Re: Virtual Memory on a //GS???????????????????

[sknkwrks@peewee.cs.unlv.edu (Scott Alfter)]

Waiter...reality check, please? :-)

In article <Pine.3.07.9401261643.A20028-a100000@cap.gwu.edu> dpfarrer@CAP.GWU.EDU (Daniel Pfarrer) writes:
>Well, I doubr you would notice the difference if you had a ZIP 14 or 16
>Mhz & a RamFast/1MB. That would increase the performance on a //GS. & With
>virtual memory, you could expand up to 32MB of RAM or so & hardly notice
>the difference.

Umm...how do you propose to access 32 megs of memory (virtual or
otherwise) on a CPU that has only a 24-bit address bus?  (2^24=16
megs.)  Don't tell me you'll do bank switching...bank switching is a
kludge that was devised for the tiny (16-bit) address spaces of 8-bit
CPUs.  Also, why would you want a RamFAST for a virtual-memory
application?  You'd be better off getting that 1 meg of cache off of
the 1-MHz expansion bus and into the CPU-speed expansion memory.  (The
RamFAST is great for file access, but I wouldn't want to use one for
VM.)  Finally, how are your apps going to use VM when you have no MMU?
You'd want, at the very least, to implement demand paging...that takes
hardware found in 80[34]86, 680[34]0, and other midrange and high-end
CPUs, and in very few other places.  Paging hardware also requires
system software that can take advantage of the capability...no IIGS
system software ever released by Apple, to my knowledge, can do this.
Unless you feel like (1) reverse-engineering a Sys601 clone that
supports VM or (2) ditching Sys601 and cobbling something that
supports VM but is incompatible with all existing apps, I don't see
what point there would be in adding VM to the GS.

Besides, who needs 32 megs on a GS anyway?  This isn't the Mac and it
definitely isn't Windoze--the GS doesn't have any apps that need
anywhere near 32 megs just to run.  (That memory hog known as
HyperCard GS, for instance, requires only 2 megs to run.)  The only
possible need for such a large amount of memory would be as data
storage--and how often do you work with 32-meg data files?  The ProDOS
filesystem won't even let you create files larger than 16 megs!  I
have 4.25 megs of RAM in my system, which is sufficient for most of my
needs.  AppleWorks has never run out of memory on this system.  I
digitize songs fairly often, usually at 22.05 kHz; if I need to sample
something that won't fit in memory at that rate (which is rare), I
just shift to a slightly lower rate (such as 17-18 kHz).  If I had 8
megs, I don't think I'd ever run out of memory at all.

Why bother with tilting at windmills, trying to obtain some capability
that you'll probably never need?

  _/_   Scott Alfter (sknkwrks@cs.unlv.edu)       Ask me about SoftDAC--digital
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