Following the release of Intel-based Macs, third-party platform virtualization software such as Parallels Desktop, VMware Fusion, and VirtualBox began to emerge. These programs allow users to run Microsoft Windows or previously Windows-only software on Macs at near native speed. Apple also released Boot Camp and Mac-specific Windows drivers that help users to install Windows XP or Vista and natively dual boot between Mac OS X and Windows. Though not condoned by Apple, it is possible to run the Linux operating system using Boot camp or other virtualization workarounds. Unlike most PCs, however, Macs are unable to run many legacy PC operating systems. In particular, Intel-based macs lack the A20 gate.
What do Macs lack that makes them unable to run many legacy PC operating systems?
the A20 gate