Following on the heels of Apple's release of release of Boot Camp, which allows Intel-based Macs to dual-boot Mac OS X and Windows, comes a virtualisation system allowing both operating systems to run at the same time.

Parallels, a Virginia-based startup, released a beta of the system, Parallels Workstation 2.1 for Mac OS X, a day after Apple's announcement of Boot Camp. The test version offers a fully functional 30-day trial, with a final release due in the next few weeks, Parallels said.

The product allows any version of Windows or DOS, or most other x86 operating systems, for that matter, to run in a virtual machine in a window on the Mac desktop, Parallels said. The company promised near-native performance.

PowerPC-based Macs have been able to run Windows for some time using software such as Virtual PC, but it has always been necessary for such software to emulate x86 hardware, creating a significant performance drag. Virtualisation, by contrast, makes the same hardware available to several different operating systems at once, and should provide near-native performance, as long as the operating systems are all compatible with the hardware.

The release of Intel-based Macs theoretically makes this option available to Mac OS X, but so far Parallels Workstation is the only near-complete product available. VMWare has said it is running its system on Mac OS X in the labs already, and Microsoft has said it plans at some point to port Virtual PC to Intel-based Macs, but neither company has given a release date. Other Mac virtualisation projects, such as Free Q and Darwine (which uses different principles), are in their early stages.

CodeWeavers has said it plans to offer a Mac OS X version of its CrossOver Office software, which allows the Windows version of Microsoft Office to run on other platforms. The product has been available on Linux for several years.

Parallels uses its own patent-pending hypervisor to provide virtualised access, a technique also used by other projects such as Xen.

The software offers support for Intel's VT - Virtualization Technology - which offers a significant performance boost, according to Parallels. The downside is that not all Intel hardware includes VT: The Intel-based MacBook Pro and iMac use VT, but the Mac Mini doesn't. VT-based hardware adds up to a 150 percent performance increase, Parallels said.

The software has some limitations, such as a 1.5GB memory limit for each virtual machine.

Industry observers have noted that dual-booting is still superior for high-performance tasks, such as gaming, since it provides direct access to hardware.

Apple has said it has no plans to offer Windows virtualisation for the Mac.