32
edits
Changes
m
→Memory Design
Process A and B are running two separate programs, so the memory map is set so that the pages used by the first program are visible when process A is running; pages used for the data used by that process are also made visible. The memory map is changed so that the pages of the second program and data space are visible when process B is running. Neither process can access the software or data of the other program.
Process B and C may be executing the same program. In this casescase, the memory map active while each process is running contains the same program, but different data pages. The program is only loaded into memory once, reducing memory requirements.
Advanced use of the MMU by the operating system enables features such as virtual memory (pages which are not in use are placed in storage (on disk) until required), demand-loading (pages of data or software are only retrieved from disk into memory when they are accessed for the first time), and copy-on-write (two processes access a copy of the same data page, until one of them writes to the page, at which point the operating system makes a copy of the page and sets up each process to access their own copy - which gives the same result as having two private copies of the page, without using additional memory until absolutely necessary; this is commonly used with a fork() system call).
