The IBM System/360 Model 91: Machine Philosophy and Instruction-Handling

The System/360 Model 91 central processing unit provides internal computational performance one to two orders of magnitude greater than that of the IBM 7090 Data Processing System through a combination of advancements in machine organization, circuit design, and hardware packaging. The circuits employed will switch at speeds of less than 3 nsec, and the circuit environment is such that delay is approximately 5 nsec per circuit level. Organizationally, primary emphasis is placed on (1) alleviating the disparity between storage time and circuit speed, and (2) the development of high speed floating-point arithmetic algorithms. This paper deals mainly with item (1) of the organization. A design is described which improves the ratio of storage bandwidth and access time to cycle time through the use of storage interleaving and CPU buffer registers. It is shown that history recording (the retention of complete instruction loops in the CPU) reduces the need to exercise storage, and that sophisticated employment of buffering techniques has reduced the effective access time. The system is organized so that execution hardware is separated from the instruction unit; the resulting smaller, semiautonomous “packages” improve intra-area communication.