Processors to control terminals and secondary memories (input-output processors)
The first three chapters of this section show the evolution of the IBM Data Channels (io processors) from 1958 (the 7094 II) to the present (the 1800, which came after the 360). The processor approach for controlling T and Ms components, while more general, should be contrasted with the specialized one-instruction controls in the B 5000 (Chap. 22) and Burroughs D825 (Chap. 36).
The fourth chapter, on the DEC 338, shows a processor that controls cathode-ray-tube display consoles. The graphic terminals are the first T's of sufficient complexity to utilize a processor of their own. The first CRT displays used the Pc (e.g., on Whirlwind); then small Pcs were adapted to the task; the DEC 338 is one of the earliest special P.displays that appeared.
There is no example in this section of a specialized P for message concentration and switching. For computer systems multiple remote inputs are still recent enough so that either the main Pc handles the task, via specialized K, or small Pc's are committed to it. However, in the telephone industry there has been a very substantial development by the Bell System of the Electronic Switching System (ESS), which uses specialized Cs to control switching (routing). In computer systems, we can expect the use of such specialized processors to increase in the near future.
The IBM 7094 II
The IBM 709, a member of the IBM 701-7094 II family, is one of the first computers to have an io processor (IBM name: Data Channel) in its structure. Chapter 41 discusses the two Data Channel types: the early 7607 and the later 7909. The 7909 Data Channel ISP, and a K which it controls, are given in Appendix 2 and 3 of Chap. 41. The principal difference is that Pc controls the Pio ('7909) which in turn controls the K, which in turn controls a T or Ms; the Pc controls the Pio ('7607) and the K; the K controls the T or Ms. The series is discussed in Part 6, Sec. 1, page 515.
The structure of System/360
Part I-outline of the logical structure
The io processors (Selector and Multiplexor Channels) in the System/360 have evolved from the IBM 701-7094 II Series. Part 6, Sec. 3 presents the ISP and PMS structures for these processors. Depending on the computer model, the implementations are realized by a microprogrammed processor interpreting a shared control program for both Pios and Pc, or by a hardwired Pio. The multiple Pio's in a 360 Multiplexor Channel, though logically independent, are implemented as a single, shared physical processor.
The IBM 1800
The Pios in this structure are presented in Chap. 33, and the structure is discussed in Part 5, Sec. 2, page 396.
The Digital Equipment Corporation DEC 338 display processor
The DEC 338 is an early P.display. It directly interprets a stored program to control a T.display. Earlier T.displays were controlled by Pc (Whirlwind, Chap. 6), or by a special K.display without stored-program capability, or by a general-purpose Pio. The last method outputs fixed length blocks containing data to be interpreted by T.display as points, vectors, characters, curved line segments, etc. The control of T.display first by Pc, then by a K, then by a Pio, and finally by a P.display has been observed as an evolution [Myer and Sutherland, 1968]. Myer and Sutherland also observe that the evolution is about to become a closed cycle because the generality of a Pc is needed to control a T.display.
Note that the 338 has a very extensive ISP. In fact, the P.displays ISP is more extensive than the companion Pc of the PDP-8 (Chap. 5). There are some display tasks which require Pc, for example, compiling programs (pictures), calculating elaborate light-pen tracking figures, making coordinate and curved lines to straight-line vector approximation transformations, and communicating with other system components.