Monday, April 10, 2017

What is inside a WCE7 BSP

The BSP (Board Support Package) is the layer that will interface the Operating System with the hardware.

It consists of the following items:
  • [Optional] Bootloader
  • OAL (OEM Abstraction layer)
  • KITL to debug the OS in development phase
  • Configuration files that specifies the board (Ex: config.bib for memory settings)
The BSP image is specific to the OS version and to the target hardware architecture.

 

Bootloader

When powered up, the microprocessor starts executing instructions from a specified memory address located in ROM. This code can contain the entire system for small footprint applications and will run until shutdown.
With WCE7, the system image (nk.bin) needs to be loaded to RAM from a storage disk or network because it can't fit into a ROM memory. In this case, these are the typical steps of a bootloader:
  • Minimal Initialization of the system (memory and com drivers)
  • Loads the OS image to RAM
  • Jumps to OS start address in RAM and calls startup fuction
  • The OAL takes over
Windows CE 7 includes a configurable and reusable bootloader named CE Boot with the following features:
  • Can load files from disks (IDE, PCI)
  • Emulates a console to allow user interraction at boot
  • Network support: DHCP, TFTP, IP

 

XLDR

In some cases, the system can be loaded in two steps : a first bootloader loads a second bootloader to RAM. The second bootloader loads the system image to RAM et jumps to it. This is necessary when:
  • The initial startup program is too small
  • The startup program on the target system is general purpose
  • The CPU has a small amount of RAM to load an image immediately and shall initialize an external RAM to host the system
XLDR is a lightweight loader that can load CE Boot in two stages.


OAL

The OAL allows the kernel to access the hardware layer (interrupts, timers, cache, IOCTL...) through an abstract interface.
When it takes over the boot phase, it accomplishes the following steps:
  • Initialize the CPU state, hardware and kernel's global variables
  • Jumps to the entry point of the kernel
  • The kernel exchanges global pointers with the OAL. From this point on, the kernel has access to all functions and variables defined in OEMGLOBAL and OAL has access to all functions and variables defined in NKGLOBAL
  • On kernel call, the OAL initializes the debug serial port (OEMInitSerialDebug) (like the bootloader did before without the help of the OAL)
  • On kernel call, the OAL initializes the rest of hardware interfaces on the device (OEMInit())
  • [Optional] On kernel call, KITL initialization (OEMKitlInit)
  • The kernel starts executing its first thread

 

Configuration files

File Description
config.bib Memory structure definition file
platform.bib BSP global settings. Selects which files need to included in the image depending on user-selected options/items
platform.dat Not used
platform.db Not used
platform.reg BSP global registry settings. Selects which keys to write in the registry depending on user-selected options/items
sources.cmn Starting point of the BSP build process. On top of source tree. Defines include directories and linked libraries
<MyBSP>.bat Project-specific environment variables to use in the build process. In particular, specifies the devices to include in the build.

To have a deeper look at the BSP's file structure, check the Build process.

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