9]"/> time requirements can be relaxed somewhat. For example, a small DEC disk system for the PDP-8 holds up to 128k 12-bit words and has an average access time of 17 µsec and a transfer rate of 16,000 12-bit words per sec. It costs $6000 for the first 32k of capacity, plus $3000 for each additional 32k, including interfacing through the direct data channel. Larger and faster versions are available. Disks (or drums) should be important in future systems. Magnetic tapes of the IBM-compatible type are valuable, especially for communication with machines at computing centers, but tape drives and interfacing are usually expensive. It often costs $25,000 or more to get a single tape drive in service, although the next few are usually less expensive. The cheapest tape drives available cost about $5000. The cost of interfacing depends greatly on the particular computer. It may be as little as $5000, but it is often in the neighborhood of $15,000 or $20,000.

7. Program Input Method

Because they provide immediate access, the most satisfactory program storage media are magnetic disks and drums, followed by the IBM tape. The most satisfactory cheap device for input of programs is the high-speed, punched-tape reader, but the advantages of using small "cartridge-type" magnetic tapes have recently been emphasized. Recently, card readers have appeared which are much cheaper than the older IBM models. They can read 200-300 cards per minute. They cost about $2000 plus interfacing. Examples: Soroban, General Devices, Uptime.

A simple means of restoring the basic loader program (other than toggling!) is desirable. Many computers have this feature, e.g., the IBM 360 series; the SDS Sigma 2, Sigma 5, and 910 PDP-9.

8. Memory Protection

Hardware memory protection is necessary in multiprogram systems. It is very helpful in any machine with a batch-processing resident monitor and in other special situations.

9. Parity Check

This feature is useful for purposes as detecting memory failures, but it is usually not worth its cost in computer speed and in capital investment in the case of a small system.

10. Ease and Cost of Interfacing

This is a big subject, partly because the organization of computers for input and output of data varies with the manufacturer. Some computers such as the Hewlett-Packard and the DEC models are especially easy to interface, whereas the automatic channels of the SDS Sigma computers and the ordinary IBM machines (e.g., the 360 series) are very difficult. The IBM machines require an expensive control unit. It is said that before a competent engineer could order plug boards for Sigma interfacing he would have to study the system for a month or two. However, once interfaced, these machines permit rapid input of data. Interfacing a $5000 Calcomp plotter to the automatic channel of an IBM or Sigma series machine may cost much more than the cost of the plotter.

11. Typewriters

Many small computers use teletype machines as console typewriters. The ASR-33 teletype has not performed well, but it has recently been improved. The ASR-35 and KSR-35 have excellent records, and the newer ASR-37 and KSR-37 (15 characters/sec) are very good. The IBM Selectric has had a mixed reliability record which is, however, improving. In every case, expert routine maintenance is required.

12. Index Registers

These are a valuable asset to efficient programming. At least one, and preferably more, is desirable, especially in the medium and large computers.

13. Line Printers

These are of great use for obtaining a permanent ("hard copy") record, especially when large volumes of output are produced; however, they are expensive, usually costing $20,000 or more (including interfacing). In order to avoid tying up a large central processor during typewriter output of masses of data, a line printer is not only very useful, it is essential for efficient operation (and to spare the typewriter). A line printer can be immensely helpful and can save much time in the process of developing and debugging programs. The cost, however, will often preclude its addition to a modest system. If the system has an IBM-compatible tape drive, the computer output can be written on tape and later carried to a computing center for printing. Several industrial concerns are known to be working on new types of printers, some being dry-copy, nonpercussive types. One type which has already been marketed, the Inktronic printer, operates by spraying ink at the paper from small tubes. The characters are well formed. It operates at about 120 characters per second and costs $5600. Conveniently, it requires standard Teletype interfacing, and it can be ordered with an optional keyboard. Although it has exhibited a few new-product ailments in its first 8 months or so of use, it shows promise of becoming a very useful device. Another printer operating on a similar principle has just appeared—the A.B. Dick Company's Videojet printer, priced at about $4900.

14. Plotters

The overwhelming favorite is still the incremental machine called the Calcomp plotter. It costs about $5000 and is easily interfaced to many computers. It is very accurate (about 0.01 in. in each direction) and provides valuable output to the experimenter. It can be programmed to plot experimental points and theoretical curves together on white paper in India ink, relieving draftsmen of considerable work and doing a more precise job. Other incremental plotters are now on the market, e.g., the Houston Instruments version. Varian has developed an electrostatic plotter to sell for about $15,000.

15. Cathode-Ray Tube Displays

At least four types are in use. The standard scheme involves the displaying of bright spots under control of the computer, which has generated appropriate words to cause x and y deflections of the spot when those words have been transformed by ADC's in the CRT unit. The pattern is rewritten continuously. A light pen held against a particular part of the display pattern can be used to signal the computer. This scheme works well but may produce a flickering image if the computer is interrupted frequently to handle higher priority jobs or if the display is so complicated that the rewriting period exceeds 1/30 sec. The expensive hardware option called a character generator is considered not worthwhile unless large amounts of text are to be displayed. On a 10 in. x 10 in. raster a matrix of dots 1024 x 1024 is sensible.

A second scheme involves a disk or drum on which the computer writes the words to generate the pattern. Separate reading heads send the words to the CRT unit. Thus the display, automatically rewritten over and over, is updated from time to time by the computer. The light-spot cursor and joy-stick method replace the light pen in this case. (In passing, it is worth remarking that a light pen is only as effective as the computer program allows it to be, that the effort of programming for light-pen control is usually not trivial, and that a substantial amount of core storage may be required. A means of display control perhaps not so popular as it should be is sense-switch control.)

A third scheme makes use of a modern storage CRT. The computer sends the pattern to the CRT only once, and the display can persist until erased. This method is flicker-free and inexpensive, but the pattern is not so distinct and sometimes not so bright as in the above schemes. However, it is cheap. Furthermore, the storage tube can be used alternately as an ordinary CRT with quite satisfactory resolution. A storage version is thus possible which reverts to the standard scheme, for high-resolution inspection,