Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!mailrus!uwmcsd1!nic.MR.NET!umn-cs!sshankar From: sshankar@umn-cs.cs.umn.edu (Subash Shankar) Newsgroups: comp.sys.apple Subject: Re: Fast/Slow screen RAM Message-ID: <6497@umn-cs.cs.umn.edu> Date: 1 Aug 88 20:09:01 GMT References: <24698@think.UUCP> Reply-To: sshankar@umn-cs.UUCP (Subash Shankar) Organization: University of Minnesota Lines: 26 In article <24698@think.UUCP> whitney@godot.think.com.UUCP (David Whitney) writes: > >As for the 2.8 MHz speed in general, it turns out that 150ns RAM >limits the processor speed to 5.6 MHz. Since I haven't yet taken the Thanks. I was hoping that somebody would mention the memory bottleneck instead of blaming Apple for not producing a 25 MHz GS. In actuality, I believe that the maximum speed would be less then 5.6 MHz due to bus delays. (note 150 ns = 5.6 MHz). I've heard estimates of around 4 Mhz. > >If a GS+ (or some such) *does* go at nice dreamy speeds of 7.x MHz or >more, then it will need faster RAM which will cost a whole lot more >unless the machine uses wait states (like the Mac family) which >effectively slows down the computer. Oh well... As if wait states on MSDOS machines weren't bad enough, they would be absolutely meaningless on the 6502 family. The 6502, unlike other processors accesses memory on every cycle either for instructions or data. The addition of wait states for higher clock rates thus has absolutely no effect on performance. In contrast the 68XXX and 80XXX have several instructions that can proceed without memory access, and these instruction (mainly arithmetic) benefit from the higher clock rates. Also, their four cycle memory access is slowed down less by a wait state (20%) when compared to the single-cycle memory access 6502 which slows down by 50%.