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
  32fffe0000        fffeffff        XScale cache flush area.  This is used
  33                                in proc-xscale.S to flush the whole data
  34                                cache.  Free for other usage on non-XScale.
  35
  36fff00000        fffdffff        Fixmap mapping region.  Addresses provided
  37                                by fix_to_virt() will be located here.
  38
  39ffc00000        ffefffff        DMA memory mapping region.  Memory returned
  40                                by the dma_alloc_xxx functions will be
  41                                dynamically mapped here.
  42
  43ff000000        ffbfffff        Reserved for future expansion of DMA
  44                                mapping region.
  45
  46VMALLOC_END     feffffff        Free for platform use, recommended.
  47                                VMALLOC_END must be aligned to a 2MB
  48                                boundary.
  49
  50VMALLOC_START   VMALLOC_END-1   vmalloc() / ioremap() space.
  51                                Memory returned by vmalloc/ioremap will
  52                                be dynamically placed in this region.
  53                                VMALLOC_START may be based upon the value
  54                                of the high_memory variable.
  55
  56PAGE_OFFSET     high_memory-1   Kernel direct-mapped RAM region.
  57                                This maps the platforms RAM, and typically
  58                                maps all platform RAM in a 1:1 relationship.
  59
  60TASK_SIZE       PAGE_OFFSET-1   Kernel module space
  61                                Kernel modules inserted via insmod are
  62                                placed here using dynamic mappings.
  63
  6400001000        TASK_SIZE-1     User space mappings
  65                                Per-thread mappings are placed here via
  66                                the mmap() system call.
  67
  6800000000        00000fff        CPU vector page / null pointer trap
  69                                CPUs which do not support vector remapping
  70                                place their vector page here.  NULL pointer
  71                                dereferences by both the kernel and user
  72                                space are also caught via this mapping.
  73
  74Please note that mappings which collide with the above areas may result
  75in a non-bootable kernel, or may cause the kernel to (eventually) panic
  76at run time.
  77
  78Since future CPUs may impact the kernel mapping layout, user programs
  79must not access any memory which is not mapped inside their 0x0001000
  80to TASK_SIZE address range.  If they wish to access these areas, they
  81must set up their own mappings using open() and mmap().
  82