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Concept
Out of memory
Summary
Out of memory (OOM) is an often undesired state of computer operation where no additional memory can be allocated for use by programs or the operating system. Such a system will be unable to load any additional programs, and since many programs may load additional data into memory during execution, these will cease to function correctly. This usually occurs because all available memory, including disk swap space, has been allocated.
Historically, the out of memory condition was more common than it is now, since early computers and operating systems were limited to small amounts of physical random-access memory (RAM) due to the inability of early processors to address large amounts of memory, as well as cost considerations. Since the advent of virtual memory opened the door for the usage of swap space, the condition is less frequent. Almost all modern programs expect to be able to allocate and deallocate memory freely at run-time, and tend to fail in uncontrolled ways (crash) when that expectation is not met; older ones often allocated memory only once, checked whether they got enough to do all their work, and then expected no more to be forthcoming. Therefore, they would either fail immediately with an "out of memory" error (OOME) message, or work as expected.
Early operating systems such as MS-DOS lacked support for multitasking. Programs were allocated physical memory that they could use as they needed. Physical memory was often a scarce resource, and when it was exhausted by applications such as those with terminate-and-stay-resident functionality, no further applications could be started until running applications were closed.
Modern operating systems provide virtual memory, in which processes are given a range of memory, but where the memory does not directly correspond to actual physical RAM. Virtual memory can be backed by physical RAM, a disk file via mmap (on Unix-derivatives) or MapViewOfFile (on Windows), or swap space, and the operating system can move virtual memory pages around as it needs.
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