MEM(4) MEM(4) NAME mem, kmem, kmemr, kUmem, mtpr - main memory DESCRIPTION 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 files kmem, kmemr, and kUmem are the same as mem except that kernel virtual memory rather than physical memory is accessed. Kmemr is read-only, and excludes system buffer pools and device addresses; it is intended as a ``safe'' and ``secure'' mechanism for accessing system tables. KUmem guarantees that reads and writes will be done in two-byte quantities; this is convenient for UNIBUS accesses. On the VAX, mtpr accesses internal processor registers. Registers are 4 bytes long; register n may be read or writ- ten at address n*4. FILES /dev/mem, /dev/kmem, /dev/kmemr, /dev/kUmem, /dev/mtpr BUGS On PDP11's and VAX's, memory files are accessed one byte at a time, an inappropiate method for some device registers. Examining and patching device and processor registers is likely to lead to unexpected results when read-only or write-only bits are present. An attempt to read a nonexistent processor register returns 0 instead of an error.