Neil George Bartow

System/360 model 85 microdiagnostics

System/360 Model 85 is a large central processing unit, (CPU), which contains a machine cycle of 80 nanoseconds and a main storage access of 1.2 microseconds. It has the capability of executing 12,500,000 add register to register type instructions per second. Its major parts are the Instruction Preparation Unit, (I Unit), Instruction Execution Unit, (E Unit), and Storage Control Unit, (SCU). In addition to these three main parts, there is also another portion of the hardware dedicated to maintenance controls. The IBM Model 85 computer has high speed buffer storage and hardware capable of initiating and executing Instruction Retry. There are two major control storage elements. Read Only Storage, (ROS), and Writeable Control Storage, (WCS). ROS consists of 2,048 decimal control words while WCS consists of 512 control words for a standard Model 85 or 1024 control words for a Model 85 with an emulator feature. Each control word is 128 bits long and consists of 33 control fields as illustrated in Figure 1---Model 85 Control Word. Approximately 450 microorders have been implemented in the Model 85 for use in microprogramming.