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From: toddpw@tybalt.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.sys.apple2
Subject: Re: Mega II (was: Accepting the Mac)
Message-ID: <1990Mar19.121731.19584@spectre.ccsf.caltech.edu>
Date: 19 Mar 90 12:17:31 GMT
References: <1848@crash.cts.com> <18491@boulder.Colorado.EDU> <90077.155936JLS139@psuvm.psu.edu>
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Organization: California Institute of Technology
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JLS139@psuvm.psu.edu (Abaddon) writes:

>In article <1990Mar18.095408.1765@spectre.ccsf.caltech.edu>,
>toddpw@tybalt.caltech.edu (Todd P. Whitesel) says:
>>
>>I never said scrap the II compatibility, I said scrap the Mega II. The Mega II
>>has to run at 1 Mhz and in the GS that's a big problem. If they had
>>implemented
>>its functions in a more distributed manner then the only part of the machine
>>that would run slow would be the actual expansion slots. When the GS was

>    Correct me if I am wrong, but under DOS 3.3 wasn't disk access based
>in software (RAM) and wasn't writing based on critical timing loops.
>Seems to me that RAM would have to be running at 1Mhz for disk access(writes).
>Or am I wrong? How do the accelerator boards for older ]['s work in this
>instince?

The RAM doesn't have to run at 1 Mhz, in fact it doesn't -- it runs at 2 mhz
so the video (or refresh in a GS) can do its thing while the CPU is not using
the RAM. If the RAM is run faster but is made available to the CPU when it asks
for it, then the CPU runs at 1 mhz and the disk access still works; the RAM
could be doing other stuff without the CPU caring or the disk messing up. Apple
has yet to do something more useful with this idea, but my proposed //f would
find plenty of uses for it (background DMA to sound and video during disk
access with no penalties -- yeah!)

All accelerators (and the GS) have to know when a disk access is occuring.
They figure out when the disk motor has been turned on, and slow down to 1 Mhz
until it is turned off. The GS actually detects the exact locations that turn
the drive motor on and off; most accelerators slow down for 50 microseconds
after any one of sixteen locations are accessed -- for disk drives and many
other peripherals that might need to run at 1 mhz this stays in effect because
the card is re-accessed and the 50 microsecond timer starts again. (AE's
transwarp for the ][+ & //e does this, if memory serves me, and one card in
particular -- forget which -- didn't work with it because it had to run at
1 mhz but it didn't reaccess its card and restart the 50 microsecond timer
fast enough... I remember reading about it in the Nibble letter column some
years ago.)

Todd Whitesel
toddpw @ tybalt.caltech.edu