How IBM mainframes stored microcode in transformers #VintageComputing #Microcode @IBM @kenshirriff

Ken Shirriff recently came across a Transformer Read-Only Storage (TROS) module that stored microcode in an IBM System/360 mainframe computer.

This unusual storage mechanism used a stack of Mylar sheets to hold 15,360 bits, equivalent to 1920 bytes. By modern standards, this is an absurdly small amount of data, but in 1964, semiconductor read-only memory chips weren’t available, so using Mylar sheets for storage was a reasonable solution. In this blog post, I explain how the TROS module worked and its role in the success of the IBM System/360.

Simplified diagram of TROS storage. Based on Model 40 Functional Units.

The diagram shows the concept behind TROS, simplified to two words of three bits each. The three transformers (square rings) each have a sense winding that generates one bit of output. Each word (A or B) has a drive line that passes either through a transformer (for a 1 bit) or around a transformer (for a 0 bit). In the diagram, drive line B (red) is activated by a current pulse. It generates a pulse (blue) from the second and third transformers, generating the bits 011 for Word B. The wiring for Word A, on the other hand, generates the bits 101. Storing more words is accomplished by threading more drive lines through (or around) the transformers, one for each word. Any bit pattern can be stored, depending on how the drive line is wired.

Ken goes into detail on this method and compares it to other data storage methods in the 1960s before other types of memory were cheap enough to consider.

See the full post here.

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