All this magic, however, requires suitable software. Most virtualization solutions rely on a software manager to separate the hardware from operating systems and applications. This manager -- also called a virtual machine monitor or hypervisor -- presents "guest" software (OSes and apps) with an abstraction or virtual processor, while selectively granting guest machines access to processor resources, physical memory, interrupt management, and I/O. As a result, each virtual machine acts as if it's running on a dedicated system, without having actual, direct access to hardware.
This design -- as well as virtualization's widespread use in high-end computing -- began in the '60s, as IBM experimented with virtual machine images of an IBM 7044. From there, Big Blue continued to develop the concept, with systems that relied on a piece of software called a Virtual Machine Monitor. That software oversaw the creation and management of each virtual machine, which could run a different operating system than the original.
Soon, not only IBM's AS/400 and Unix solutions but high-end Unix vendors such as Sun Microsystems and HP were offering RISC processor-based servers that supported virtualization or partitioning -- divvying up a CPU just as disk partitions do a hard drive, so jobs could be dynamically assigned to more processors or portions thereof in a load-balancing arrangement. The next step was to bring virtualization to the x86 platform.
This has historically been done entirely through software. Probably the best-known server and workstation virtualization packages hail from EMC's VMware, although recent years have seen a surge of alternatives. Microsoft is also a player, having acquired Connectix Corp. and the latter's Virtual PC programs for both running multiple operating environments on a PC and running PC programs on a Mac.
Microsoft's Virtual Server competes with VMware as a solution for high-end x86 servers (although Microsoft limits its blessing or tech support to multiple instances or versions of Windows instead of a mix of Windows and other operating systems). And IBM remains firmly entrenched in this space, with ties to both VMware and the open-source Xen.
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