Back in the 70s, computers were quite expensive and hence not many people had it. David Hagelbarger of Bell Laboratories designed CARDIAC (CARDboard Illustrative Aid to Computation) back then. It was actually devised as an educational device to help people without access to computers. It helped them to learn how computers worked.
The CARDIAC computer is a single-accumulator single-address machine which means that instructions function only on the accumulator or on the accumulator and a memory location. The machine executes ten instructions and each of these instructions is assigned a three digit decimal opcode. The instruction set architecture contains instructions which are common to simple Von Neumann processors like store, load, conditional branch and add/subtract.
And, operating this computer is really a piece of cake. There are the cardboard slides which will guide a person through the operation of the ALU and instruction decoder and the flow chart will display which stage one must go to next. A cardboard ladybug represents the program counter. It is manually moved through the program memory after the completion of each instruction.
Although CARDIAC is kind of outdated and too unsophisticated, it still serves as a really important tool to train how exactly microprocessors work. Well, even though microprocessors include multi-stage pipelines, finely-turned branch predictors and many other enhancements, the basic principles remain the same.
Via: Hack A Day