memory.txt

		Kernel Memory Layout on ARM Linux

		Russell King <rmk@arm.linux.org.uk>
			May 21, 2004 (2.6.6)

This document describes the virtual memory layout which the Linux
kernel uses for ARM processors.  It indicates which regions are
free for platforms to use, and which are used by generic code.

The ARM CPU is capable of addressing a maximum of 4GB virtual memory
space, and this must be shared between user space processes, the
kernel, and hardware devices.

As the ARM architecture matures, it becomes necessary to reserve
certain regions of VM space for use for new facilities; therefore
this document may reserve more VM space over time.

Start		End		Use
--------------------------------------------------------------------------
ffff8000	ffffffff	copy_user_page / clear_user_page use.
				For SA11xx and Xscale, this is used to
				setup a minicache mapping.

ffff1000	ffff7fff	Reserved.
				Platforms must not use this address range.

ffff0000	ffff0fff	CPU vector page.
				The CPU vectors are mapped here if the
				CPU supports vector relocation (control
				register V bit.)

ffc00000	fffeffff	DMA memory mapping region.  Memory returned
				by the dma_alloc_xxx functions will be
				dynamically mapped here.

ff000000	ffbfffff	Reserved for future expansion of DMA
				mapping region.

VMALLOC_END	feffffff	Free for platform use, recommended.

VMALLOC_START	VMALLOC_END-1	vmalloc() / ioremap() space.
				Memory returned by vmalloc/ioremap will
				be dynamically placed in this region.
				VMALLOC_START may be based upon the value
				of the high_memory variable.

PAGE_OFFSET	high_memory-1	Kernel direct-mapped RAM region.
				This maps the platforms RAM, and typically
				maps all platform RAM in a 1:1 relationship.

TASK_SIZE	PAGE_OFFSET-1	Kernel module space
				Kernel modules inserted via insmod are
				placed here using dynamic mappings.

00001000	TASK_SIZE-1	User space mappings
				Per-thread mappings are placed here via
				the mmap() system call.

00000000	00000fff	CPU vector page / null pointer trap
				CPUs which do not support vector remapping
				place their vector page here.  NULL pointer
				dereferences by both the kernel and user
				space are also caught via this mapping.

Please note that mappings which collide with the above areas may result
in a non-bootable kernel, or may cause the kernel to (eventually) panic
at run time.

Since future CPUs may impact the kernel mapping layout, user programs
must not access any memory which is not mapped inside their 0x0001000
to TASK_SIZE address range.  If they wish to access these areas, they
must set up their own mappings using open() and mmap().