[avr-libc-dev] Re: [avr-gcc-list] Re: What Happened to the sbi() and cbi() Macros????

[E. Weddington]

Kevin Neff wrote:

> On Sun, 30 Jan 2005, E. Weddington wrote:
>  
>
> I have trouble remembering things that I don't use regularly, so I don't
> find this argument particularly convincing.
>
> I am against intentionally making tools hard to use for beginners but I
> think that the "improvements" should have technical merit beyond making
> things easier for newbies.
>
>  
>
> > - The typecasts are in there to workaround the C language Standard of 
> > promoting the operands of bit operaters to ints, which are 16 bits for 
> > the AVR (see the avr-libc FAQ for more discussion of this). If these 
> > macros are used, then users don't have to remember to do the typecasts, 
> > which should, in theory, help with the size of their code.
> >    
>
> A good point... but isn't there a way to do this during the build process?
>
>  
Not that I know of. The problem stems from the fact that the C Language 
Standard specifies that the operands of bit operations are promoted to 
an int. GCC is usually very good about adhering to standards. So the 
extra typecasts are in there to help the compiler understand that this 
operation is done on 8-bit values. This *should* help it to optimize the 
assembly better. Admittedly, I can understand where some people would 
look at this and kind of "glaze over":

PORTA &= (uint8_t)~bit(0);

But this is eaiser to see at a glance:

bit_clear_8(PORTA, bit(0));


> > I would also like to propose two other macros for this file:
> >
> > #define BIT(x)   (1 << (x))
> > #define LONGBIT(x)   ((uint32_t)1 << (x))
> >
> > Yes, the BIT() macro is the same as _BV(). The _BV() macro is confusing 
> > in two ways: one has to remember that "BV" stands for Bit Value. The 
> > name BIT is slightly more descriptive. And _BV() has a leading 
> > underscore, which technically is supposed to be reserved for the 
> > "implementation" according to the C Standard (in this case the library). 
> > Having to type out the underscore is annoying. I feel that it's easier 
> > to just write out "BIT".
> >    
>
> As an alternative to this suggestion, I would advocate adding some simple
> bit value macros.  They are fewer characters to type and do not require
> special treatement of larger words. 
>
>  #define B0 0x01
>  #defind B1 0x02
>   ...
>
>  
I slightly disagree.

The individual IO header files (and through the <avr/portpins.h> header 
which is automatically included) already define *bit numbers* for all 
the registers in the device. I would rather not have to define *bit 
masks* too. You should always be able to easily construct an expression 
such as:

bit_set_8(PORTB, bit(PB0) | bit(PB6));

------------

When it comes down to it, if you want to operate on individual bits, 
you're going to have to learn the C language bit operators to a certain 
extent. That's what they're there for.

Eric