the 320 and 540 are further limited by having PCI slots (albeit two 66 MHz and one 33 MHz slot) that operated at 3.3V, out of step with the 5v slots used by most manufacturers. This limits the number of accessories that can be added. The other Visual Workstations are built to compete with the new Intel processor based workstations that are considerably cheaper than SGI's line of MIPS workstations. They are little more than standard PCs, and use many parts that are also available in the aftermarket. Operating system The 320 and 540 Visual Workstations shipped originally with Microsoft Windows NT 4.0. Due to the ARCloader PROM, a custom Hardware Abstraction Layer (HAL) for Windows is necessary. Windows 2000 is the last release which included the required SGI-specific HAL. Because of that, and because SGI ceased supporting the Visual Workstation series, installation of later Windows versions such as XP is unsupported. The 230, 330, and 550 also supported Windows NT and 2000. In addition, SGI offered these systems pre-loaded with release 6.2 of the Red Hat Linux distribution. These systems have the letter "L" appended to their model numbers. Because of the various SGI enhancements, in particular for the 320 and 540, Visual Workstations often out-perform Intel PCs of similar configuration in graphically-intensive or memory bound applications. However, due to the hefty upgrade costs for the non-standard components, it was more cost effective to purchase an entire new higher-spec non-SGI PC rather than purchase upgrades to a Visual Workstation. Displays and matching monitor All the Visual Workstations support conventional CRT monitors and have VGA display connectors. As the 230 330, and 550 models use Nvidia Quadro cards they also support DVI. The SGI 320 and 540 models shipped with the groundbreaking and stylistically matched 1600SW LCD display, using an OpenLDI display connector that requires an accessory flat panel adapter. The SGI O2 also supports the 1600SW with an adapter. Models and configurations The model numbers of the Visual Workstations: Visual Workstation 320 - Dual processor Pentium II/III (Slot 1) Visual Workstation 540 - Quad processor Pentium II/III Xeon (Slot 2) Visual Workstation 230/230L - Single processor Pentium III (FCPGA Socket 370) Visual Workstation 330/330L - Dual processor Pentium II/III (FCPGA Socket 370) Visual Workstation 550/550L - Dual processor Pentium III Xeon (Slot 2) Visual Workstations were initially equipped with either a single Pentium II or Pentium III processor or