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