In
computing
Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, e ...
base and bounds refers to a simple form of
virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...
where access to
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term ''primary storage ...
is controlled by one or a small number of sets of
processor registers
A processor register is a quickly accessible location available to a computer's Processor (computing), processor. Registers usually consist of a small amount of fast Computer storage, storage, although some registers have specific hardware functi ...
called ''base and bounds registers''.
In its simplest form each user
process
A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic.
Things called a process include:
Business and management
*Business process, activities that produce a specific se ...
is assigned a single contiguous segment of main memory. The
operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also in ...
loads the physical address of this segment into a ''base register'' and its size into a ''bound register''. ''Virtual'' addresses seen by the program are added to the contents of the base register to generate the ''
physical address
In computing, a physical address (also real address, or binary address), is a memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a ''particular'' storage cell ...
''. The address is checked against the contents of the bounds register to prevent a process from accessing memory beyond its assigned segment.
The operating system is not constrained by the hardware and can access all of physical memory.
This technique protects memory used by one process against access or modification by another. By itself it does not protect memory from erroneous access by the owning process. It also allows programs to be easily
relocated in memory, since only the base and bounds registers have to be modified when the program is moved.
Some computer systems extended this mechanism to multiple segments, such as the ''i bank'' and ''d bank'' for instructions and data on the
UNIVAC 1100 series
The UNIVAC 1100/2200 series is a series of compatible 36-bit computer systems, beginning with the UNIVAC 1107 in 1962, initially made by Sperry Rand. The series continues to be supported today by Unisys Corporation as the ClearPath Dorado Series. ...
computers or the separation of memory on the DEC
PDP-10
Digital Equipment Corporation (DEC)'s PDP-10, later marketed as the DECsystem-10, is a mainframe computer family manufactured beginning in 1966 and discontinued in 1983. 1970s models and beyond were marketed under the DECsystem-10 name, especi ...
system into a read/write "low" segment for the user process and a read-only "high" segment for sharable code.
Segmented
virtual memory
In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very l ...
is a further generalization of this mechanism to a large number of segments. Usually the ''segment table'' is kept in memory rather than registers.
See also
*
Memory management (operating systems) In operating systems, memory management is the function responsible for managing the computer's primary memory.
The memory management function keeps track of the status of each memory location, either ''allocated'' or ''free''. It determines how m ...
*
Memory management unit
A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit having all memory references passed through itself, primarily performing the translation of virtual memory addresses to physical ad ...
References
{{Operating System
Memory management
Virtual memory