On Fri, 2 May 2014, Dwayne Reid wrote:

> At 07:36 PM 5/1/2014, John J. McDonough wrote:
>
>> I'm kind of surprised nobody compared the generated code.  The code
>> produced by XC16 is identical:
>>
>>     if ( myBits & 0x0002 )
>>         myTestWord =3D 87;
>>
>>     if (myBitfield.b1)
>>         myTestWord =3D 83;
>>
>> I would argue that the mask code is only clearer if 1) You are doing
>> some bogus example like this or 2) You are an assembler programmer
>>
>> With the bitfield you can give the bits names, so for example you could
>> say something like:
>>
>>   if ( LCDstatus.LCDisBusy )
>>
>>   rather than
>>
>>   if ( LCDstatus & 0b0000000000100000 )
>
> This has been an informative thread for me.  I'm a diehard assembler
> programmer and my brain simply refuses to grok "C".  But I keep
> poking at it, hoping that the AH HA moment will occur.
>
> One of the many things I learned from PIC assembler guru Andrew
> Warren was to define all of my bit fields with names that are used
> later in my program.  Your example above would be done as:
>
>   #define LCDisBusy LCDstatus,1
> where LCDstatus has previously been defined in exactly the same
> way.  In use, I simply reference the defined name:
>   btfsc  LCDisBusy
>     do something
>
> One of the things that really bugs me about C programs is that there
> didn't appear to be any easy way to do this simple function.  This
> thread has shown me that there is something relatively close.
>
> Many thanks!

Something else you might consider is using the absolute address of the=20
bit. In your example this would become:

const int LCDisBusy =3D (&LCDstatus << 3) | 1;

if ((*(char *)(LCDisBusy >> 3) & (1 << (LCDisBusy & 7))) !=3D 0)
{
   do something
}

A good C compiler should compile this to:

	btfsc LCDstatus,1
	goto lab1
	do something
lab1

Although the high level benefits are not immediately obvious consider what=
=20
happens when the constant is used across functions calls :-)

I use the above "absolute bit addressing" in the XCSB libraries and the=20
compiler is able to track the use of constants and optimise to "btfsc" and
"bsf" etc., even through function calls.

E.g.

const LED1 =3D (&PORTA << 3) | 1
const LED2 =3D (&PORTB << 3) | 2
const RELAY =3D (&PORTC << 3) | 6

proc TURNON(int port)
	*(char *)(port >> 3) |=3D 1 << (port & 7)
end proc

proc main()

	TURNON(LED1)
	TURNON(LED2)
	TURNON(RELAY)

end proc

This compiles to:

	BSF	PORTA,1
	BSF	PORTB,2
	BSF	PORTC,6

BTW note the lack of semicolons in the example XCSB code.

Regards
Sergio Masci
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.