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CDC 6000 series

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The CDC 6000 series was a family of mainframe computers manufactured by Control Data Corporation in the 1960s. It consisted of CDC 6400, CDC 6500, CDC 6600 and CDC 6700 computers, which all were extremely rapid and efficient for their time. Each was a large, solid-state, general-purpose, digital computer that performed scientific and business data processing as well as multiprogramming, multiprocessing, time-sharing, and data management tasks under the control of the operating system called SCOPE (Supervisory Control Of Program Execution).

CDC 6600 computer. Display console shown in the foreground, main system cabinet in background, with memory/logic/wiring to the left and middle, and power/cooling generation and control to the right.

The CDC 6000 series computer is composed of four main functional devices: the central memory, one or two high speed central processors, seven to ten peripheral processors, and a display console. The four computer types differ primarily in the number of and kind of central processor. It had a distributed architecture and was a reduced instruction set (RISC) machine many years before such a term was invented.

History

The first member of the CDC 6000 series was the first supercomputer CDC 6600, designed by Seymour Cray and James E. Thornton in Chippewa Falls, Wisconsin. It was introduced in September, 1964 and performed up to three million instructions per second, three times faster than the IBM Stretch, the speed champ for a couple years. It remained the fastest machine for five years until the CDC 7600 was launched. The machine was Freon refrigerant cooled. Control Data manufactured about 100 machines of this type, selling for $6 to $10 million each.

The next system to be introduced was the CDC 6400, delivered in April, 1966. The 6400 central processor was a slower, less expensive, implementation with serial processing - rather than the 6600s parallel functional units. All other aspects of the 6400 were identical to the 6600. Then followed a machine with dual 6400-style central processors, the CDC 6500, designed principally by James E. Thornton, in October, 1967. And finally, the CDC 6700 - with both a 6600-style CPU and a 6400-style CPU, was released in October, 1969.

Subsequent modifications to the series in 1969 included the extension to 20 peripheral and control processors with 24 channels. Control Data also marketed a CDC 6400 with a smaller number of peripheral processors, the CDC 6415-7 with 7 peripheral processors to reduce cost.

Central processor

The central processor was the high-speed arithmetic unit that functioned as the workhorse of the computer. It performed the addition, subtraction, and logical operations and all of the multiplication, division, incrementing, indexing, and branching instructions for user programs. Note that in the CDC 6000 architecture, the central processing unit performed no I/O operations. I/O was totally asynchronous, and performed by peripheral processors.

A 6000 series CPU contained 24 operating registers, designated X0-X7, A0-A7, and B0-B7. The eight X registers were each 60-bits long, and used for most data manipulation - both integer and floating point. The 8 B registers were 18-bits long, and generally used for indexing and address storage. Register B0 was hard-wired to always return 0. By software convention, register B1 was generally set to 1. (This often allowed the use of 15-bit instructions instead of 30-bit instructions.) The eight 18-bit A registers were 'coupled' to their corresponding X registers in an interesting way: setting an address into any of registers A1 through A5 caused a memory load of the contents of that address into the corresponding X registers. Likewise, setting an address into registers A6 and A7 caused a memory store into that location in memory. Registers A0 and X0 were not coupled in this way, so could be used as scratch registers. However A0 and X0 were used when addressing CDCs Extended Core Storage (ECS).

Instructions were either 15- or 30-bits long, so there could be up to 4 instructions per 60-bit word. The op codes were 6 bits long. The remainder of the instruction was either three 3-bit register fields (two operands and one result), or two registers with a 18-bit immediate constant. All instructions were 'register to register'. For example the following COMPASS code loads two values from memory, performs a 60-bit integer add, then stores the result: 

        SA1     X           "SET" REGISTER A1 TO THE ADDRESS OF X (30 BIT INSTRUCTION)
        SA2     Y           "SET" REGISTER A2 TO THE ADDRESS OF Y (30 BIT INSTRUCTION)
        IX6     X1+X2       LONG INTEGER ADD X AND Y (15 BIT INSTRUCTION)
        SA6     Z           "SET" REGISTER A6 TO THE ADDRESS OF Z (30 BIT INSTRUCTION)

The central processor used in the CDC 6400 series contained a unified arithmetic element which performed one machine instruction at a time. Depending on instruction type, an instruction could take anywhere from a relatively fast 5 clock cycles (18-bit integer arithmetic) to as many as 68 clock cycles (60-bit population count). The CDC 6500 was identical to the 6400, but included two identical 6400 CPUs. Thus the CDC 6500 could nearly double the computational throughput of the machine.

The CDC 6600 computer, like the CDC 6400, has just one central processor. However, its central processor offered much greater efficiency. The processor was divided into 10 individual functional units - each of which was designed for a specific type of operation. The function units provided were: branch, Boolean, shift, long integer add, floating add, floating divide, two floating multipliers, and two increment (18-bit integer add) units. Functional unit latencies were between a very fast 3 clock cycles (increment add) and 29 clock cycles (floating divide).

The 6600 processor could issue a new instruction every clock cycle - assuming that various processor (functional unit, register) resources were available. These resources were kept track of by a scoreboard mechanism. Also contributing to keeping the issue rate high was an instruction stack - which cached the contents of several instruction words. Small loops could reside entirely within the stack - eliminating memory latency from instruction fetches.

Both the 6400 and 6600 CPUs had a cycle time of 100 ns (10 MHz). Due to the serial nature of the 6400 CPU, its exact speed was heavily dependent on instruction mix, but generally around 1 MIPS. Floating adds were fairly fast at 11 clocks, however floating multiply was very slow at 57 clocks. Thus its floating point speed would depend heavily on the mix of operations and could be under 200k FLOPS. The 6600 was, of course, much faster. With good compiler instruction scheduling, the machine could approach its theoretical peak of 10 MIPS. Floating point adds took 4 clocks, and floating point multiplies took 10 clocks (but there were two multiply functional units - so two operations could be processing at the same time.) The 6600 could therefore have a peak floating point speed of 2-3 MFLOPS.

The CDC 6700 computer combined the best futures of the other three computers. Like the CDC 6500, it had two central processors. One was a CDC 6400/CDC 6500 central processor with the unified arithmetic section; the other was the more efficient CDC 6600 central processor. The combination made the CDC 6700 the fastest and the most powerful of the four CDC 6000 series.

Central memory

In all the CDC 6000 series computers, the central processor communicates with up to seven simultaneously active programs (jobs), which reside in central memory. Instructions from these programs are read into the central processor registers and are executed by the central processor at scheduled intervals. The results are then returned to central memory.

Information is stored in central memory in the form of words. The length of each word is 60 binary digits (bits). The highly efficient address and data control mechanisms involved permit a word to be moved into or out of central memory every 100 nanoseconds.

An extended core storage unit (ECS) provides additional memory storage and enhances the powerful computing capabilities of the CDC 6000 series computers.

Peripheral processors

The central memory shares access to central memory with ten peripheral processors. Unlike the central processor, each peripheral processor is an individual computer with its own memory. Peripheral processors are used primarily for input/output, the transfer of information between central memory and peripheral devices such as disks and magnetic tape unit. They relieve the central processor of all input/output tasks, so that it can perform calculations while the peripheral processors are engaged in input/output functions. This feature promotes rapid overall processing of user programs. Each peripheral processor can add, subtract, and perform logical operations. Each executes SCOPE routines independently of the others.

Data channels

For input or output, each peripheral processor accesses a peripheral device over a communication link called a data channel. One peripheral device can be connected to each data channel; however, a channel can be modified with hardware to service more than one device.

Each peripheral device can communicate with any peripheral device if another peripheral processor is not using the data channel connected to that device. In other words, only one peripheral processor at a time can use a particular data channel.

Display console

Console for CDC 6600

In addition to communication between peripheral devices and peripheral processors, communication takes place between the computer operator and the operating system. This was made possible by the computer console, which had two CRT screens.

The console screens were calligraphic, not raster based. Analog circuitry actually steered the electron beams to draw the individual characters on the screen. One of the peripheral processors ran a dedicated program called "DSD" (Dynamic System Display), which drove the console. Coding in DSD needed to be fast - as it needed to continually redraw the screen fast enough to avoid visible flicker.

DSD displayed information about the system and the jobs in process. The console also included a keyboard through which the operator could enter requests to modify stored programs and display information about jobs in or awaiting execution.

A full screen editor, called "O26" (after the IBM model 026 key punch, with the first character made alphabetic due to operating system restrictions), could be run on the operator console. This text editor appeared in 1966 - years before full screen editing was available on other systems. (Unfortunately it took CDC another 15 years to offer FSE - a full screen editor for normal time-sharing users on CDCs Network Operating System.)

There were also a variety of games that were written using the operator console. These included BAT (a baseball game), KAL (a kaleidoscope), DOG (Snoopy flying his doghouse across the screens), ADC (Andy Capp strutting across the screens), EYE (changed the screens into giant eyeballs, then winked them), PAC (a Pac-man-like game), and more.

Minimum configuration

The minimum hardware requirements of a CDC 6000 series computer system consists of the computer, including 32,768 words of central memory storage, any combination of disks, disk packs or drums to provide 24 million characters of mass storage, a punched card reader, punched card punch, printer with controllers, and two 7-track magnetic tape units. Larger systems can be obtained by including optional equipment such as: additional central memory, extended core storage, (ECS), additional card readers, punches, printers, and tape units. Graphic plotters and microfilm recorders are also available.

References

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