1		Kernel Memory Layout on ARM Linux
2
3		Russell King <rmk@arm.linux.org.uk>
4		     November 17, 2005 (2.6.15)
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				VMALLOC_END must be aligned to a 2MB
41				boundary.
42
43VMALLOC_START	VMALLOC_END-1	vmalloc() / ioremap() space.
44				Memory returned by vmalloc/ioremap will
45				be dynamically placed in this region.
46				VMALLOC_START may be based upon the value
47				of the high_memory variable.
48
49PAGE_OFFSET	high_memory-1	Kernel direct-mapped RAM region.
50				This maps the platforms RAM, and typically
51				maps all platform RAM in a 1:1 relationship.
52
53TASK_SIZE	PAGE_OFFSET-1	Kernel module space
54				Kernel modules inserted via insmod are
55				placed here using dynamic mappings.
56
5700001000	TASK_SIZE-1	User space mappings
58				Per-thread mappings are placed here via
59				the mmap() system call.
60
6100000000	00000fff	CPU vector page / null pointer trap
62				CPUs which do not support vector remapping
63				place their vector page here.  NULL pointer
64				dereferences by both the kernel and user
65				space are also caught via this mapping.
66
67Please note that mappings which collide with the above areas may result
68in a non-bootable kernel, or may cause the kernel to (eventually) panic
69at run time.
70
71Since future CPUs may impact the kernel mapping layout, user programs
72must not access any memory which is not mapped inside their 0x0001000
73to TASK_SIZE address range.  If they wish to access these areas, they
74must set up their own mappings using open() and mmap().
75