Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 Kernel Memory Layout on ARM Linux 2 3 Russell King <rmk[AT]arm.linux.org[DOT]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 ffff1000 ffff7fff Reserved. 25 Platforms must not use this address range. 26 27 ffff0000 ffff0fff CPU vector page. 28 The CPU vectors are mapped here if the 29 CPU supports vector relocation (control 30 register V bit.) 31 32 ffc00000 fffeffff DMA memory mapping region. Memory returned 33 by the dma_alloc_xxx functions will be 34 dynamically mapped here. 35 36 ff000000 ffbfffff Reserved for future expansion of DMA 37 mapping region. 38 39 VMALLOC_END feffffff Free for platform use, recommended. 40 VMALLOC_END must be aligned to a 2MB 41 boundary. 42 43 VMALLOC_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 49 PAGE_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 53 TASK_SIZE PAGE_OFFSET-1 Kernel module space 54 Kernel modules inserted via insmod are 55 placed here using dynamic mappings. 56 57 00001000 TASK_SIZE-1 User space mappings 58 Per-thread mappings are placed here via 59 the mmap() system call. 60 61 00000000 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 67 Please note that mappings which collide with the above areas may result 68 in a non-bootable kernel, or may cause the kernel to (eventually) panic 69 at run time. 70 71 Since future CPUs may impact the kernel mapping layout, user programs 72 must not access any memory which is not mapped inside their 0x0001000 73 to TASK_SIZE address range. If they wish to access these areas, they 74 must set up their own mappings using open() and mmap().