Base and bounds

In computing base and bounds refers to a simple form of virtual memory where access to computer memory is controlled by one or a small number of sets of processor registers called base and bounds registers.^[1]

In its simplest form each user process is assigned a single contiguous segment of main memory. The operating system 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. 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 computers or the separation of memory on the DEC PDP-10 system into a read/write "low" segment for the user process and a read-only "high" segment for sharable code.

Apple Computer's MultiFinder of 1987 is a more modern use of this technique. Programs shipped with a requested bounds figure stored in the resource fork and the operating system attempted to move the program to an area in memory with that amount free. The user could also adjust this figure using the Get Info dialog, typically to increase the amount of memory for programs with large needs, like Photoshop.

Segmented virtual memory is a further generalization of this mechanism to a large number of segments. Usually the segment table is kept in memory rather than registers.

References

^ Pfleeger, Charles P.; Pfleeger, Shari Lawrence (2013). Security in Computing. Prentice Hall Professional. p. 185. ISBN 978-0-13-035548-5.

[1] Pfleeger, Charles P.; Pfleeger, Shari Lawrence (2013). Security in Computing. Prentice Hall Professional. p. 185. ISBN 978-0-13-035548-5.

[1]

v t e Memory management
Memory management as a function of an operating system
Hardware	Memory management unit (MMU) Translation lookaside buffer (TLB) Input–output memory management unit (IOMMU)
Virtual memory	Demand paging Memory paging Page table Virtual memory compression
Memory segmentation	Protected mode Real mode Virtual 8086 mode x86 memory segmentation
Memory allocator	dlmalloc Hoard jemalloc libumem mimalloc ptmalloc
Manual memory management	Static memory allocation C dynamic memory allocation new and delete (C++)
Garbage collection	Automatic Reference Counting Boehm garbage collector Cheney's algorithm Concurrent mark sweep collector Finalizer Garbage Garbage-first collector Mark–compact algorithm Reference counting Tracing garbage collection Strong reference Weak reference
Memory safety	Buffer overflow Buffer over-read Dangling pointer Stack overflow
Issues	Fragmentation Memory leak Unreachable memory
Other	Automatic variable International Symposium on Memory Management Region-based memory management Memory pool
Memory management Virtual memory Automatic memory management Memory management algorithms Memory management software

Base and bounds

See also

References