Atmel CAVR-4 Manual De Usuario

CAVR-4

Part 2. Compiler reference

asm, _ _asm

The 

asm

 and 

_ _asm

 extended keywords both insert an assembler instruction. However, 

when compiling C source code, the 

asm

 keyword is not available when the option 

--strict_ansi

 is used. The 

_ _asm

 keyword is always available.

Note: Not all assembler directives or operators can be inserted using this keyword.

asm ("string");

The string can be a valid assembler instruction or an assembler directive, but not a 
comment. You can write several consecutive inline assembler instructions, for example:

asm ("Label:

nop\n"

     "

jmp Label");

where 

\n

 (new line) separates each new assembler instruction. Note that you can define 

and use local labels in inline assembler instructions.

For more information about inline assembler, see Mixing C and assembler, page 93.

_ _eeprom

Controls the storage of data objects in eeprom memory space.

The 

_ _eeprom

 memory attribute allows you to place initialized and non-initialized 

variables in the built-in EEPROM of the AVR microcontroller. These variables can be 
used like any other variable and provide a convenient way to access the built-in 
EEPROM.

You can also override the supplied support functions to make the 

_ _eeprom

 memory 

attribute access an EEPROM or flash memory that is placed externally but not on the 
normal data bus, for example on an I2C bus.

To do this, you must write a new EEPROM library routines file and include it in your 
project. The new file must use the same interface as the 

eeprom.s90

 file in the 

avr\src\lib\

 directory (visible register use, the same entry points and the same 

semantics).

Note: Variables declared 

_ _eeprom

 are initialized only when a downloadable linker 

output file is downloaded to the system, and not every time the system startup code is 
executed.

1) --eeprom_size ≤ 256 bytes
2) --eeprom_size ≥ 512 bytes 

0-0xFF

  (255 bytes)

255 bytes

8 bits

Eeprom

 1)

0-0xFFFF

 (64 kbytes)

65535 bytes

16 bits

Eeprom

 2)