While SCD closely monitored the progress of an early array processor, the ILLIAC IV, it became clear by 1970 that extending computing capability at NCAR beyond the Control Data Corporation (CDC) 3600 would be provided by either a CDC 7600 or an IBM System 360, Model 195. The CDC 7600 won by a slight margin in benchmark tests run in early 1970. It was delivered in May 1971, just before the computer facility complex was expanded in July.
The CDC 7600 had a small-core memory of 65,536 60-bit words and a a clock speed of 27 nanoseconds. It generally ran at five times the speed of the CDC 6600. This represented the last time that large a general increase of speed was available from a single processor.
The 7600 supported the FORTRAN 70 compiler, and because the operating system had been developed by SCD, users had a uniform software environment on both the 6600 and 7600 (which was difficult because the machines' architectures differed substantially). Most users could run on either system without changes.
(Such a uniform operating environment would not return again until the late 1980s when UNICOS began to be used on the CRAY supercomputers at NCAR. The uniform operating environment would again disappear in the late 1990s with the advent of distributed shared memory computer architectures.)
However, the CDC 7600 was unstable. Its low mean-time-to-failure forced SCD to put extremely good job- and file-recovery procedures into the system. This took a few years to accomplish, by which time the aggravation level was high for users.
The CDC 7600 was replaced in 1983 by a used CRAY-1A.