CDC 7600 - Relationship With The CDC 6600

Relationship With The CDC 6600

Although the 7600 shared many features of the 6600, including hardware, instructions, and its 60-bit word size, it was not object-code compatible with the CDC 6600. In addition, it was not entirely source-code (COMPASS) compatible, as some instructions in the 7600 did not exist in the 6600, and vice-versa. It had originally been named the CDC 6800, but was changed to 7600 when Cray decided it could not be completely compatible. However, due to the 7600's operating system design, the 6600 and 7600 shared a "uniform software environment" despite the low-level differences.

In fact, from a high-level perspective, the 7600 was quite similar to the 6600. At the time computer memory could be arranged in blocks with independent access paths, and Cray's designs used this to their advantage. While most machines would use a single CPU to run all the functionality of the system, Cray realized that this meant each memory block spent a considerable amount of time idle while the CPU was processing instructions and accessing other blocks. In order to take advantage of this, the 6600 and 7600 left mundane housekeeping tasks, printing output or reading punched cards for instance, to a series of ten smaller 12-bit machines based on the CDC 160A known as Peripheral Processors or PP's. For any given cycle of the machine one of the PP's was in control, feeding data into the memory while the main processor was crunching numbers. When the cycle completed, the next PP was given control. In this way the memory always held up-to-date information for the main processor to work on (barring delays in the external devices themselves), eliminating delays on data as well as allowing the CPU to be built for mathematical performance and nothing else. The PPU could have been called a very smart "communications channel".

Like the 6600, the 7600 used 60-bit words with instructions that were generally 15-bits in length (although there were longer versions). However the instruction set itself had changed to reflect the new internal memory layout, thereby rendering it incompatible with the earlier 6600. The machines were similar enough to make porting of compilers and operating systems possible without too much trouble. The machine initially did not come with software; sites had to be willing to write their own operating system, like NLTSS, NCAROS, and others; and compilers like LRLTRAN .

CDC also manufactured two multi-processor computers based on the 7600, with the model number 7700. They consisted of two 7600 machines in an asymmetric configuration, a central and an adjunct machine. They were used for missile launch and inbound tracking of USSR ICBMs. The radar simulator was a real-time simulator with a CDC 6400 for input/output front-end. These systems were to be used in the Pacific Missile Range. One computer was installed at TRW in Redondo Beach CA (later moved to Kwajalein Atoll, South Pacific), and the second one was installed at McDonnell Douglas in Huntington Beach CA. They were actual 7600s connected by chassis 25 to make them a 7600 MP. The operating system was TOS/BOS (Tactical Operating System/Basic Operating System).