Atmel CAVR-4 ユーザーズマニュアル

CAVR-4

Part 1. Using the compiler

This section presents some general techniques used in the system startup code, including 
background information that might be useful if you need to modify it.

Note: Do not modify the file 

cstartup.s90

 unless required by your application. Your 

first option should be to use a customized version of 

_ _low_level_init

 for 

initialization code. 

The source files are well commented and are not described in detail in this guide.

For information about assembler source files, see the AVR® IAR Assembler Reference 
Guide.

To understand how the startup code is designed, you must have a clear understanding of 
modules and segment parts, and how the IAR XLINK Linker treats them.

An assembler module starts with a 

MODULE

 directive and ends with an 

ENDMOD

 directive. 

Each module is logically divided into segment parts, which are the smallest linkable 
units. There will be segment parts for constants, code bytes, and for reserved space for 
data. Each segment part begins with an 

RSEG

 directive. 

When XLINK builds an application, it starts with a small number of modules that have 
either been declared using the 

_ _root

 keyword or have the program entry label 

_ _program_start

. The linker then continues to include all modules that are referred 

from the already included modules. XLINK then discards unused segment parts.

The system startup code has been designed to use segment parts so that as little as 
possible of unused code will be included in the linked application.

A piece of code or data is not included if it is not used or referred to. To make the linker 
always include a piece of code or data, the assembler directive 

REQUIRE

 can be used.

The segment parts defined in the system startup code are guaranteed to be placed 
immediately after each other. There are two reasons for this. First, the alignment 
requirement of the segment parts is every two bytes. Because the size of all assembler 
instructions are multiples of two, this does not allow pad bytes to be placed between 
code sections. Second, XLINK will not change the order of the segment parts or 
modules, because the segments holding the system startup code are placed using the 

-Z

 

option.