RAM 6.0 BUX II Series Guía de usuario descargar pdf (Pagina 19)

2 Overview of a BSP

2.2 Boot Sequence

2.2.2 Boot Sequence Configurations

There are several boot sequence configurations that are commonly supported. For

BSPs intended for specific applications only, one or more of the methods are

supported. General-purpose BSPs support all boot methods.

For development, the most common boot method is through the presence of a

ROM-based boot loader. The purpose of this boot loader is to load the final

VxWorks image from a remote development host (or possibly from a local file

system) and then to start running the newly downloaded image. The boot image

is a specially-crafted version of VxWorks with a boot loader as its only application.

Because this boot image is largely independent of development changes, it is

seldom necessary to re-program the flash, which saves development time. The

boot image is referred to as bootrom; the OS image that is loaded is referred to as

vxWorks. These images are described later in the document.

Another alternative boot method is to put the VxWorks image into flash directly.

Typically, both the core OS and the application code reside in the same image. In

this case, there is no intermediate step between loading the boot loader and

loading the application image. Also, no file system is needed to hold the final

VxWorks image file. Although this boot method can be used for development,

progress can be hindered by the need to re-program the flash often. However, this

is a very common boot mechanism in final product delivery.

Whether loading a VxWorks image from flash or loading a boot loader from flash,

there are special requirements. The processor’s reset vector causes it to start

execution of the code in flash memory. This flash-resident code can do one of

several things. It can:

■

Continue running from flash (that is, fetch instructions from the flash memory

and execute them).

■

Copy itself from flash to RAM and branch to an appropriate place in the RAM

copy.

■

Decompress a compressed image contained in flash and put the image into

RAM, then branch to an appropriate place in the RAM copy.

The names of the different image types are determined both by the build method

used and by the behavior of the image.

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RAM 6.0 BUX II Series Guía de usuario Pagina 19