Groups > comp.sys.apple2.programmer > #1464

Re: Vertical BLanking (VBL) for Apple II

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From	D Finnigan <dog_cow@macgui.com>
Newsgroups	comp.sys.apple2, comp.sys.apple2.programmer
Subject	Re: Vertical BLanking (VBL) for Apple II
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Date	Sat, 3 Jan 2015 07:34:12 +0000 (UTC)
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Cross-posted to 2 groups.

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David Schmenk wrote:
> On Thursday, 1 January 2015 16:04:20 UTC-8, mmphosis  wrote:
>> http://hoop-la.ca/apple2/2015/vbl/
> 
> This is basically the code the MouseCard uses to initialize the 6522 timer
> for VLB interrupts on an Apple ][ and ][+. Don't try enabling the VLB
> interrupts from code residing in the $2000 to $4000 range, like in a
> ProDOS
> system program. Ask me how I know :-)

High-Resolution Graphics page 1 gets cleared by the mouse firmware so your
program gets nuked. I found this out the hard way when writing the
Multitasking Mouse Draw Demo. Fortunately, I had an inkling of reading about
this side effect long ago, so I pulled out the AppleMouse manual, and sure
enough, there was the warning. This memory clear only happens on the
original II and II Plus

-- 
]DF$
Apple II Book: http://macgui.com/newa2guide/
Usenet: http://macgui.com/usenet/  <-- get posts by email!
Apple II Web & Blog hosting: http://a2hq.com/

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Re: Vertical BLanking (VBL) for Apple II D Finnigan <dog_cow@macgui.com> - 2015-01-03 07:34 +0000

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