avr-libc-1.7.1/doc/api/sfr.dox

/* Copyright (c) 2002, Joerg Wunsch
   Copyright (c) 2008, Atmel Corporation
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/* $Id: sfr.dox 1631 2008-03-17 17:59:10Z arcanum $ */

/** \defgroup avr_sfr <avr/sfr_defs.h>: Special function registers

When working with microcontrollers, many tasks usually consist
of controlling internal peripherals, or external peripherals that are connected 
to the device. The entire IO address space is made available as 
<em>memory-mapped IO</em>, i.e. it can be accessed using all the MCU 
instructions that are applicable to normal data memory.  For most AVR devices, 
the IO register space is mapped into the data memory address space with an 
offset of 0x20 since the bottom of this space is reserved for direct access to 
the MCU registers.  (Actual SRAM is available only behind the IO register
area, starting at some specific address depending on the device.)

For example the user can access memory-mapped IO registers as if they were 
globally defined variables like this:

\code
	PORTA = 0x33;
    unsigned char foo = PINA;
\endcode

The compiler will choose the correct instruction sequence to generate based
on the address of the register being accessed.

The advantage of using the memory-mapped registers in C programs is
that it makes the programs more portable to other C compilers for the
AVR platform.

Note that special care must be taken when accessing some of the 16-bit
timer IO registers where access from both the main program and within
an interrupt context can happen.  See \ref faq_16bitio.

\par Porting programs that use the deprecated sbi/cbi macros

Access to the AVR single bit set and clear instructions
are provided via the standard C bit manipulation commands. The sbi and cbi
macros are no longer directly supported. 
sbi (sfr,bit) can be replaced by  sfr |= _BV(bit) .

i.e.: sbi(PORTB, PB1);  is now PORTB |= _BV(PB1);

This actually is more flexible than having sbi directly, as the optimizer 
will use a hardware sbi if appropriate, or a read/or/write operation if not 
appropriate. You do not need to keep track of which registers sbi/cbi will 
operate on.

Likewise, cbi (sfr,bit) is now   sfr &= ~(_BV(bit));

*/