ARM VERSION 1.2 Guide de l'utilisateur télécharger pdf (Page 80)

See the code in the Examples\embedded\cache directory for examples of cache initialization code.

Initializing the stack pointers

The initialization code initializes the stack pointer registers. You might have to initialize some or all of the following

stack pointers, depending on the interrupts and exceptions you use:

sp_SVC

This must always be initialized.

sp_IRQ

This must be initialized if IRQ interrupts are used. It must be initialized before interrupts are

enabled.

sp_FIQ

This must be initialized if FIQ interrupts are used. It must be initialized before interrupts are

enabled.

sp_ABT

This must be initialized for Data and Prefetch Abort handling.

sp_UND

This must be initialized for Undefined Instruction handling.

Generally, sp_ABT and sp_UND are not used in a simple embedded system. However, you might want to initialize

them for debugging purposes.

You can set up the stack pointer sp_USR when you change to User mode to start executing the application.

Caution

If you are using scatter loading, you must retarget the function __user_initial_stackheap() to place the stack

and heap. If you do not, there might be link errors because the default implementation provided by the C library

attempts to use Image$$ZI$$Limit that is not defined when scatter loading is used. See Loading the ROM image

at address 0 for more information on retargeting this function.

Initializing any critical I/O devices

Critical I/O devices are any devices that you must initialize before you enable interrupts. Typically, you must initialize

these devices at this point. If you do not, they might cause spurious interrupts when interrupts are enabled.

Changing processor mode

At this stage the processor is still in Supervisor mode. If your application runs in User mode, change to User mode

and initialize the User mode sp register, sp_USR.

Changing processor state

All ARM cores, including Thumb-capable processors, start up in ARM state on reset. The initialization code (at least

the reset handler) must be ARM code. If the application is compiled for Thumb, main() is Thumb code. The linker

can add ARM to Thumb interworking veneers automatically to change state between the ARM initialization code and

the Thumb application.

6.3.2 Initializing the application

An application is initialized by:

• initializing the nonzero writable data by copying the initializing values to the writable data region

• setting to zero the ZI writable data region.

After memory initialization, control is passed to the entry point of the application in, for example, C library code.

Initializing memory required by C code

The initial values for any initialized variables (RW) must be copied from ROM to RAM. All other ZI variables must be

initialized to zero. The library initialization code called at __main performs the copying and initialization.

Note

The linker assigns memory addresses for RO code, RW data, and ZI data. If a scatter-load description file is not

used, the linker uses one of the default layouts. Scatter-loading examples are given in Loading the ROM image at

address 0 and Using both scatter loading and remapping.

Using the main function

When the compiler compiles a function called main(), it generates a reference to the symbol __main to force the

linker to include the basic C run-time system from the ANSI C library. (The symbol __main is marked as an entry

point.)

Writing Code for ROM

1 2 ... 75 76 77 78 79 80 81 82 83 84 85 ... 132 133

Commentaires sur ces manuels

Pas de commentaire

ARM VERSION 1.2 Guide de l'utilisateur Page 80

Commentaires sur ces manuels