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Re: [avr-libc-dev] Bit number/mask, Part 3000
From: |
Joerg Wunsch |
Subject: |
Re: [avr-libc-dev] Bit number/mask, Part 3000 |
Date: |
Mon, 10 Apr 2006 07:05:49 +0200 |
User-agent: |
Mutt/1.5.11 |
As Vic Wagner wrote:
> It's getting tiresome to keep reminding people that C (and C++, of
> course) has always had a mechanism for dealing with single bits
> (multiple bit fields, too) called, _big surprise_ bit fields go look
> it up on your favorite elementary book on your fave of the two
> languages.
It's tiresome to remind people that bitfields in C are a completely
useless thing when it comes to hardware abstractions, as about
everything about them is implementation-defined so you cannot predict
their behaviour for different compilers.
I've heard that Ada did a better job here.
Also, with bit-fields, it's getting even harder to set unrelated bits
in a single byte at the same time. As the object they are referring
to is `volatile', the compiler would not be allowed to optimize two
adjacent settings into a single one:
ADCSRA.ADEN = 1;
ADCSRA.ADSC = 1;
while without bitfields, you can have both forms:
ADCSRA = _BV(ADEN) | _BV(ADSC);
or
ADCSRA = _BV(ADEN);
ADCSRA |= _BV(ADSC);
--
cheers, J"org .-.-. --... ...-- -.. . DL8DTL
http://www.sax.de/~joerg/ NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)