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2008-11-24 11:40:14

In order to boot ARM Linux, you require a boot loader, which is a small program that runs before the main kernel. The boot loader is expected to initialise various devices, and eventually call the Linux kernel, passing information to the kernel.

Essentially, the boot loader should provide (as a minimum) the following:

  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: MANDATORY
New boot loaders: MANDATORY

The boot loader is expected to find and initialise all RAM that the kernel will use for volatile data storage in the system. It performs this in a machine dependent manner. (It may use internal algorithms to automatically locate and size all RAM, or it may use knowledge of the RAM in the machine, or any other method the boot loader designer sees fit.)

2. Initialise one serial port

Existing boot loaders: OPTIONAL, RECOMMENDED

The boot loader should initialise and enable one serial port on the target. This allows the kernel serial driver to automatically detect which serial port it should use for the kernel console (generally used 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, and serial format options as described in


3. Detect the machine type

Existing boot loaders: OPTIONAL
New boot loaders: MANDATORY

The boot loader should detect the machine type its running on by some method. Whether this is a hard coded value or some algorithm that looks at the connected hardware is beyond the scope of this document. The boot loader must ultimately be able to provide a MACH_TYPE_xxx value to the kernel. (see linux/arch/arm/tools/mach-types).

4. Setup the kernel tagged list

New boot loaders: MANDATORY

The 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 tag has the size field set to '2' (0x00000002). The ATAG_NONE must set the size field to zero.

Any number of tags can be placed in the list. It is undefined whether a repeated tag appends to the information carried by the previous tag, or whether it replaces the information in its entirety; some tags behave as the former, others the latter.

The boot loader must pass at a minimum the size and location of the system memory, and root filesystem location. Therefore, the minimum tagged list should look:

base ->	| ATAG_CORE |  |
	+-----------+  |
	| ATAG_MEM  |  | increasing address
	+-----------+  |
	| ATAG_NONE |  |
	+-----------+  v
The tagged list should be stored in system RAM.

The tagged list must be placed in a region of memory where neither the kernel decompressor nor initrd 'bootp' program will overwrite it. The recommended placement is in the first 16KiB of RAM.

5. Calling the kernel image

Existing boot loaders: MANDATORY
New boot loaders: MANDATORY

There are two options for calling the kernel zImage. If the zImage is 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 flash directly.

The zImage may also be placed in system RAM (at any location) and called there. Note that the kernel uses 16K of RAM below the image to 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 and must not contain any stale data.
  • Devices
    • DMA to/from devices should be quiesced.
  • The boot loader is expected to call the kernel image by jumping directly to the first instruction of the kernel image.
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