I 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 19641, 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 ro...