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From: lim@iris.ucdavis.edu (Lloyd Lim)
Newsgroups: comp.sys.mac.programmer
Subject: Re: "TN.248.DAs in Need of Time" in need of proofreader.
Keywords: Sloppy, careless, wrong.
Message-ID: <5281@ucdavis.ucdavis.edu>
Date: 8 Sep 89 11:01:55 GMT
References: <15462@dartvax.Dartmouth.EDU> <34579@apple.Apple.COM>
Sender: uucp@ucdavis.ucdavis.edu
Reply-To: lim@iris.ucdavis.edu (Lloyd Lim)
Organization: U.C. Davis - Department of Electrical Engineering and Computer Science
Lines: 69

In article <34579@apple.Apple.COM> pvh@Apple.COM (Pete Helme) writes:
>
>Unfortunately for me, I used THINK C (not MPW C) to test the statement:
>
> dce->dCtlFlags ^= dNeedTime;
> dce->dCtlFlags |= dNeedTime;
>
>which THINK C compiles to:
>
> BCHG    #5,4(A0)          
> BSET    #5,4(A0)          
>
>BSET #5,4(A0) is OK for what we want since it's the high byte we're changing, 
>but it's not correctly compiled as far as the operators are concerned.  
>THINK C actually included stuff like dce->dCtlFlags |= dNeedTime in 
>their old samples I believe.  That's why I probably left it that way.
>BCHG 5,4(A0) is just plain wrong.  A pox on me and my 4 slice toaster.
>
>
>FYI, THINK C compiled the following lines:
>
> dce->dCtlFlags &= ~(1 << (dNeedTime + 8)); 
> dce->dCtlFlags |= (1 << (dNeedTime + 8));  
>
>to:
>
> BCLR    #0,4(A0)          
> BSET    #0,4(A0)          
>
>Huh?

If you look through the THINK C header files, you'll see what's going on.
Inside Mac says dNeedTime is 5 while THINK C has #define dNeedTime 0x2000.
Earle's code is correct for the Inside Mac values.

The correct code for THINK C is:

dce->dCtlFlags &= ~dNeedTime;
dce->dCtlFlags |= dNeedTime;

The output of the THINK C compiler (the resulting bit instructions) can be
explained by something I heard on the net sometime somewhere (probably in a
dream.  I remember the statement that the THINK C compiler optimizes bit
operations with single bit masks to bit instructions.  Is this true, Rich?

BTW, I was stuck for an hour today with code that essentially looked like
this once you evaluated all the #defines and stuff:

switch (x) {
0:
  /* */
  break;
1:
  /* */
  break;
default:
  Debugger();
  break;
}

For the life of me, I couldn't see what was wrong.  Then I noticed the missing
cases.  Arrrggghhh!  Sometimes I wonder why these things compile.  TMON wasn't
too enlightening since the code for the fake cases was in there and it looked
okay.

+++
Lloyd Lim     Internet: lim@iris.ucdavis.edu (128.120.57.20)
              Compuserve: 72647,660
              US Mail: 146 Lysle Leach Hall, U.C. Davis, Davis, CA 95616