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Parallel Operation in the Control Data66001
James E. Thornton
In the summer of 1960, Control Data began a project which culminated October, 1964 in the delivery of the first 6600 Computer. In 1960 it was apparent that brute force circuit performance and parallel operation were the two main approaches to any advanced computer.
This paper presents some of the considerations having to do with the parallel operations in the 6600. A most important and fortunate event coincided with the beginning of the 6600 project. This was the appearance of the high-speed silicon transistor, which survived early difficulties to become the basis for a nice jump in circuit performance.
The computing system envisioned in that project, and now called the 6600, paid special attention to two kinds of use, the very large scientific problem and the time sharing of smaller problems. For the large problem, a high-speed floating point central processor with access to a large central memory was obvious. Not so obvious, but important to the 6600 system idea, was the isolation of this central arithmetic from any peripheral activity.
It was from this general line of reasoning that the idea of a multiplicity of peripheral processors was formed (Fig. 1). Ten such peripheral processors have access to the central memory on one side and the peripheral channels on the other. The executive control of the system is always in one of these peripheral processors, with the others operating on assigned peripheral or control tasks. All ten processors have access to twelve input-output channels and may "change hands," monitor channel activity, and perform other related jobs. These processors have access to central memory, and may pursue independent transfers to and from this memory.
Each of the ten peripheral processors contains its own memory for program and buffer areas, thereby isolating and protecting the more critical system control operations in the separate processors. The central processor operates from the central memory with relocating register and file protection for each program in central memory.
Peripheral and Control Processors
The peripheral and control processors are housed in one chassis of the main frame. Each processor contains 4096 memory words of 12 bits length. There are 12- and 24-bit instruction formats to provide for direct, indirect, and relative addressing. Instructions provide logical, addition, subtraction, and conditional branching. Instructions also provide single word or block transfers to and from any of twelve peripheral channels, and single word or block transfers to and from central memory. Central memory words of 60 bits length are assembled from five consecutive peripheral words. Each processor has instructions to interrupt the central processor and to monitor the central program address.
To get this much processing power with reasonable economy and space, a time-sharing design was adopted (Fig. 2). This design contains a register "barrel" around which is moving the dynamic information for all ten processors. Such things as program address, accumulator contents, and other pieces of information totalling 52 bits are shifted around the barrel. Each complete trip around requires one major cycle or one thousand nanoseconds. A "slot" in the barrel contains adders, assembly networks, distribution network, and interconnections to perform one step of any peripheral instruction. The time to perform this step or, in other words, the time through the slot, is one minor cycle or one hundred nanoseconds. Each of the ten processors, therefore, is allowed one minor cycle of every ten to perform one of its steps. A peripheral instruction may require one or more of these steps, depending on the kind of instruction.
In effect, the single arithmetic and the single distribution and assembly network are made to appear as ten. Only the memories are kept truly independent. Incidentally, the memory read-write cycle time is equal to one complete trip around the barrel, or one thousand nanoseconds.
Input-output channels are bi-directional, 12-bit paths. One 12-bit word may move in one direction every major cycle, or 1000 nanoseconds, on each channel. Therefore, a maximum burst rate of 120 million bits per second is possible using all ten peripheral processors. A sustained rate of about 50 million bits per second can be maintained in a practical operating system. Each channel may service several peripheral devices and may interface to other systems, such as satellite computers.
Peripheral and control processors access central memory through an assembly network and a dis-assembly network. Since
1AFIPS Proc. FJCC, p5. 2, vol. 26, 1964, pp. 33-40.
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