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- Kernel Memory Layout on ARM Linux
- Russell King <rmk@arm.linux.org.uk>
- November 17, 2005 (2.6.15)
- 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.
- ffff4000 ffffffff cache aliasing on ARMv6 and later CPUs.
- 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.)
- fffe0000 fffeffff XScale cache flush area. This is used
- in proc-xscale.S to flush the whole data
- cache. (XScale does not have TCM.)
- fffe8000 fffeffff DTCM mapping area for platforms with
- DTCM mounted inside the CPU.
- fffe0000 fffe7fff ITCM mapping area for platforms with
- ITCM mounted inside the CPU.
- fff00000 fffdffff Fixmap mapping region. Addresses provided
- by fix_to_virt() will be located here.
- ffc00000 ffefffff 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_START VMALLOC_END-1 vmalloc() / ioremap() space.
- Memory returned by vmalloc/ioremap will
- be dynamically placed in this region.
- Machine specific static mappings are also
- located here through iotable_init().
- VMALLOC_START is based upon the value
- of the high_memory variable, and VMALLOC_END
- is equal to 0xff000000.
- 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.
- PKMAP_BASE PAGE_OFFSET-1 Permanent kernel mappings
- One way of mapping HIGHMEM pages into kernel
- space.
- MODULES_VADDR MODULES_END-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().
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