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Chapter 18

The IBM 1401

The second-generation transistor-technology IBM 1401 has been included both because a large number1 have been produced and because it differs from common fixed word length binary and decimal computers. IBM 1401s are used in business data-processing applications requiring variable-length character strings or fields and rather limited calculating ability. Two specific applications are as a card processor in making a transition from plugboard programmed calculators to full-scale automatic computations and for converting data from one medium to another, for example, from card to tape. The 1401 was little used by the scientific, engineering, and scientific business data-processing communities, probably because of the limited Mp size, the low overall processing speed, and the lack of concurrent I/O operation in the smaller configurations. However, it did achieve considerable use as a stand-alone Cio in C('7090) installations, perhaps because of the speed and quality of the T('1403; line; printer).

Although undoubtedly influenced by machines outside the IBM organization, the IBM 1401 is derived primarily from the IBM 702 and 705, which are variable word length decimal machines. The relationship of the various IBM decimal computers to one another is shown in Fig. 1. (RCA's early computers2 also use a combination of fixed-length and variable-length 7-bit character strings and may have influenced the 1401.)

The IBM 1401's ISP was the first to be adopted by another company. Honeywell defined its H-200 ISP to be a superset of the IBM 1401 ISP. The ISP of the H-200 is more complex and increases performance by organizing Mp by both characters and words.

The IBM 1401, 1440, and 1460 are the only IBM computers to be completely character-string oriented. That is, both instructions and data are stored in variable-length character strings; these strings are addressed by a pointer register to the string. The address integer is fixed at three characters. The encoding process for addresses is given in Appendix 1 of this chapter. The 3-character address (3 x 6 bits) is assigned as 3 x 4 bcd characters for encoding addresses 0:999; 2 x 2 bits for selecting 16 x 1,000 addresses; and 2 bits for selecting one of the three index registers.

The IBM 1620 processes variable-length data strings, although the instruction length is a fixed 12-digit string corresponding to a word in Mp. The 1620, though not identical to the 1401, is almost a member of the same family.

The 1401 evolved. Figure 1 shows the evolution of "features" which have created new computers. The 1401's optional features are mainly design afterthoughts; they sometimes increase performance, sometimes make certain operations possible, and sometimes provide substantive change. There are approximately 19 features in the 1401: memory expansion beyond the anticipated 4,000 characters and index registers required encoding the field bits of the A and B addresses; store A-Address and store B-Address register

Fig. 1. IBM decimal and character-string computer relationships.

1Up to 1966, more 1401s were produced than any other model. An estimated 7,500 1401s, 1,500 1401 G's (card-only system), 3,600 1440s, and 1,500 1460s were produced. About 1,800 1620s were produced.
2RCA 301, 501, and 601.


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