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RealView Linker and Utilities Guide
Preface Introduction The Linker Command Syntax Using the Basic Linker Functionality Specifying the image structure
Building blocks for objects and images Load view and execution view of an image Specifying an image memory map Image entry points Section placement Ordering input sections by attribute Using FIRST and LAST to place sections Aligning sections Ordering execution regions containing Thumb code Optimizations and modifications Common debug section elimination Common group or section elimination Unused section elimination Unused function elimination Linker feedback RW data compression PI to absolute veneers Veneer generation Reuse of veneers with overlay execution regions Branch inlining Using command-line options to create simple images Type 1, one load region and contiguous execution r Type 2, one load region and non contiguous executi Type 3, two load regions and non contiguous execut Using command-line options to handle C++ exception Getting information about images Using image-related information Accessing Image Symbols Using Scatter-loading Description Files Using the ARM Librarian Using fromelfImage entry points
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| Home > Using the Basic Linker Functionality > Specifying the image structure > Image entry points | |||
An entry point in an image is a location where program execution can start. There are two distinct types of entry point:
- Initial entry point
The initial entry point for an image is a single value that is stored in the ELF header file. For programs loaded into RAM by an operating system or boot loader, the loader starts the image execution by transferring control to the initial entry point in the image.
An image can have only one initial entry point. The initial entry point can be, but is not required to be, one of the entry points set by the
ENTRYdirective.- Entry points set by the ENTRY directive
These are entry points that are set in the assembly language sources with the
ENTRYdirective. In embedded systems, this directive is typically used to mark code that is entered through the processor exception vectors, such as RESET, IRQ, and FIQ.You can specify multiple entry points in an image with the
ENTRYdirective. The directive marks the output code section with anENTRYkeyword that instructs the linker not to remove the section when it performs unused section elimination.For C and C++ programs, the
__main()function in the C library is also an entry point.See the Assembler Guide for more information on the
ENTRYdirective.If an embedded image is to be used by a loader, it must have a single initial entry point specified in the header. See Specifying an initial entry point for more information.
You can specify an initial entry point with the ‑‑entry linker option. You can specify the ‑‑entry option only once. See the description in armlink command syntax for more information.
For embedded applications with ROM at zero use ‑‑entry 0x0, or optionally 0xFFFF0000 for processors that have high vectors.
The initial entry point must meet the following conditions:
the image entry point must always lie within an execution region
the execution region must be non‑overlay, and must be a root execution region (the load address is the same as the execution address).
If you do not use the ‑‑entry option to specify the initial entry point then:
if the input objects contain only one entry point set by the
ENTRYdirective, the linker uses that entry point as the initial entry point for the imagethe linker generates an image that does not contain an initial entry point when either:
more than one entry point has been specified by using the
ENTRYdirectiveno entry point has been specified by using the
ENTRYdirective.
In both these situations, the linker issues a warning.
Specify a unique entry point with
‑‑entryto fix this warning.
| Copyright © 2007 ARM Limited. All rights reserved. | ARM DUI 0377A |
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