Editing IBM 701 (section)

===Memory===
[[Image:Williams tube.agr.jpg|thumb|upright|Williams tube from an IBM 701 at the [[Computer History Museum]]]]
The system used [[vacuum tube]] logic circuitry and electrostatic storage, consisting of 72 [[Williams tube]]s with a capacity of 1024 [[bit]]s each, giving a total memory of 2048 [[Word (data type)|words]] of 36 bits each. Each of the 72 Williams tubes was 3 inches in diameter. Memory could be expanded to a maximum of 4096 words of 36 bits by the addition of a second set of 72 Williams tubes or (later) by replacing the entire memory with magnetic-[[core memory]]. The Williams tube memory and later core memory each had a memory cycle time of 12 microseconds. The Williams tube memory required periodic refreshing, mandating the insertion of [[Memory refresh|refresh cycles]] into the 701's timing. An addition operation required five 12-microsecond cycles, two of which were refresh cycles, while a multiplication or division operation required 38 cycles (456 microseconds). In addition, magnetic drum and magnetic tape were utilized for secondary storage.<ref name="comp-arch-n-org">{{cite book |title=Computer Architecture and Organization |last1=Hayes |first1=John P. |isbn=0-07-027363-4 |date=1978 |page=22}}</ref>