View source for IBM 650

The '''IBM 650''' (formally the '''Type 650 Magnetic Drum Data-Processing Machine''') was one of [[International Business Machines|IBM]]'s first computers. It was a very low-end (and thus less expensive) design, using a [[drum]] for [[main memory]], and was thus fairly slow, but it provided an entry into the computing world for those who needed more than a [[calculating punch]] (such as the [[IBM 604]]) but whose computing requirements did not justify the higher end (and more expensive) machines such as the early [[IBM 700 series]].

It was announced in July, 1953, and first shipment was in December, 1954; production continued until 1962. (It was actually IBM's first computer design project, but dithering by IBM management, and conflicting cost and performance goals, delayed it so that it was overtaken to market by the [[IBM 701]].)

Due in part to its low cost, it was the first computer to have a production run of larger than the low ten's; eventually almost two thousand were produced (over 800 by the beginning of 1958 alone), at a peak production rate of one per day. One source opines that "it is safe to say that, for several years, far more computer scientists had their first experience on that machine than on any other".

Internally, it was decimal, rather than [[binary]], using a [[bi-quinary]] representation; although inefficient in storage terms, this representation allowed extensive 'validity checking', since only 10 codes out of the 128 available (with 7 [[bit]]s per digit) were valid. (The machine gained a reputation for reliability, due in part due to the validity checking.) The [[word]] length was 10 digits, although multiplication and division could use 20-digit values (for output and input, respectively).

It used a [[serial computer|serial]] [[arithmetic logic unit|ALU]] (digit-serial, not bit-, though). The two internal [[register]]s in the [[Central Processing Unit|CPU]] (a 10-digit 'distributor', and a 20-digit [[accumulator]]) used [[capacitor]]-based [[memory]].

The [[instruction set]] was double-address, with a two-digit [[opcode]], and 4 digits each of data [[address]], and next [[instruction]]. (The [[fetch]] of the next instruction was overlapped with the [[execute|execution]] of the previous one.)

The drum was 4" in diameter, 16" long, and ran at 12,500 RPM; with a complete rotation time of 4.8 msec, average [[access time]] was thus 2.4 msec (although [[optimum programming]] would reduce that for instructions). There were initially two models, with 10,000 and 20,000 digit memory capacity. Each 'band' (containing 5 tracks) contained 50 words; thus, 20 bands in the former, and 40 in the latter. The time to read a word, once it was at the [[head]]s, was only 100 μsec.

Initially, the only [[input/output|I/O]] was via [[punched card]]s, making it a small step for organizations whose data processing was already built around punched cards; the 650 could be used to simplify an existing punched card operation, and remove clerical steps. The reader punch could read 200 cards per minute, and punch 100, using 10-word [[buffer]]s stored (but not addressable by the CPU) on the drum.

A [[printer]] option was soon added; and eventually (September, 1956) also [[disk]]s, initially with a capacity of 6 million digits per [[drive]], with up to four drives.

==Further reading==

* Charles J. Bashe, Lyle R. Johnson, John H. Palmer, Emerson W. Pugh, ''IBM's Early Computers'', MIT Press, Cambridge, 1986

==External links==

* [http://www.bitsavers.org/pdf/ibm/650/ 650] - documentation at [[Bitsavers]]
** [http://www.bitsavers.org/pdf/ibm/650/24-5000-0_GeneralInfo.pdf 650 Data Processing System Bulletin - General Information]
** [http://www.bitsavers.org/pdf/ibm/650/22-6060-2_650_OperMan.pdf 650 magnetic drum data-processing machine - manual of operation]
** [http://www.bitsavers.org/pdf/ibm/650/22-6270-1_RAM.pdf 650 Data-processing System with 355 Random Access Memory - Manual of Operation] - system with disk drive(s)
* [http://s3data.computerhistory.org/brochures/ibm.650.1955.102646125.pdf The 650 Magnetic Drum Data Processing Machine] - marketing brochure from IBM
* [https://onlineonly.christies.com/s/firsts-history-computing-paul-g-allen-collection/ibm-650-magnetic-drum-data-processing-machine-106/230045 An IBM 650 Magnetic Drum Data Processing Machine] - from Paul Allen's collection at the [[Living Computer Museum|LCM]], includes several images

[[Category: IBM Small Computers]]