Memory management (operating systems)

In operating systems, memory management is the function responsible for managing the computer's primary memory.:pp-105–208Rollout/rollin allows the temporary, dynamic expansion of a particular job beyond its originally specified region. When a job needs more space, rollout/rollin attempts to obtain unassigned storage for the job's use. If there is no such unassigned storage, another job is rolled out -- i.e., is transferred to auxiliary storage -- so that its region may be used by the first job. When released by the first job, this additional storage is again available, either (1) as unassigned storage, if that was its source, or (2) to receive the job to be transferred back into main storage (rolled in). In operating systems, memory management is the function responsible for managing the computer's primary memory.:pp-105–208 The memory management function keeps track of the status of each memory location, either allocated or free. It determines how memory is allocated among competing processes, deciding which gets memory, when they receive it, and how much they are allowed. When memory is allocated it determines which memory locations will be assigned. It tracks when memory is freed or unallocated and updates the status. This is distinct from application memory management, which is how a process manages the memory assigned to it by the operating system. Single allocation is the simplest memory management technique. All the computer's memory, usually with the exception of a small portion reserved for the operating system, is available to the single application. MS-DOS is an example of a system which allocates memory in this way. An embedded system running a single application might also use this technique. A system using single contiguous allocation may still multitask by swapping the contents of memory to switch among users. Early versions of the MUSIC operating system used this technique. Partitioned allocation divides primary memory into multiple memory partitions, usually contiguous areas of memory. Each partition might contain all the information for a specific job or task. Memory management consists of allocating a partition to a job when it starts and unallocating it when the job ends. Partitioned allocation usually requires some hardware support to prevent the jobs from interfering with one another or with the operating system. The IBM System/360 used a lock-and-key technique. Other systems used base and bounds registers which contained the limits of the partition and flagged invalid accesses. The UNIVAC 1108 Storage Limits Register had separate base/bound sets for instructions and data. The system took advantage of memory interleaving to place what were called the i bank and d bank in separate memory modules.:3-3 Partitions may be either static, that is defined at Initial Program Load (IPL) or boot time or by the computer operator, or dynamic, that is automatically created for a specific job. IBM System/360 Operating System Multiprogramming with a Fixed Number of Tasks (MFT) is an example of static partitioning, and Multiprogramming with a Variable Number of Tasks (MVT) is an example of dynamic. MVT and successors use the term region to distinguish dynamic partitions from static ones in other systems.:73 Partitions may be relocatable using hardware typed memory, like the Burroughs Corporation B5500, or base and bounds registers like the PDP-10 or GE-635. Relocatable partitions are able to be compacted to provide larger chunks of contiguous physical memory. Compaction moves 'in-use' areas of memory to eliminate 'holes' or unused areas of memory caused by process termination in order to create larger contiguous free areas.:94