Chapter 41 ½ The Structure of SYSTEM/360 715
amount of hardware provided for other channels is somewhere in between these extremes.
The disparity in the amount of channel hardware reflects the extent to which the channels share CPU hardware in accomplishing their functions. Such sharing is done at the expense of increased interference with the CPU, of course. This interference ranges from complete lock-out of CPU operations at high data rates on some of the smaller models, to interference only in essential references to main storage by the channel in the large models.
Channel/System Relationship
When the channels are viewed in their relationship to the whole system, the three factors of speed, size, and simultaneity take on a different aspect. The channel is viewed as a system component, and its effect on system throughput and other system capabilities is of concern. The speeds of the channels vary from a maximum rate of about 16 thousand bytes per second (byte interleaved mode) on the multiplexor channel of Model 30 to a maximum rate of about 1250 thousand bytes per second on the channels of Models 60, 62, and 70. The size of each of the channels is the same, in the sense that each handles an 8-bit byte at a time and each can connect to eight different control units. A slight size difference exists among multiplexor channels in terms of the maximum number of subchannels.
The degree of channel simultaneity differs considerably among the various models of SYSTEM/360. For example, operation of the Model 30 or 40 multiplexor channels in burst mode inhibits all other activity on the system, as does operation of the special high-speed channel on Model 50. At the other extreme, as many as six selector channels can be operating concurrently with the CPU on Models 60, 62, or 70. A second type of simultaneity is present in the multiplexor channels available on Models 30, 40, and 50. When operating in byte interleaved mode, one of these channels can control a number of concurrently operating input/output devices, and the CPU can also continue operation.
Differences in Application Emphasis
The models of SYSTEM/360 differ not only in throughput but also in the relative speeds of the various operations. Some of these relative differences are simply a result of the design choices described in this paper, made to achieve the desired overall performance. The more basic differences in relative performance of the various operations, however, were intentional. These differences in emphasis suit each model to those applications expected to comprise its largest usage.
Thus the smallest system is particularly aimed at traditional commercial data processing applications. These are characterized by extensive input/output operations in relation to the internal processing, and by more character handling than arithmetic. The fast selector channels and character-oriented data paths of Model 30 result from this emphasis. But despite this emphasis, the general-purpose instruction set of SYSTEM/360 results in much better scientific application performance for Model 30 than for its comparable predecessors.
On the other hand, the large systems are expected to find particularly heavy use in scientific computation, where the emphasis is on rapid floating-point arithmetic. Thus Models 60, 62, and 70 contain registers and adders that can handle the frill length of a long format floating-point operand, yet do character operations one byte at a time.
No particular emphasis on either commercial or scientific applications characterizes the intermediate models. However, Models 40 and 50 are intended to be particularly suitable for communication-oriented and real-time applications. For example, Model 50 includes a multiplexor channel, storage protection, and a timer as standard features, and also provides the ability to share main storages between two CPU's in a multiprocessing arrangement.
References
Peacock [n.d.].