In other systems, such as IBM's MXT, the compression process occurs in a dedicated processor that handles transfers between a local cache and RAM.
Once compressed, they are either stored in a swap file in main memory, or written to auxiliary storage, such as a hard disk.
[1][7] Specifically, in (the data segment -- the WK algorithms are not suitable for instruction compression[1]) target code generated by most high-level programming languages, both integers and pointers are often present in records whose elements are word-aligned.
Additionally, common data patterns such as a word of all zeroes can be encoded in the compressed output by a very small code (two bits in the case of WKdm).
[1][7] By reducing the I/O activity caused by paging requests, virtual memory compression can produce overall performance improvements.
In an opposite state, where enough physical memory is available and paging activity is low, compression may not impact performance enough to be noticeable.
In a typical virtual memory implementation, paging happens on a least recently used basis, potentially causing the compression algorithm to use up CPU cycles dealing with the lowest priority data.
In this case the bar for compression is higher, since the I/O cycle it is attempting to eliminate is much shorter, particularly on flash memory devices.
The price and speed of RAM and external storage have plummeted due to Moore's Law and improved RAM interfaces such as DDR3, thus reducing the need for virtual memory compression, while multi-core processors, server farms, and mobile technology together with the advent of flash based systems make virtual memory compression more attractive.
Acorn Computers' Unix variant, RISC iX, was supplied as the primary operating system for its R140 workstation released in 1989.
However, the principal motivation for providing compressed executable files was to accommodate a complete Unix system in a hard disk of relatively modest size.
[18] Helix Software Company pioneered virtual memory compression in 1992, filing a patent application for the process in October of that year.
[21] In its 8 April 1997 issue, PC Magazine published a comprehensive test of the performance enhancement claims of several software virtual memory compression tools.
Subsequent testing of the technology by Intel showed 5–20% overall system performance improvement, similar to the results obtained by PC Magazine with Hurricane.