N4000 was upgraded with newer processors throughout its life, with models called N4000-36, N4000-44 and N4000-55 indicating microprocessor clock frequencies of 360, 440, and 550 MHz, respectively. It was renamed to the rp7400 series in 2001. L-class The L-class servers are 7U rackmount machines with up to 4 CPUs (depending on model). They have 12 PCI slots, but only 7 slots are enabled in the entry-level L1000 system. Two of the PCI slots are occupied by factory integrated cards and cannot be utilized for I/O expansion by the end-user. The L1000 and L2000 are similar to the A400 and A500, being based on an Astro/Elroy combination. They initially shipped with 360 MHz and 440 MHz PA-8500 and were upgraded with 540 MHz PA-8600. The L3000 is similar to the N4000, being based on a DEW/Ike/Elroy combination. It shipped only with 550 MHz PA-8600 CPUs. The L-class family was renamed to the rp5400 series in 2001. A-class The A180 and A180C were 32-bit, single-processor, 2U servers based on the PA-7300LC processor with the Lasi and Dino ASICs. The A400 and A500 servers were 64-bit, single and dual-processor 2U servers based on the PA-8500 and later processors, using the Astro IOMMU and Elroy PCI adapters. The A400-36 and A500-36 machines used the PA-8500 processor running at 360 MHz; the A400-44 and A500-44 are clocked at 440 MHz. The A500-55 uses a PA-8600 processor running at 550 MHz and the A500-75 uses a PA-8700 processor running at 750 MHz. The A-class was renamed to the rp2400 series in 2001. S/X-class The S- and X-class were Convex Exemplar SPP2000 supercomputers rebadged after HP's acquisition of Convex Computer in 1995. The S-class was a single-node SPP2000 with up to 16 processors, while the X-class name was used for multi-node configurations with up to 512 processors. These machines ran Convex's SPP-UX operating system. V-class The V-class servers were based on the multiprocessor technology from the S-class and X-class. The V2200 and V2250 support a maximum of 16 processors, and the V2500 and V2600 support a maximum of 32 processors. The V-class systems are physically large systems that need extensive cooling and three-phase electric power to operate. They provided a transitional platform between the T-class and the introduction of the Superdome. Operating systems Apart from HP-UX and Domain/OS (on the 400), many HP 9000s can also run the Linux operating system. Some PA-RISC-based models are able to