Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!think!whitney From: whitney@think.COM (David Whitney) Newsgroups: comp.sys.apple Subject: Re: Fast/Slow screen RAM Message-ID: <24698@think.UUCP> Date: 31 Jul 88 23:10:24 GMT References: Sender: usenet@think.UUCP Reply-To: whitney@godot.think.com.UUCP (David Whitney) Organization: Thinking Machines Corporation, Cambridge, MA Lines: 53 In article jm7e+@ANDREW.CMU.EDU ("Jeremy G. Mereness") writes: > >It has been talked about (in Open-Apple for one) that the gs could be sped up >tremendously by some simple(?) hardware restructuring. > >Thinking about this, I reasoned that one of the biggest bottlenecks would be >the slow RAM dedicated to the display, as even in 16-bit mode, the processor >must slow down to access it. > ... >If the slow RAM could be filled with fast RAM chips, what would happen (besides >the price going up... damned boycott; where are the American companies to take >advantage of this)? > >If RAM could be accessed at any speed so long as it is not beyond its >capability (yes/no?), then why not fill the slow RAM with fast chips and >arrange for the 65816 to slow down to 1 MHz only when in 8-bit mode? Would this >work? Would it trash every 16-bit program that's been written so far? I think the reason for slowing down when accessing video RAM was to keep timing with the video circuitry simple. That's what I heard was the real trouble. It makes for a pretty good bottleneck, as there tends to be a lot of video activity in any useful program. Note also that the processor slows when the 5.25" disk is on. Try this: call-151, C0E9 (drive turns on - make sure it doesn't have disk inside!), 300L - note the printout speed - C0E8 (drive turns off), 300L - zing! much faster. It may also slow down for the 3.5" but I don't think so (have absolutely no idea, really). 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 appropriate class, I can't tell you the formula used to compute this, but a friend who TA'd the class tells me 5.6 MHz is the top. Now, 150ns RAM is relatively cheap, so the clock speed restriction basically kept the price of the machine down. Also, since the max speed can be 5.6MHz, that's why the two accelerators promised sometime soon (AE's, which was demoed at the Boston Applefest, and one from MDIdeas that I've read about in The Apple //GS Buyer's Guide) promise a speed of 5.6 MHz. If they were to go faster, they would need faster onboard RAM which would cost a whole lot. 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... David Whitney, MIT '90 DISCLAIMER: Nobody knows what I'm up {out there}!harvard!think!whitney to. Don't blame them for my actions whitney@think.com nor me for theirs. ^^^^^ will be changing before 1989 is here. Don't depend on it after 1/1/89.