Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.csd.uwm.edu!cs.utexas.edu!uunet!microsoft!brianw From: brianw@microsoft.UUCP (Brian Willoughby) Newsgroups: comp.sys.apple Subject: Re: GS--running from ROM is faster Summary: Why GS ROM is faster Message-ID: <7508@microsoft.UUCP> Date: 25 Aug 89 18:10:37 GMT References: <8908212244.aa15584@SMOKE.BRL.MIL> <34254@apple.Apple.COM> Reply-To: brianw@microsoft.UUCP (Brian Willoughby) Organization: Microsoft Corp., Redmond WA Lines: 56 In article <34254@apple.Apple.COM> dlyons@Apple.COM (David Lyons) writes: >In article <8908212244.aa15584@SMOKE.BRL.MIL> SEWALL@UCONNVM.BITNET (Murph Sewall) writes: >>[...] UNLESS (to improve performance) applications routinely copy the Rev 3 >>ROM into the RAM space it would occupy in a Rev 1 machine. That (if I >>grasp the concept I only read about last week) is the "shadow RAM" >>technique some PC clone vendors are using to improve performance. In the >>MS-DOS World RAM is faster memory than ROM; is that true of the IIgs? > >Nope. In the GS, running from ROM is actually faster than running from >RAM. (So copying the ROM into RAM wouldn't help, even if it were not a >no-no of mammoth proportions, which it is, and even if it would work, >which it won't.) > >(Why? About 8% of the time, RAM is getting refreshed. If you're *running* >from RAM, you have to stop and wait for it. If you're running from ROM, >you don't. This explanation is missing some details, but the idea is there.) > > > --Dave Lyons, Apple Computer, Inc. | DAL Systems The only reason that is true, Dave, is that currently the GS is not running anywhere near the limits of RAM access speeds. If it was, then the ROM access speed would be more of a speed limiting factor. This may not be true of the latest and greatest ROMs, but in my experience the fastest RAMs are much faster (even pausing for refresh) than ROM. As an example, when my II+ is running at 3.58 MHz, the ROMs are much slower than the 120 nsec RAMs, and that's why AE uses ROM shadowing on their TransWarp. An 8 MHz 65C816 (which I benchmarked as equivalent to a 16 MHz 68000, for my best code on each processor) might run faster from RAM than from ROM. And 10 MHz 65C802s are easily available (I have one) these days. I believe that the TransWarp GS has to cache the ROM (i.e. copy recent accesses to ROM into RAM) to achieve its full speed. And since everyone keeps comparing the speed of the GS to other 68000 or 80x86 CPUs, I thought I would add this: The 65xxx doesn't need to be clocked as fast to get the same amount of computing done as an 80x86. The 68000 and 8086 eat 4 cycles reading a memory address, and the newer 80x86s eat 3 or 2 cycles (the 80386 takes the fewest, of course). Compare this to 1 cycle for the 65xxx. This means that a slower 65xxx is an awful lot closer to the limits of today's RAM access speeds than a faster uP from "the other guys". Of course, this 4 times factor (between a 68000 and 65816) is somewhat misleading. The plus for the 65xxx is that its instructions take much fewer cycles to complete. The drawback is the 8 bit bus, and that you can't operate on 32 bit data (yet). Moral: don't wait for Apple to come out with a 33 MHz Apple II, it would run circles around a 33 MHz PC (if you could find fast enough RAM :-) Brian Willoughby UUCP: ...!{tikal, sun, uunet, elwood}!microsoft!brianw InterNet: microsoft!brianw@uunet.UU.NET or: microsoft!brianw@Sun.COM Bitnet brianw@microsoft.UUCP Microsoft no longer cares about the Apple II, why should they care what I say about the Apple II?