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From: imp@crayview.msi.umn.edu (Chuck Lukaszewski)
Newsgroups: comp.sys.mac.programmer
Subject: VBLS are OK above BufPtr
Keywords: VBL BufPtr
Message-ID: <10606@umn-cs.CS.UMN.EDU>
Date: 31 Dec 88 02:05:52 GMT
Sender: news@umn-cs.CS.UMN.EDU
Lines: 16

The .Sony and .Sound drivers, which are located in ROM (i.e. above $400000)
both install VBLs during the course of their operation.  I have looked at
the code for both and neither relocates the VBL task.  Rather, the actual
address of the routine is put into the Vertical Retrace Queue element.

I am loathe to believe that the VBL Task Handler has a special test that
permits VBLs below BufPtr and above ROMBase.  Not only does it seem pointless
to create a VBL desert anywhere in memory, but also the instruction waste
would be ludicrous, even for Apple :-).  So it seems safe to say that
VBLs are safe above BufPtr.  Anyone from Apple care to comment?

It also occurs to me that I have seen _VInstall and _VRemove in the .MPP or
.ATP driver, which are also in ROM.

______________________________________________________________________________
Chuck Lukaszewski          imp@crayview.msi.umn.edu               612 789 0931