The Cray Y-MP was a supercomputer sold by Cray Research from 1988, and the successor to the company's X-MP. The Y-MP retained software compatibility with the X-MP, but extended the address registers from 24 to 32 bits. High-density VLSI ECL technology was used and a new liquid-cooling system was devised. The Y-MP ran the Cray UNICOS operating system. The Y-MP could be equipped with two, four or eight vector processors, with two functional units each and a clock cycle time of 6 ns (167 MHz). Peak performance was thus 333 megaflops per processor. Main memory comprised 128, 256 or 512 MB of SRAM. The original Y-MP (otherwise known as the Y-MP Model D) was housed in a chassis similar to the horseshoe-shaped X-MP, but with an extra rectangular cabinet added in the middle (containing the CPU boards), thus forming a "Y" shape in plan view. The system could be configured with one or two Model D IOSs (Input/Output Subsystems) and an optional Solid State Disk (SSD) of 256 MB to 4GB capacity. The Y-MP had a measured GFLOPS of 2.144 and a peak GFLOPS of 2.667 in both 1988 and 1989. The Model D Y-MP was superseded in 1990 by the Y-MP Model E, which replaced IOS Model D with IOS Model E, providing twice the I/O throughput. The Y-shaped chassis was dropped in favor of one or two rectangular cabinets (each with a separate connected cabinet containing the liquid-cooling system), depending on configuration. Maximum RAM was increased to 2 GB and up to eight IOSs were possible. Model E variants included the Y-MP 2E, Y-MP 4E, Y-MP 8E and Y-MP 8I, the latter being a single-cabinet (I for Integrated) version of the two-cabinet 8E. The 2E and 4E were later available with optional secondary air cooling. The Y-MP M90 was a large-memory variant of the Y-MP Model E introduced in 1992. This replaced the SRAM of the Y-MP with up to 32 GB of slower, but physically smaller DRAM devices. The Y-MP M90 was also available in variants with up to two, four or eight processors (M92, M94 and M98 respectively). Later, the model name was abbreviated to the Cray M90 series.The Y-MP C90 series is described separately. Y-MP EL In 1992, Cray launched the cheaper Y-MP EL (Entry Level) model. This was a reimplementation of the Y-MP architecture in CMOS technology, based on the S-2 design acquired by