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BSD 2.11 - man page for mem (bsd section 4)

MEM(4)							     Kernel Interfaces Manual							    MEM(4)

mem, kmem - main memory
major device number(s): raw: 1 minor device encoding: mem: 0; kmem: 1; null: 2
Mem is a special file that is an image of the main memory of the computer. It may be used, for example, to examine (and even to patch) the system. Byte addresses in mem are interpreted as physical memory addresses. References to non-existent locations cause errors to be returned. The file kmem is the same as mem except that kernel virtual memory rather than physical memory is accessed. Only kernel virtual addresses that are mapped to memory are allowed. Examining and patching device registers is likely to lead to unexpected results when read-only or write-only bits are present. On PDP-11s, the I/O page begins at location 0160000 of kmem and the per-process data segment for the current process begins at 0140000 and is USIZE clicks (64 bytes each) long.
/dev/mem /dev/kmem /dev/MAKEDEV script to create special files /dev/MAKEDEV.local script to localize special files
On PDP-11's, specifying an odd kernel or user address, or an odd transfer count is [generally] slower than using all even parameters. On machines with ENABLE/34(tm) memory mapping boards the I/O page can be accessed only through kmem. 3rd Berkeley Distribution January 28, 1988 MEM(4)