30b0b12e
From: uhclemLOSE.apr09@nemesis.lonestar.org
On Mar 20, 6:48?am, "Mike Y" wrote:
> But on a Tandy 1000 why would you be accessing the memory via the
> 'shared' area? ?You should always be accessing it 'as if' it were in the
> video area. ?Doing that on a 1000 makes the machine just like a PC-XT
> or whatever.
Jim Leonard wrote:
: Because I want to use two video pages for smoother/faster animation, a
: visible page and a hidden page, and switch between them. Both systems
: are capable of it; I just need to work out how to determine where the
: graphics area is in the Tandy.
Be cautious of doing this, because certain video modes in the
1000 family have a lot of wait states on access to the RAM that is
the considered video RAM, even if not currently shown on screen.
(Wait states in the various video modes is probably not the same
as the Jr, a point no hardware engineer would have thought to
make the same.)
Depending on the mode there can be something like 12 wait states
per read/write in video-accessible memory, both on 8086 and 80286
models. So a read/write instruction that takes five clocks anywhere
else in RAM takes 15 clocks in video RAM, that sort of thing.
Mis-aligned accesses could cost even more time.
We used to have to build the Deskmate MUSIC and SOUND
programs so that critical code would avoid getting loaded
in any of the video-accessible RAM areas, because if it was there
it would not run fast enough to keep the audio streaming.
Depending on what you are doing, it may be faster to prepare your
image in non-video RAM, then block transfer it in full to the idle
page, then switch to that region as the active display page,
rather than to build/update the page in video RAM areas.
Frank Durda IV - send mail to this address and remove the "LOSE":
http://nemesis.lonestar.org
Copyright 2009, ask before reprinting.
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