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- Path: sparky!uunet!elroy.jpl.nasa.gov!ames!data.nas.nasa.gov!taligent!apple!dlyons
- From: dlyons@Apple.COM (David A Lyons)
- Newsgroups: comp.sys.apple2
- Subject: Re: When is the tick counter incremented?
- Message-ID: <71718@apple.Apple.COM>
- Date: 27 Aug 92 12:58:30 GMT
- References: <1992Aug26.185150.22795@julian.uwo.ca>
- Organization: Apple Computer Inc., Cupertino, CA
- Lines: 37
-
- In article <1992Aug26.185150.22795@julian.uwo.ca> hackett@gaul.csd.uwo.ca (Mike Hackett) writes:
- >[...] after
- >reading TN #39 several times, I'm not entirely sure exactly when the tick
- >counter is incremented.
-
- From page 14-22 of Toolbox Reference 1, "GetTick":
-
- Returns the current value of the tick counter. The value will be
- incremented only if the Heartbeat Interrupt handler is installed
- (always true if the Event Manager is active) and VBL interrupts are
- enabled. [...]
-
- From this you can (correctly) conclude that the same routine that dispatches
- control to Heartbeat tasks increments the tick count.
-
- >The note seems to suggest that the vertical
- >blanking interrupt occurs at line 192. (Actually, it often refers to the
- >vertical blanking actually occurring at 192, but this is not true.) [...]
-
- You have the July 1989 versions of GS TN #39 and #40, right? Near the end,
- 39 says outright, "Note that the VBL interrupts always trigger at scan line
- 192, even in the Super Hi-Res display mode."
-
- Once you forgive both notes for not distinguishing between *actual* vertical
- blanking and the point where the VBL interrupt triggers & the $C019 switch
- twiddles, I believe they are accurate.
-
- (I thought so in April 1989, too, when I pulled my first all-nighter in DTS
- playing with various little mini-assembler programs to do strange things to
- the border color based on $C02E and $C019 to answer a developer's question
- and then revise the notes.)
- --
- David A. Lyons, Apple Computer, Inc. | DAL Systems
- Apple II System Software Engineer | P.O. Box 875
- Internet:dlyons@apple.com | Cupertino, CA 95015-0875
-
- My opinions are my own, not Apple's.
-