id="pgepubid00011">2. Small Computers

The small computer system can handle the jobs of data acceptance, data manipulation, and output characteristic of the simple Class 1 operations, but they are suitable for very few jobs involving floating-point arithmetic. In fact, we must usually be skeptical about the use of small machines for any of the Class 2 operations except those of the process-control type, which in many cases would involve little if any arithmetic. (Process-control applications have been rather few to date, but a rapid increase can be expected in this field, especially because of the convenience and low cost of small modern computers.) It is apparent that these machines have been designed as economical instruments specifically intended to handle Class 1 jobs. The smallest word length of a machine in this group, 12 bits, is sufficient for storing in one word the output of a 4096-channel ADC unit, but it is not quite so convenient for handling the output of a typical scaler, which would likely require the use of two words. The capability of even a small computer system to convert experimental information into digital form, to transfer it into memory, to manipulate it, and to present it for inspection in a digested, convenient form, all at a high rate and essentially without error, is of immense value to an experimenter who has to cope with the abundant outflow of data from a modern nuclear experiment.

3. Medium-Sized Computers

The capabilities of medium-sized computers are less clear. These machines are superior to the small ones mainly in two respects: they have a more flexible command structure (i.e., they have a larger set of wired-in operations), and, usually, they have a longer word length. These features make them easier to program and give them a limited, but important, capability to execute floating-point operations sufficiently quickly and accurately for many purposes, even though these operations must in most cases be programmed, in the absence of floating-point hardware. We can reasonably conclude that the medium-sized machines will serve for any use listed in Classes 1 and 2. Certain simpler calculations of Class 3a are also expected to prove feasible, but few, if any, of those of Class 3b.

E. ON CHARACTERISTIC FEATURES OF COMPUTERS AND RELATED EQUIPMENT

The value of any feature depends on its need in the application involved; therefore detailed, absolute statements regarding each characteristic usually cannot be made. However, the Panel has discussed various features at some length, and we present here some general comments on the pros and cons of these features. Among the items discussed are some, such as word length and cycle time, that represent basic, inherent properties of the computer; while a great many others, such as priority interrupts, are customarily offered as options.

1. Word Length

The shorter the word length the cheaper the hardware, generally speaking, but the less the accuracy in calculations unless multiple precision is used. For example, although the 12-bit words of the PDP-8 match the accuracy of data from most ADC's, they are too small not to match the output data from most counters; furthermore, indirect addressing is often required because a single word is too short to include both the operation code and the absolute address of a memory location. Apart from addressing considerations, a 12-bit word is too small for many uses, e.g., in general-purpose pulse-height analyzer applications where 16 bits or, better, 18 bits should be considered a minimum. Fortran programs for numerical calculations are in general best run on machines having at least 32-bit words, although 24-bit words are usually acceptable here when double precision can be used.

2. Number of Memory Words

In general the more words that a system can retain the better; but the greater the memory, the greater the expense. The cost must be weighed against the need. For simple handling of data, a 4k memory may be adequate, but in a large shared-time general-purpose machine a 16k or greater memory is essential. In the latter case, the resident shared-time monitor will probably occupy at least 6k of the memory, so with a 16k memory only 10k would be left accessible to users, and experience has shown that this much can be taken up completely by one user compiling a Fortran IV program. A 4k memory is adequate for many process-control applications, but it is too small for many other applications such as general-purpose pulse-height analyzer use, where an 8k memory is highly desirable. Adding a supplemental rotating memory device (disk or drum), at a cost per word about 1 percent that of core storage, is often preferable to adding core memory. See 6 below.

3. Cycle Time

For most purposes the typical memory cycle time of 1 to 2 µsec is quite adequate. Some of the modern computers have cycle times under 1 µsec.

4. Direct Data Channels

These allow sequential depositing of digital data from external devices directly into blocks of computer memory without intervention of the central processor (direct memory access, DMA). Such input may require only one computer cycle per word, that being the next cycle after the one during which the interrupt signal arrives. This is the fastest means of getting data into memory, but it requires more external hardware and more complex interfacing than input through an accumulator of the central processor. Most data-acquisition machines provide both possibilities. Direct data channels can be valuable for interfacing to magnetic disks, drums, and tapes.

5. Priority Interrupts (Nested)

These can be very useful. They may cost as little as $125 each, depending on the machine, and can be used to reduce greatly the overhead running time losses of the computer. In complicated data-taking applications many interrupt lines are desirable; 8 to 16 priority levels are generally adequate. The usual Fortran compiler cannot compile programs that respond properly to interrupts, although a relocatable object code generated by the compiler can always be assembled with a machine-language subroutine designed to handle interrupts. Enlargement of Fortran compilers for data-acquisition use to include statements designed to handle interrupts is desirable. (See, for example, the discussion of the Yale-IBM system, Chapter 2, Section E.)

6. Mass Storage

Magnetic media—drums, disks, and standard magnetic tapes—are employed here. DEC tapes are useful and reliable, but they have only a small capacity. The use of such microtapes is also limited by their incompatibility with typical computer-center equipment. Reliable, inexpensive incremental magnetic tape units are now available which can be operated asynchronously at about 300 Hz, too slow for many purposes. Some of them can also be run much faster in a synchronous mode. Drums and disks are highly desirable because they provide program-controlled rapid access to great volumes of data. Typically, access times are of the order of 17 µsec. In the past few years, good and inexpensive disks have been developed which are now on the market. Some suppliers are IBM, CDC, Datadisk, Burroughs, DEC, and SDS. Disk storage is cheaper per word than core storage by two orders of magnitude; therefore, it is preferable for applications where data can be organized serially and where access and transfer