Editing IBM 650 (section)

===Main memory===
Rotating [[drum memory]] provided 1,000, 2,000, or 4,000 [[Word (computer architecture)|words]] of memory at addresses 0000 to 0999, 1999, or 3999 respectively. Each word had 10 [[Bi-quinary coded decimal#IBM650code|bi-quinary coded decimal digits]], representing a signed 10-digit number or five characters. (Counting a bi-quinary coded digit as seven bits, 4000 words would be equivalent to 35 kilobytes.)<ref>{{cite web |url=https://www.ibm.com/ibm/history/exhibits/650/650_ph09.html |title=IBM Archives: IBM 650 Magnetic Drum |website=IBM |archive-url=https://web.archive.org/web/20230207023510/https://www.ibm.com/ibm/history/exhibits/650/650_ph09.html |archive-date=2023-02-07 |url-status=dead}}</ref><ref>{{cite web |url=https://www.ibm.com/ibm/history/exhibits/650/650_pr4.html |title=IBM Archives: IBM 650 Model 4 announcement |website=IBM |archive-url=https://web.archive.org/web/20231023160555/https://www.ibm.com/ibm/history/exhibits/650/650_pr4.html |archive-date=2023-10-23 |url-status=dead}}</ref>  Words on the drums were organized in bands around the drum, fifty words per band, and 20, 40, or 80 bands for the respective models. A word could be accessed when its location on the drum surface passed under the read/write heads during rotation (rotating at 12,500 [[Revolutions per minute|rpm]], the non-optimized average access time was 2.5 [[millisecond|ms]]).  Because of this timing, the second address in each instruction was the address of the next instruction.  Programs could then be [[Optimum programming|optimized]] by placing instructions at addresses that would be immediately accessible when execution of the previous instruction was completed. IBM provided a form with ten columns and 200 rows to allow programmers to keep track of where they put instructions and data. Later an [[assembly language|assembler]], SOAP (Symbolic Optimal Assembly Program), was provided that performed rough optimization.<ref>{{cite web|url=https://www.drdobbs.com/the-ibm-650/184404809|title=The IBM 650|first=Herb|last=Kugel|date=October 22, 2001|publisher=Dr. Dobb's}}</ref><ref name=soap />

The [[LGP-30]], [[Bendix G-15]] and [[IBM 305 RAMAC]] computers used vacuum tubes and drum memory too, but they were quite different from the IBM 650.

Instructions read from the drum went to a ''program register'' (in current terminology, an [[instruction register]]). Data read from the drum went through a 10-digit ''distributor.'' The 650 had a 20-digit [[Accumulator (computing)|''accumulator'']], divided into 10-digit lower and upper accumulators with a common sign.  Arithmetic was performed by a one-digit adder.  The console (10 digit switches, one sign switch, and 10 bi-quinary display lights), distributor, lower and upper accumulators were all addressable; 8000, 8001, 8002, 8003 respectively.