Slow GS video thanks to: Apple sucks

[nathan@visi.com (Nathan Mates)]

In article <5upqgv$ltr$1@client3.news.psi.net>,
Steve Mentzer <!smentzer@pacbell.net> wrote:
>The IIgs experiences a cool feature that also acts as a huge
>achilles heel.  This feature is backwards compatibility. The IIgs
>bus speed runs at ~1Mhz and unfortunately, this is totally
>inadequate considering that video memory and the like run at that
>speed, regardless of how fast your accelerator card is.

   Bzzzzt! Thank you for playing, but you lost. The GS's super hires
screen is writable only at 1Mhz, true, but most of the rest of ram
(non-I/O space, also all of banks E0/E1) is accessible at the system
speed (1Mhz or 2.5Mhz depending on what it's set to). Since the SHR
screen is in bank E1, it's accessible at the slow speed. Blame Apple
for cheapskating on the RAM and using part of slow ram for that screen
rather than doing it properly and used fast ram or a dedicated bank of
ram.

   The SHR 'shadow' screen in bank 01 can be read at the full 2.5Mhz,
but writes to it drop the system speed to 1Mhz while it updates the
memory location in both banks. Even so, there is still some serious
hardware voodoo going on-- PEI slamthrus of the screen from 01 to E1
work better with a NOP every 13 PEIs simply to get the dualspeed bus
time to recover better.

   Had Apple's engineers had the time (and Management backing, which
was the most lacking and therefore most at fault), they could have
isolated the 1Mhz parts of the bus (i.e. mostly the slots) behind an
interface unit, and ran the GS-specific features properly at 2.5Mhz.
And actually, for another management-killed feature of the GS, the
engineers originally wanted 10Mhz 65816s under the hood. Making that
the default with a 10Mhz bus to video would have made the original GS
actually useful. However, those who'd rather whitewash and suck up to
Macintosh, Inc would rather just deny that and all other backstabbings
of the Apple II line.

   The annoyances of this design took Apple's engineers several years
to figure out how to get some decent performance out of the hardware
they designed; GS/OS 5.x and up finally managed to use the shadow SHR
screen effectively and speed up Quickdraw drawing in general.

Nathan Mates
--
<*> Nathan Mates http://www.visi.com/~nathan/      <*>
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