View source for Szwg Spec Typicalboot

== Introduction ==
* Embedded Linux boot procedure is very different from enterprise/pc linux. First of all, in most cases, there is no mass storage devices. Typically, Linux kernel is stored in ROM/Flash on the target system. In order to save the area where Linux kernel is stored, the kernel is compressed as zImage.
* The following is the procedure.
### Boot loader copies zImage kernel from ROM/Flash to the specified RAM area and let zImage kernel execute.
### zImage kernel (precisely speaking, the routine attached to zImage kernel) decompress the zImage Kernel and load the image on the specified RAM address.
### Boot loader let the newly loaded kernel start.
* Depending on the conditions, several different file systems are used. We picked ext2 as typical file system and as normalizer.
== Rational ==
* This method saves ROM size (a little bit) because the kernel is stored compressed.
== Specifications ==
  < Recommended implementation for boot loader >
*** Boot loader SHALL initialize accessible RAM
**** Boot loader SHALL initialize CPU - cache,MMU,interrupt
**** Boot loader SHALL initialize memory controller,if necessary
**** Boot loader MAY initialize serial controller to get debug message
*** Boot loader MAY check Linux kernel stored in ROM/Flash whether it is XIP executable image or zImage
*** Boot loader SHALL copy zImage kernel from ROM/Flash to the specified RAM area
*** Boot loader SHALL let zImage kernel (in RAM) start.
  < Specification for the Kernel >
*** zImage Linux kernel SHALL copy the executable Linux image to the specified address.
*** zImage Linux kernel SHALL let the executable Linux image start.
== Notes(informational and non-normative) ==
Pros and Cons:
* This method saves Kernel ROM size because the kernel is stored compressed, but require RAM space for Kernel image to run.
* It takes time to make kernel copy(from ROM to RAM) and decompress (RAM to RAM) before kernel boot.
* Kernel execution speed shall be fast as the kernel code is located on RAM. note memory access speed to/from RAM is faster than ROM/Flash.
* In this configuration, File System is located in ROM/Flash as ext2 file system and it shall be used as Read Only,  unless the driver has special implementation.

 Actual data is shown in Appendix  ( [wiki:[[Appendix A_R2]]])

== References ==
** This feature is implemented in CELF 1.0 as well as master Linux (2.4 kernel at least)
== Remaining Issues ==
** Some data on the platform are still missing.

<pre>#noprint
   * TimRiker asks: why ext2? It's much larger and slower than romfs or cramfs
   * TimRiker comments: the inclusion of this specification in the CELF spec seems to forbid XIP. which is '''required''' elsewhere in the spec. This conflict should be resolved.
#noprint</pre>

[[Category:Consumer Electronics Linux]]