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From: ephraim@think.com (Ephraim Vishniac)
Newsgroups: comp.sys.mac.programmer
Subject: Re: How's the screen cleaned after _SysError?
Message-ID: <34396@news.Think.COM>
Date: 1 Mar 90 15:10:41 GMT
References: <34252@news.Think.COM> <6959@internal.Apple.COM>
Sender: news@Think.COM
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 34

In article <6959@internal.Apple.COM> lsr@Apple.COM (Larry Rosenstein) writes:
>In article <34252@news.Think.COM> ephraim@Think.COM (Ephraim Vishniac) 
>writes:
>> How is the screen restored after a call to _SysError?

>There's a low memory global called DSAlertRect.  In GetMouse, if the high 
>bit of DSWndUpdate is 1, then the system refreshes the rectangle stored in 
>DSAlertRect.

GetMouse doesn't do this on my Mac II (6.02, Multifinder off).  I
disassembled it and found that the only subroutine call was to a
journaling routine which exits immedidately if journaling is off. 

Also, DSWndUpdate seems to be $FF most of the time (i.e., the high bit
is always set). 

>Since this doesn't seem to be documented, use at your own risk.

Not documented?  Not so!  Please refer to "Low Memory in Alphabetical
Order" dated 12 Apr 85.  (You did keep every scrap of paper from the
Macintosh Software Supplements, right?) DSWndUpdate is at least as
well documented as the famous MrMacHook.  Notice the line that reads:

DSWndUpdate .EQU $15D ;01 GNE not to paintBehind DS AlertRect? [byte]

This suggests that it's GetNextEvent, not GetMouse, and painting will
occur when the bit is cleared, not set.  I haven't actually tried this
out yet - it took me a while to find the right documentation.

--
Ephraim Vishniac    ephraim@think.com   ThinkingCorp@applelink.apple.com
 Thinking Machines Corporation / 245 First Street / Cambridge, MA 02142
        One of the flaws in the anarchic bopper society was
        the ease with which such crazed rumors could spread.