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On-Line Manuals
Linker User Guide
Preface
Overview of the Linker
Linking Models Supported by armlink
Image Structure and Generation
The structure of an ARM ELF image
Views of the image at each link stage
Input sections, output sections, regions, and prog
Load view and execution view of an image
Methods of specifying an image memory map with the
Image entry points
Simple images
Types of simple image
Type 1 image structure, one load region and contig
Type 2 image structure, one load region and non-co
Type 3 image structure, multiple load regions and
Section placement with the linker
Default section placement
Section placement with the FIRST and LAST attribut
Section alignment with the linker
Linker support for creating demand-paged files
Linker reordering of execution regions containing
Linker-generated veneers
What is a veneer?
Veneer sharing
Veneer types
Generation of position independent to absolute ven
Reuse of veneers when scatter-loading
Command-line options used to control the generatio
Weak references and definitions
How the linker performs library searching, selecti
How the linker searches for the ARM standard libra
Specifying user libraries when linking
How the linker resolves references
The strict family of linker options
Linker Optimization Features
Getting Image Details
Accessing and Managing Symbols with armlink
Scatter-loading Features
Scatter File Syntax
Linker Command-line Options
Linker Steering File Command Reference
Via File Syntax
Image entry points
| Non-Confidential |  PDF version | ARM DUI0377H | ||
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| Home > Image Structure and Generation > The structure of an ARM ELF image > Image entry points | ||||
3.1.5 Image entry points
An entry point in an image is the location that is loaded into the PC. It is the location where program execution starts. Although there can be more than one entry point in an image, you can specify only one when linking.
Not every ELF file has to have an entry point. Multiple entry points in a single ELF file
are not permitted.
Note
For embedded Cortex-M programs, the program starts at whatever value is loaded into the PC from the Reset vector. Typically, the Reset vector points to the CMSISReset_Handler function.Types of entry point
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
ENTRYdirective - You can select one of many possible entry points for an image. An image can have only one entry point.You create entry points in objects with the
ENTRYdirective in an assembler file. In embedded systems, typical use of this directive is to mark code that is entered through the processor exception vectors, such as RESET, IRQ, and FIQ.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.If an embedded image is to be used by a loader, it must have a single initial entry point specified in the header. Use the--entrycommand-line option to select the entry point.
The initial entry point for an image
There can be only one initial entry point for an image, otherwise linker warning L6305W is output.
The initial entry point must meet the following conditions:
- The image entry point must always lie within an execution region.
- The execution region must not overlay another execution region, and must be a root execution region. That is, where 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 image. - The 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
ENTRYdirective. - No entry point has been specified by using the
ENTRYdirective.
For embedded applications with ROM at zero use
--entry 0x0, or optionally
0xFFFF0000Related concepts
Related reference
| Non-Confidential | PDF version | ARM DUI0377H |
| Copyright © 2007, 2008, 2011, 2012, 2014-2016 ARM. All rights reserved. |
| Home > Image Structure and Generation > The structure of an ARM ELF image > Image entry points |
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