1 Booting ARM Linux 2 ================= 3 4Author: Russell King 5Date : 18 May 2002 6 7The following documentation is relevant to 2.4.18-rmk6 and beyond. 8 9In order to boot ARM Linux, you require a boot loader, which is a small 10program that runs before the main kernel. The boot loader is expected 11to initialise various devices, and eventually call the Linux kernel, 12passing information to the kernel. 13 14Essentially, the boot loader should provide (as a minimum) the 15following: 16 171. Setup and initialise the RAM. 182. Initialise one serial port. 193. Detect the machine type. 204. Setup the kernel tagged list. 215. Call the kernel image. 22 23 241. Setup and initialise RAM 25--------------------------- 26 27Existing boot loaders: MANDATORY 28New boot loaders: MANDATORY 29 30The boot loader is expected to find and initialise all RAM that the 31kernel will use for volatile data storage in the system. It performs 32this in a machine dependent manner. (It may use internal algorithms 33to automatically locate and size all RAM, or it may use knowledge of 34the RAM in the machine, or any other method the boot loader designer 35sees fit.) 36 37 382. Initialise one serial port 39----------------------------- 40 41Existing boot loaders: OPTIONAL, RECOMMENDED 42New boot loaders: OPTIONAL, RECOMMENDED 43 44The boot loader should initialise and enable one serial port on the 45target. This allows the kernel serial driver to automatically detect 46which serial port it should use for the kernel console (generally 47used for debugging purposes, or communication with the target.) 48 49As an alternative, the boot loader can pass the relevant 'console=' 50option to the kernel via the tagged lists specifing the port, and 51serial format options as described in 52 53 linux/Documentation/kernel-parameters.txt. 54 55 563. Detect the machine type 57-------------------------- 58 59Existing boot loaders: OPTIONAL 60New boot loaders: MANDATORY 61 62The boot loader should detect the machine type its running on by some 63method. Whether this is a hard coded value or some algorithm that 64looks at the connected hardware is beyond the scope of this document. 65The boot loader must ultimately be able to provide a MACH_TYPE_xxx 66value to the kernel. (see linux/arch/arm/tools/mach-types). 67 68 694. Setup the kernel tagged list 70------------------------------- 71 72Existing boot loaders: OPTIONAL, HIGHLY RECOMMENDED 73New boot loaders: MANDATORY 74 75The boot loader must create and initialise the kernel tagged list. 76A valid tagged list starts with ATAG_CORE and ends with ATAG_NONE. 77The ATAG_CORE tag may or may not be empty. An empty ATAG_CORE tag 78has the size field set to '2' (0x00000002). The ATAG_NONE must set 79the size field to zero. 80 81Any number of tags can be placed in the list. It is undefined 82whether a repeated tag appends to the information carried by the 83previous tag, or whether it replaces the information in its 84entirety; some tags behave as the former, others the latter. 85 86The boot loader must pass at a minimum the size and location of 87the system memory, and root filesystem location. Therefore, the 88minimum tagged list should look: 89 90 +-----------+ 91base -> | ATAG_CORE | | 92 +-----------+ | 93 | ATAG_MEM | | increasing address 94 +-----------+ | 95 | ATAG_NONE | | 96 +-----------+ v 97 98The tagged list should be stored in system RAM. 99 100The tagged list must be placed in a region of memory where neither 101the kernel decompressor nor initrd 'bootp' program will overwrite 102it. The recommended placement is in the first 16KiB of RAM. 103 1045. Calling the kernel image 105--------------------------- 106 107Existing boot loaders: MANDATORY 108New boot loaders: MANDATORY 109 110There are two options for calling the kernel zImage. If the zImage 111is stored in flash, and is linked correctly to be run from flash, 112then it is legal for the boot loader to call the zImage in flash 113directly. 114 115The zImage may also be placed in system RAM (at any location) and 116called there. Note that the kernel uses 16K of RAM below the image 117to store page tables. The recommended placement is 32KiB into RAM. 118 119In either case, the following conditions must be met: 120 121- CPU register settings 122 r0 = 0, 123 r1 = machine type number discovered in (3) above. 124 r2 = physical address of tagged list in system RAM. 125 126- CPU mode 127 All forms of interrupts must be disabled (IRQs and FIQs) 128 The CPU must be in SVC mode. (A special exception exists for Angel) 129 130- Caches, MMUs 131 The MMU must be off. 132 Instruction cache may be on or off. 133 Data cache must be off. 134 135- The boot loader is expected to call the kernel image by jumping 136 directly to the first instruction of the kernel image. 137 138