Alpha, originally known as Alpha AXP, is a 64-bit reduced instruction set computing (RISC) instruction set developed by Digital Equipment Corporation (DEC), designed to replace their 32-bit VAX complex instruction set computer (CISC) instruction set. Alpha was implemented in microprocessors originally developed and fabricated by DEC. These microprocessors were most prominently used in a variety of DEC workstations and servers, which eventually formed the basis for almost all of their mid-to-upper-scale lineup. Several third-party vendors also produced Alpha systems, including PC form factor motherboards. Operating systems that supported Alpha included OpenVMS (previously known as OpenVMS AXP), Tru64 UNIX (previously known as DEC OSF/1 AXP and Digital UNIX), Windows NT (discontinued after 4.0 SP6), and Windows 2000 RC1), Linux (Debian GNU/Linux, SUSE Linux, Gentoo Linux and Red Hat Linux), BSD UNIX (NetBSD, OpenBSD and FreeBSD up to 6.x), Plan 9 from Bell Labs, as well as the L4Ka::Pistachio kernel. The Alpha architecture was sold, along with most parts of DEC, to Compaq in 1998. Compaq, already an Intel customer, decided to phase out Alpha in favor of the forthcoming Hewlett-Packard/Intel Itanium architecture, and sold all Alpha intellectual property to Intel in 2001, effectively killing the product. Hewlett-Packard purchased Compaq later that same year, continuing development of the existing product line until 2004, and promising to continue selling Alpha-based systems, largely to the existing customer base, until October 2006 (later extended to April 2007).