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From: sokoloff@athena.mit.edu (James T Sokoloff)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Direct-to-screen drawing
Message-ID: <1991Apr29.214814.12004@athena.mit.edu>
Date: 29 Apr 91 21:48:14 GMT
References: <91113.105146CXT105@psuvm.psu.edu> <1991Apr24.200807.8602@santra.uucp>
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> Actually, Inside Mac V says that bit 15 is used and that bits 13 and 14
> should always be 0. Develop I, page 9 says that 32 bit QD allows rowBytes
> to be up to $3FFE, so they have obviously made some changes.

Only stripping the most significant bit is dangerous.  It is *most correct*
to strip the two most significant bits.  (Obviously, if bit 14 is zero,
this yields the same result.  However, who is to say that bit 14 will be
forever zero.  Apple has told us that these two bits are not to be included
in calculations with rowbytes, let's just believe them and follow their
code examples...)

Jim Sokoloff
sokoloff@athena.mit.edu