linux/Documentation/arm/memory.txt
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   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