472 Part 5 The PMS level Section 4 Network computers and computer networks

why we consider the 6600 to be fundamentally a network. Each Cio (actually a general-purpose, 12-bit C) can easily serve the specialized Pio function for Cc. The Mp of Cc is an Ms for a Cio, of course. By having a powerful Cio, more complex input-output tasks can be handled without Cc intervention. These tasks can include data-type conversion, error recovery, etc. The K's which are connected to a Cio can also be less complex. Figure 2 has about the same information as Thorton's Fig. 1 block diagram (Chap. 39).

A detailed PMS diagram for the C('6400, '6416, '6500, and '6600) is given in Fig. 3. The interesting structural aspects can be seen from this diagram. The four configurations, 6400 ~ 6600, are included just by considering the pertinent parts of the structure. That is, a 6416 has no large Pc; a 6400 has a single straightforward Pc; a 6500 has two Pc's; and the 6600 has a single powerful Pc. The 6600 Pc has 10 D's, so that several parts of a single instruction stream can be interpreted in parallel. A 6600 Pc also has considerable M.buffer to hold instructions so that Pc need not wait for Mp fetches.

The implementation of the 10 Cio's can be seen from the PMS diagram (Fig. 3). Here, only one physical processor is used on a time-shared basis. Each 0.1 m s a new logical P is processed by the physical P. The 10 Mp's are phased so that a new access occurs each 0.1 m s. The 10 Mp's are always busy. Thus the i.rate is 10 x 12 b/m s or 120 megabits/s. This process of shifting a new Pc state into position each 0.1 m s has been likened to a barrel by CDC. A diagram of the process is shown in Fig. 4.

The T's, K's, and M's are not given, although it should be mentioned that the following units are rather unique: a K for the management of 64 telegraph lines to be connected to a Cio; an Ms(disk) with four simultaneous access ports, each at 1.68 megachar/s data transfer rate, and a capacity of 168 megachar; an Ms(magnetic tape) with a K( # 1:4) and S to allow simultaneous transfers to 4 Ms; the T (display) for monitoring the system's operation; K's to other C's and Ms's; and conventional T(card reader, punch, line printer, etc.).

The ISP description of the Pc is given in Appendix 1, Chap. 39. The Pc has a very clean, straightforward scientific-calculation-oriented ISP. We can consider it a variation on the general-register structure because the Pc state has three sets of general registers. Their use is explained both in Chap. 39 and its Appendix 1. This structure assumes that a program consists of several read accesses to a large array(s), a large number of operations on these accessed elements, followed by occasional write accesses to store results. We would agree that this is a valid assumption for scientific programs (e.g., look at a FORTRAN arithmetic statement), and it is probably valid for most other programs as well.

Cc has provisions for multiprogramming in the form of a protection and relocation address. The mapping is given in the ISP description for both Mp and Ms('Extended Core Storage /ECS).

Appendix 2, Chap. 39, has an ISP description of the PCP. Appendix 2 includes a figure which shows the instruction decoding and execution as well. The 6600 PCP is about the same as the early CDC 160. The PCP has an 18-bit A register because it has to process addresses for the large Cc.

One interesting aspect of the 6600 which we question is the lack of communication among all components at the ISP (programming) level. When Pc stops, it has no way of explicitly informing any other components. There are no interprocessor interrupts. An io device cannot interrupt a Pio, nor can Pio's communicate with one another except by polling. The state switching for Pc is, however, elegant, since a Pio can request Pc to stop a job, store Mps, and resume a new task in one instruction. (The t.save + t.restore ~ 2 m s.)

The Cio's functions are data transmission between a peripheral device and the large Cc via the Cio's Mp with some data transformation or conversions; complete task management, including initiation, termination, and error handling; and management of Pc. The Cio's perform in about the same manner as the C('Attached Support Processor) in the N('360 ASP) (Chap. 40, page 506). The operating-system software is managed by a single fixed Cio. The remaining nine Cio's are free, and as io tasks arise in the system, the Cio's assign themselves to particular tasks, carry out the tasks, and then free themselves to take on other tasks. The operating-system software resides in Mp(Pc) (that is, Cc) accessible to all Cio's and includes: