Documentation/arm/memory.txt

   1                 Kernel Memory Layout on ARM Linux
   2
   3                 Russell King <rmk@arm.linux.org.uk>
   4                      November 17, 2005 (2.6.15)
   5
   6 This document describes the virtual memory layout which the Linux
   7 kernel uses for ARM processors.  It indicates which regions are
   8 free for platforms to use, and which are used by generic code.
   9
  10 The ARM CPU is capable of addressing a maximum of 4GB virtual memory
  11 space, and this must be shared between user space processes, the
  12 kernel, and hardware devices.
  13
  14 As the ARM architecture matures, it becomes necessary to reserve
  15 certain regions of VM space for use for new facilities; therefore
  16 this document may reserve more VM space over time.
  17
  18 Start           End             Use
  19 --------------------------------------------------------------------------
  20 ffff8000        ffffffff        copy_user_page / clear_user_page use.
  21                                 For SA11xx and Xscale, this is used to
  22                                 setup a minicache mapping.
  23
  24 ffff4000        ffffffff        cache aliasing on ARMv6 and later CPUs.
  25
  26 ffff1000        ffff7fff        Reserved.
  27                                 Platforms must not use this address range.
  28
  29 ffff0000        ffff0fff        CPU vector page.
  30                                 The CPU vectors are mapped here if the
  31                                 CPU supports vector relocation (control
  32                                 register V bit.)
  33
  34 fffe0000        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
  38 fffe8000        fffeffff        DTCM mapping area for platforms with
  39                                 DTCM mounted inside the CPU.
  40
  41 fffe0000        fffe7fff        ITCM mapping area for platforms with
  42                                 ITCM mounted inside the CPU.
  43
  44 fff00000        fffdffff        Fixmap mapping region.  Addresses provided
  45                                 by fix_to_virt() will be located here.
  46
  47 ffc00000        ffefffff        DMA memory mapping region.  Memory returned
  48                                 by the dma_alloc_xxx functions will be
  49                                 dynamically mapped here.
  50
  51 ff000000        ffbfffff        Reserved for future expansion of DMA
  52                                 mapping region.
  53
  54 VMALLOC_END     feffffff        Free for platform use, recommended.
  55                                 VMALLOC_END must be aligned to a 2MB
  56                                 boundary.
  57
  58 VMALLOC_START   VMALLOC_END-1   vmalloc() / ioremap() space.
  59                                 Memory returned by vmalloc/ioremap will
  60                                 be dynamically placed in this region.
  61                                 VMALLOC_START may be based upon the value
  62                                 of the high_memory variable.
  63
  64 PAGE_OFFSET     high_memory-1   Kernel direct-mapped RAM region.
  65                                 This maps the platforms RAM, and typically
  66                                 maps all platform RAM in a 1:1 relationship.
  67
  68 PKMAP_BASE      PAGE_OFFSET-1   Permanent kernel mappings
  69                                 One way of mapping HIGHMEM pages into kernel
  70                                 space.
  71
  72 MODULES_VADDR   MODULES_END-1   Kernel module space
  73                                 Kernel modules inserted via insmod are
  74                                 placed here using dynamic mappings.
  75
  76 00001000        TASK_SIZE-1     User space mappings
  77                                 Per-thread mappings are placed here via
  78                                 the mmap() system call.
  79
  80 00000000        00000fff        CPU vector page / null pointer trap
  81                                 CPUs which do not support vector remapping
  82                                 place their vector page here.  NULL pointer
  83                                 dereferences by both the kernel and user
  84                                 space are also caught via this mapping.
  85
  86 Please note that mappings which collide with the above areas may result
  87 in a non-bootable kernel, or may cause the kernel to (eventually) panic
  88 at run time.
  89
  90 Since future CPUs may impact the kernel mapping layout, user programs
  91 must not access any memory which is not mapped inside their 0x0001000
  92 to TASK_SIZE address range.  If they wish to access these areas, they
  93 must set up their own mappings using open() and mmap().