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1 Kernel Memory Layout on ARM Linux
2
3 Russell King <rmk@arm.linux.org.uk>
4 May 21, 2004 (2.6.6)
5
6This document describes the virtual memory layout which the Linux
7kernel uses for ARM processors. It indicates which regions are
8free for platforms to use, and which are used by generic code.
9
10The ARM CPU is capable of addressing a maximum of 4GB virtual memory
11space, and this must be shared between user space processes, the
12kernel, and hardware devices.
13
14As the ARM architecture matures, it becomes necessary to reserve
15certain regions of VM space for use for new facilities; therefore
16this document may reserve more VM space over time.
17
18Start End Use
19--------------------------------------------------------------------------
20ffff8000 ffffffff copy_user_page / clear_user_page use.
21 For SA11xx and Xscale, this is used to
22 setup a minicache mapping.
23
24ffff1000 ffff7fff Reserved.
25 Platforms must not use this address range.
26
27ffff0000 ffff0fff CPU vector page.
28 The CPU vectors are mapped here if the
29 CPU supports vector relocation (control
30 register V bit.)
31
32ffc00000 fffeffff DMA memory mapping region. Memory returned
33 by the dma_alloc_xxx functions will be
34 dynamically mapped here.
35
36ff000000 ffbfffff Reserved for future expansion of DMA
37 mapping region.
38
39VMALLOC_END feffffff Free for platform use, recommended.
40
41VMALLOC_START VMALLOC_END-1 vmalloc() / ioremap() space.
42 Memory returned by vmalloc/ioremap will
43 be dynamically placed in this region.
44 VMALLOC_START may be based upon the value
45 of the high_memory variable.
46
47PAGE_OFFSET high_memory-1 Kernel direct-mapped RAM region.
48 This maps the platforms RAM, and typically
49 maps all platform RAM in a 1:1 relationship.
50
51TASK_SIZE PAGE_OFFSET-1 Kernel module space
52 Kernel modules inserted via insmod are
53 placed here using dynamic mappings.
54
5500001000 TASK_SIZE-1 User space mappings
56 Per-thread mappings are placed here via
57 the mmap() system call.
58
5900000000 00000fff CPU vector page / null pointer trap
60 CPUs which do not support vector remapping
61 place their vector page here. NULL pointer
62 dereferences by both the kernel and user
63 space are also caught via this mapping.
64
65Please note that mappings which collide with the above areas may result
66in a non-bootable kernel, or may cause the kernel to (eventually) panic
67at run time.
68
69Since future CPUs may impact the kernel mapping layout, user programs
70must not access any memory which is not mapped inside their 0x0001000
71to TASK_SIZE address range. If they wish to access these areas, they
72must set up their own mappings using open() and mmap().