CPU configurations


The 1131 CPU was built into a free-standing, desk-like enclosure. It included a standard keypunch-type keyboard and Selectric printing mechanism, which wasn't usually used much unless the system ran APL. The CPU logic and core memory were in the back of the "desk" and the power supply and space for a single Ramkit disk drive were at the right, where the desk drawers would be. Above the keyboard, in the casing for the Selectric printer, were 16 toggle switches that could be used for direct entry of data into core (for example, to load a program without a working card reader). They could also be interrogated by running programs, and were often used for debugging or other program mode changes.

Models of the 1131 were available with or without disk storage, with memory cycle speeds of 2.2 microseconds or 3.6 microseconds, and with 4KW to 32KW core memory (that is, 8KB to 64KB). Normally the CPU cycle time (time required to do one machine step, normally several of those per instruction) was the same as the memory cycle time, but as an very-low-end model IBM also sold a 5.85-microsecond CPU-cycle-time system (with 3.6-microsecond core). This system, the Model 4, was actually too slow to service the 1132 Printer's interrupts; see the Printers page for a story about that.

The CPU is constructed with hard-coded (that is, not microcoded) logic based on Solid Logic Technology (SLT) modules, which IBM also used in construction of the System/360; monolithic integrated circuits in 1965 were considered too new and too risky, especially since System/360 was already a bet-the-company project. SLT modules consist of small printed circuit boards with edge connectors, several discrete components, and several 1/2 inch square metal SLT circuit cans. These circuit cans are hybrid modules, consisting of transistor and diode dice placed onto ceramic plates printed with connective wiring. Connections within the SLT can were made by placing tiny solder balls at the junctions, then heating the entire assembly to melt the solder. (SLT technology is quite an interesting topic in itself; see for example the 1964 IBM Journal article announcing SLT and this review of IBM circuit-packaging technologies.)

Core memory, used in the 1131, was the highest-performance memory device available when the 1130 was introduced. Another benefit for IBM was that the company had developed highly efficient production techniques for producing it, and its profit margins were enormous. 1131s with up to 8KW core had the shape shown in this photo; models with more core had it mounted in an extension of the "desk" to the left.