Booting ARM Linux			=================Author:	Russell KingDate  : 18 May 2002The following documentation is relevant to 2.4.18-rmk6 and beyond.In order to boot ARM Linux, you require a boot loader, which is a smallprogram that runs before the main kernel.  The boot loader is expectedto initialise various devices, and eventually call the Linux kernel,passing information to the kernel.Essentially, the boot loader should provide (as a minimum) thefollowing:1. Setup and initialise the RAM.2. Initialise one serial port.3. Detect the machine type.4. Setup the kernel tagged list.5. Call the kernel image.1. Setup and initialise RAM---------------------------Existing boot loaders:		MANDATORYNew boot loaders:		MANDATORYThe boot loader is expected to find and initialise all RAM that thekernel will use for volatile data storage in the system.  It performsthis in a machine dependent manner.  (It may use internal algorithmsto automatically locate and size all RAM, or it may use knowledge ofthe RAM in the machine, or any other method the boot loader designersees fit.)2. Initialise one serial port-----------------------------Existing boot loaders:		OPTIONAL, RECOMMENDEDNew boot loaders:		OPTIONAL, RECOMMENDEDThe boot loader should initialise and enable one serial port on thetarget.  This allows the kernel serial driver to automatically detectwhich serial port it should use for the kernel console (generallyused for debugging purposes, or communication with the target.)As an alternative, the boot loader can pass the relevant 'console='option to the kernel via the tagged lists specifing the port, andserial format options as described in       linux/Documentation/kernel-parameters.txt.3. Detect the machine type--------------------------Existing boot loaders:		OPTIONALNew boot loaders:		MANDATORYThe boot loader should detect the machine type its running on by somemethod.  Whether this is a hard coded value or some algorithm thatlooks at the connected hardware is beyond the scope of this document.The boot loader must ultimately be able to provide a MACH_TYPE_xxxvalue to the kernel. (see linux/arch/arm/tools/mach-types).4. Setup the kernel tagged list-------------------------------Existing boot loaders:		OPTIONAL, HIGHLY RECOMMENDEDNew boot loaders:		MANDATORYThe boot loader must create and initialise the kernel tagged list.A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE.The ATAG_CORE tag may or may not be empty.  An empty ATAG_CORE taghas the size field set to '2' (0x00000002).  The ATAG_NONE must setthe size field to zero.Any number of tags can be placed in the list.  It is undefinedwhether a repeated tag appends to the information carried by theprevious tag, or whether it replaces the information in itsentirety; some tags behave as the former, others the latter.The boot loader must pass at a minimum the size and location ofthe system memory, and root filesystem location.  Therefore, theminimum tagged list should look:	+-----------+base ->	| ATAG_CORE |  |	+-----------+  |	| ATAG_MEM  |  | increasing address	+-----------+  |	| ATAG_NONE |  |	+-----------+  vThe tagged list should be stored in system RAM.The tagged list must be placed in a region of memory where neitherthe kernel decompressor nor initrd 'bootp' program will overwriteit.  The recommended placement is in the first 16KiB of RAM.5. Calling the kernel image---------------------------Existing boot loaders:		MANDATORYNew boot loaders:		MANDATORYThere are two options for calling the kernel zImage.  If the zImageis stored in flash, and is linked correctly to be run from flash,then it is legal for the boot loader to call the zImage in flashdirectly.The zImage may also be placed in system RAM (at any location) andcalled there.  Note that the kernel uses 16K of RAM below the imageto store page tables.  The recommended placement is 32KiB into RAM.In either case, the following conditions must be met:- CPU register settings  r0 = 0,  r1 = machine type number discovered in (3) above.  r2 = physical address of tagged list in system RAM.- CPU mode  All forms of interrupts must be disabled (IRQs and FIQs)  The CPU must be in SVC mode.  (A special exception exists for Angel)- Caches, MMUs  The MMU must be off.  Instruction cache may be on or off.  Data cache must be off.- The boot loader is expected to call the kernel image by jumping  directly to the first instruction of the kernel image.