Atmel CAVR-4 ユーザーズマニュアル

CAVR-4

Part 1. Using the compiler

In ISO/ANSI C all static variables—variables that are allocated at a fixed memory 
address—have to be initialized by the run-time system to a known value. This value is 
either an explicit value assigned to the variable, or if no value is given, it is cleared to 
zero.

In the 

AVR IAR C/C++ Compiler, there are two exceptions to this rule and they both 

use keywords for ISO/ANSI C language extensions. Variables declared 

_ _no_init

 

are not initialized at all. Variables declared 

_ _eeprom

 are allocated in the EEPROM 

memory and because the EEPROM memory can typically be used for storing 
configuration data—data that need to live through a reset—these variables will not be 
initialized by the runtime system. Instead, these initializers are stored in a separate 
segment, which can be loaded to the EEPROM as part of the program download.

When an application is started, the system startup code initializes static and global 
variables in three steps:

1 It clears the memory of the variables that should be initialized to zero; these 

variables are located in segments with the suffix 

_Z

.

2 It initializes the non-zero variables by copying a block of ROM to the location of the 

variables in RAM. This means that the data in the ROM segment with the suffix 

ID

 

is copied to the corresponding 

I

 segment.

This works when both segments are placed in continuous memory. However, if one 
of the segments is divided into smaller pieces, it is important that:

●

The other segment is divided in exactly the same way

●

It is legal to read and write the memory that represents the gaps in the sequence.

For example, if the segments are assigned the following ranges, the copy will fail:

However, in the following example, the linker will place the content of the segments 
in identical order, which means that the copy will work appropriately:

Note that the gap between the ranges will also be copied. Note also that the 

NEAR_ID

 

segment holding the initializers is always located in flash memory and that these 
initializers are only used once, that is before reaching the 

main

 function.

NEAR_I

0x1000-0x10FF

 and 

0x1200-0x12FF

NEAR_ID

0x4000-0x41FF

NEAR_I

0x1000-0x10FF

 and 

0x1200-0x12FF

NEAR_ID

0x4000-0x40FF

 and 

0x4200-0x42FF