3.11 __io Keyword

The IAR __io keyword specifies that the object aliases memory mapped SFRs and that access will be performed on the AVR's IO registers. This keyword is used for accessing SFRs at addresses below 0x100.

Suggested Replacement

There are several MPLAB XC8 attributes that perform similar tasks to this keyword, but there are some differences in their usage and effect.

Use one of __attribute__((io_low(n))), __attribute__((io(n))) or


                    __attribute__((address((n))))

, depending on the desired address of the SFR, where n is the mapped address in data memory. If the SFR is in the bit-addressable IO range (IO memory address 0x0-0x1F), then use the io_low attribute. If the SFR is in the IN/OUT range (IO memory address 0x20-0x3F), then use the io attribute; otherwise, use the address attribute.

The compiler will use the in and/or out instructions whenever possible to access the symbol.

Caveats

An object specified with this IAR keyword is assigned memory, like with any ordinary definition; however, this keyword ensures that this memory is in the I/O memory space. By comparison, MPLAB XC8 does not assign memory to the symbol that uses these attributes, assuming that the symbol represents a peripheral register. The symbol defined with this attribute is merely equated with an address.

A warning will be issued if the declaration for a symbol using either the io or io_low attributes is not volatile.

Examples

Consider migrating IAR code such as:

__io int userMode;

to MPLAB XC8 codes similar to:

// declare userMode as being at the specified address in upper I/O memory
volatile int userMode __attribute__((io(0x42)));

// or declare userMode as being in the lower area in I/O memory
volatile int userMode __attribute__((io_low(0x28)));

// or declare userMode as being at the specified address in I/O memory
volatile int userMode __attribute__((address(0x50)));

Further Information

See the Attributes section in the MPLAB XC8 C Compiler User's Guide for AVR MCUs for more information on these attributes.