The Architecture of IBM's Early Computers
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C. J. Bashe W. Buchholz G. V. Hawkins J. J. lngram N. Rochester The Architectureof IBM’s Early Computers Most of the early computers made by IBM for commercial productionare briefly described with an emphasis on archi- tecture and performance. The description covers a period offifteen years, starting with the design of an experimental machine in 1W9 and extending to, but not including, the announcement in 1964 of Systeml360. Introduction This is a technical account of IBM’s principal computers Table 1 illustrates the chronology of the machines dis- during the fifteen-year period beginning in late 1949 and cussed in this paper. The membersof a family or series of leading up to the announcement in 1964 of Systeml360. machines appear vertically in a column; the later entries They wereall electronic stored-program computers. Each are more or less upward compatible with earlier ones. had a random-access memory that was reasonably large (There is nofamily relationship, however, among thema- by the standards of the time, could do arithmetic and chines in the column labeled “Others.”) All machines up logic, could call subroutines, could-and did-translate to 1957 used vacuum-tube electronics; from 1958 on, all from various symbolic languages to its own machine lan- of IBM’s computers were built with transistors. Every guage, and eachhad a substantial setof input/output (I/O) machine listed is discussed orreferred to in the text. equipment. This accountemphasizes architecture and performance, that is, the programmer’s view at the level Except for a discussion of IBM’s precursor machines, of machine language. no attempt is made here to acknowledge adequately the manypioneering projectsand individuals in IBMand The paper begins with a section on machines which elsewhere which made these developmentspossible. The were precursors of the computer era. This is followed 701 clearly owed much to themachine at the Institute for by descriptions of several series of related computers, Advanced Studies, to prior work at the University of each named after its first member: 701, 702, 650, and Pennsylvania,and to Whirlwind at MIT;the 702 was 1401. Next is an essay on the IBM Stretch system, and largely stimulated by the Eckert-Mauchly UNIVAC; and the final section concerns I/O techniques for all these the 650 had early roots in work at Engineering Research machines. Associates. Cathode-ray tube (CRT) memory, as used in the 701 and 702, and index registers are well known to Not all of IBM’s stored-program computers of this pe- have originated atthe University of Manchester. The riod are included; because of space limitations, several names of J. P. Eckert and J. W. Mauchly, of J. von Neu- interesting and important machines are omitted. The se- mann, who was a consultant on the701, and of F. C. Wil- lection is based on a subjective judgment of which ma- liamsreadily cometo mind. To give sufficient credit chines had technically the most impact during those fif- beyondthe obvious, however, would go outsidethe teen years and beyond. scope of this paper. Copyright 1981 by International Business Machines Corporation. Copying is permitted without payment of royalty provided that (1) each reproduction is done without alteration and (2) the Journal reference and IBM copyright notice are included on the first page. The title and abstract may be used without further permission in computer-based and other information-service systems. Permission to republish other excerpts should be obtained from the Editor. 363 IBM J. RES, DEVELOP. 0 VOL. 25 0 NO. 5 0 SEPTEMBER 1981 C. J. BASHE ET AL. Table 1 Chronology of principal IBM computers before Systed360. 701 702 650 1401 Others Date Series Series Series Series 70: 111 9 3 157 announced I I VACUUM TUBES Precursors The decade of the 1950s was a significant one in the his- sity in 1944 and became known as the Harvard Mark I, tory of IBM computers. All of the electronic stored-pro- was based on electromechanical devices for both calcu- gram computers designed for regular production were an- lation andsequence control. The 604 electronic cal- nounced and first delivered after 1950. All earlier devices culating punch, which IBM started shipping in 1948, used that were more than electromechanical accounting ma- electronic counters for calculationand was controlled chines were either one-of-a-kind machines or combina- by a plugboard program of up to60 three-address instruc- tions of electromechanical and electroniccalculating tions. units, which did not incorporate an electronic form of stored program [ 11. A major step was the dedication, also in 1948, of the SelectiveSequence Electronic Calculator (SSEC)at There were, tobe sure, earlier examples of the greater IBM’s headquarters in New York City. It had a hierar- logical power obtained from large numbers of calculating chical memory system consisting of 20 OOO words of pa- elements operating underelaborate sequence controls per-tape storage, 150 words of relay memory, and eight and of the increased speed resulting from substitution of words of electronic memory, each word consisting of 19 electroniccircuits for electromechanicalrelays and digits and a sign. Its electronic arithmetic unit could add, counters. The IBM Automatic Sequence Controlled Cal- subtract, divide, and multiply, the latter in 20 ms. The 364 culator, which was presentedby IBM to HarvardUniver- paper-tape storageconsisted of 66 readers, eachholding a C. J. BASHE ET AL. 1BM J. RES. DEVELOP. VOL. 25 NO. 5 0 SEPTEMBER 1981 I loop of tape as wide and thick as an IBM card. These 0 two pairs of 1250-word-per-second magnetic-tape tapes could hold tables, data to be operated on, or se- drives, adapted from designs developed for the TPM quences of 78-bit instructions. The SSECwas the first op- (702) project, erating computer capable of treating its own stored in- four 2048-word magnetic drums, adapted from earlyde- structions exactly like data, modifying them, and acting signs for the 650 computer, on the result. a 150-line-per-minute printer, 0 a 150-card-per-minute card reader, and In 1949 IBM introduced the Card Programmed Calcu- a 100-card-per-minute card punch. lator (CPC), the concept forwhich originated at Northrop Aircraft. The CPC had a 604 as its arithmetic unit, a card Instructions could address 18-bit half words or 36-bit reader to read the program from punched cards, a 480- full words. The instruction set was simple but carefully digit electromechanical memory, a card punch, and a tab- polished for efficiency and consistency. A 36-bit binary ulating machine to print the output. Itrequired a sorter, a addition took five 12-ps cycles, while multiply and divide collator, and a cart to move decks of cards around. This instructions each took 38 cycles. machine provided the means to do computing on a hith- erto impractical scale, enabled users to develop computing The 701 was made binary, instead of decimal, because methods and expertise, and increased the awareness of fast binaryarithmetic was simpler and thereforeless and interest in the potential of fully electronic, stored-pro- costly, yet it did not mean giving up the advantages of gram computers, which were just coming into existence. decimal and alphabetic input and output. Conversion be- tween external decimal and internal binary representa- The final precursor, the Test Assembly, was such a tions and between punched-card andmagnetic-tape codes computer. This experimental unit was built in the IBM could readily be handled by programming in a binary ma- Poughkeepsie laboratory starting in 1949. It used a 604 as chine and did not require special translation circuits. The its electronic arithmetic unit, a second machine frame choice of binary arithmetic with its superior character- with 604-type circuits as logical elements to exercise con- istics right at the start also set an important precedent: It trol, a 250-word CRT memory that was a prototype of the might have been very difficult to introduce a binary ma- 701 and 702 memories, a magnetic drum memory to store chine later on if all of IBM’s early computers had been both data and instructions, and a card reader-punch for decimal. 110. By solving a differential equation in 1950, the Test Assembly demonstrated that these components could be The 704 used to build a full-fledged, stored-program computer. In the 704, magnetic-core memory with from 4096 to 32768 words replaced theCRT memory of the 701, The 701 series thereby eliminating the most difficult maintenance prob- The 701 was the first large-scale electronic computer that lem and providing users with a more efficient way to run IBM put into production. To make the machine quicker large programs. In the early 1950s, there was a widely and easier to design, implement, and maintain, the archi- held belief among theorists that 1000 words or so of mem- tecture was kept simple and spartan. Many desirable fea- ory should suffice. However, practical programmers tures, such as floating-point arithmetic, were left to be seemed to need more than that. Also, the cost of provid- done by programming. One quarter of the central pro- ing larger memories kept going down as engineering and cessing unit (CPU) remained empty to give flexibility for production methods advanced. correcting errors or adding function. Thesuccessor of the 701, the 704, provided the added functionand filled up the To accommodate the largeraddress space and other re- space. Thenthere followed a succession of larger andfas- quirements, the instruction length was expanded to 36 ter, upward-compatible machines with new features as bits. The instructions were similar in function to the 701 they were invented, culminating in the 7094 I1 and its instructions but were incompatible, and their number was smaller, simplified offshoots, the 7040 and 7044.