The primary operating system for the Cray-1 was the Cray Operating System (COS), which was a batch processing system. COS was specifically designed to exploit the hardware capabilities of the Cray-1, focusing on high-speed computation rather than on features like multi-user support. Given its focus on scientific and mathematical computations, COS supported compilers for languages like FORTRAN, which was the dominant language for scientific computing at the time. The Cray FORTRAN Compiler was highly optimized to take advantage of the Cray-1's vector processing capabilities. There was also a set of mathematical libraries optimized for its architecture. These libraries included routines for linear algebra, Fourier transforms, and other mathematical operations critical in scientific computing.