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
24ffff4000	ffffffff	cache aliasing on ARMv6 and later CPUs.
25
26ffff1000	ffff7fff	Reserved.
27				Platforms must not use this address range.
28
29ffff0000	ffff0fff	CPU vector page.
30				The CPU vectors are mapped here if the
31				CPU supports vector relocation (control
32				register V bit.)
33
34fffe0000	fffeffff	XScale cache flush area.  This is used
35				in proc-xscale.S to flush the whole data
36				cache. (XScale does not have TCM.)
37
38fffe8000	fffeffff	DTCM mapping area for platforms with
39				DTCM mounted inside the CPU.
40
41fffe0000	fffe7fff	ITCM mapping area for platforms with
42				ITCM mounted inside the CPU.
43
44fff00000	fffdffff	Fixmap mapping region.  Addresses provided
45				by fix_to_virt() will be located here.
46
47ffc00000	ffefffff	DMA memory mapping region.  Memory returned
48				by the dma_alloc_xxx functions will be
49				dynamically mapped here.
50
51ff000000	ffbfffff	Reserved for future expansion of DMA
52				mapping region.
53
54VMALLOC_START	VMALLOC_END-1	vmalloc() / ioremap() space.
55				Memory returned by vmalloc/ioremap will
56				be dynamically placed in this region.
57				Machine specific static mappings are also
58				located here through iotable_init().
59				VMALLOC_START is based upon the value
60				of the high_memory variable, and VMALLOC_END
61				is equal to 0xff000000.
62
63PAGE_OFFSET	high_memory-1	Kernel direct-mapped RAM region.
64				This maps the platforms RAM, and typically
65				maps all platform RAM in a 1:1 relationship.
66
67PKMAP_BASE	PAGE_OFFSET-1	Permanent kernel mappings
68				One way of mapping HIGHMEM pages into kernel
69				space.
70
71MODULES_VADDR	MODULES_END-1	Kernel module space
72				Kernel modules inserted via insmod are
73				placed here using dynamic mappings.
74
7500001000	TASK_SIZE-1	User space mappings
76				Per-thread mappings are placed here via
77				the mmap() system call.
78
7900000000	00000fff	CPU vector page / null pointer trap
80				CPUs which do not support vector remapping
81				place their vector page here.  NULL pointer
82				dereferences by both the kernel and user
83				space are also caught via this mapping.
84
85Please note that mappings which collide with the above areas may result
86in a non-bootable kernel, or may cause the kernel to (eventually) panic
87at run time.
88
89Since future CPUs may impact the kernel mapping layout, user programs
90must not access any memory which is not mapped inside their 0x0001000
91to TASK_SIZE address range.  If they wish to access these areas, they
92must set up their own mappings using open() and mmap().
93