The principal designer was Steve Chen. The X-MP shared the 'horseshoe' design of the earlier machine. The processors ran on a 8.5 ns clock compared to 12.5 ns for the Cray-1A, delivering around 55 MFLOPS per processor and 235 MFLOPS for the four processor 1982 machine. The processors also had better chaining support, parallel arithmetic pipes, and shared memory access with multiple pipelines per processor. The system initially ran COS with UniCOS (a System V derivation) running through the guest operating system facility. Unicos became the main OS from 1984.
The X-MP was sold with one, two or four processors and from one to sixteen megawords (8-128 MB) of main memory (initial memory capacity was limited to 16 megawords with a 24 bit address register, the extended memory architecture upgrade raised addressable memory to a theoretical 2 GB). A 1982 X-MP/48 was about $15 million plus the cost of disks.
The Cray-2, a completely new design, was introduced 1985. A very different compact four processor design with from 512 MB to 4 GB of main memory, it was clocked at 500 MFLOPS but was slower than the X-MP on certain calculations due to its high memory latency. In 1986 a X-MP/48 achieved a speed of 713 MFLOPS on the standardized Linpack tests. The Cray Y-MP series was sold from 1988, it was an evolutionary improvement of the X-MP with a new processor and the capacity for up to sixteen.