share my views !