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Re: [avr-gcc-list] efficiency of assigning bits

From: David Brown
Subject: Re: [avr-gcc-list] efficiency of assigning bits
Date: Mon, 14 Mar 2005 13:07:36 +0100

> >
> > Hello,
> > To turn two bits off, the method I would adopt is
> >
> >  PORTD &= 0B11101101;
> >
> > This is the fastest method.
> What does that work out to in assembly?  A cbi/sbi assembly instruction is
2 clock cycles.  I still don't understand why these instructions would be
taken out of winavr, they obviously are useful for them to have been
included as opcodes in the AVR! :)
> Is it possible to do inline assembly with cbi/sbi?
> Something like this:
> asm("sbi,x,y)"
> --gives an error though
> cheers,
> Jamie

It's tempting to reply with RTFM, but I'll elaborate a little (although you
should still read the documentation and FAQ).

Nobody has taken "instructions out of winavr".  A few macros were removed
from the standard headers - these macros were non-standard, confusingly
named, useful only in the distant past when avr-gcc had limited
optomisation, and had been marked as pending removable for a long time
(several years, IIRC).  Instead of incorrectly pretending that the C
compiler is an assembler, or that it needs assembler to access hardware, use
standard C bit manipulation operators:

PORTD &= ~PD1; PORTD &= ~PD4;
PORTD &= ~(PD1 | PD4);

The first method will be faster, since it compiles to two cbi instructions,
but it does not turn the bits off simultaneously.  The second method gives
you simultaneous switch-off, but takes slightly longer.

If you want to know what assembly is generated, use the gcc flags to
generate a listing file  (such as -Wa,-ahlsd=listing.txt ).  If you want to
use in-line assembly (which is very seldom necessary, and certainly not
appropriate here), there is plenty of documentation in the avr-libc manual.


> >
> > The cbi and sbi instructions are removed in the latest
> > version of winavr and can not be used. Moreover the
> > two instructions take more time and more space
> > compared to the above example.
> >
> > Nayani
> >
> >
> >
> >
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> >
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