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CARDIAC (CARDboard Illustrative Aid to Computation) is a learning aid developed by David Hagelbarger and Saul Fingerman for Bell Telephone Laboratories in 1968 to teach high school students how computers work. The kit consists of an instruction manual and a die-cut cardboard "computer".

The computer "operates" by means of pencil and sliding cards. Any arithmetic is done in the head of the person operating the computer. The computer operates in base 10 and has 100 memory cells which can hold signed numbers from 0 to ±999. It has an instruction set of 10 instructions which allows CARDIAC to add, subtract, test, shift, input, output and jump.

Hardware

The “CPU” of the computer consists of 4 slides that move various numbers and arrows to have the flow of the real CPU (the user's brain) move the right way. They have one flag (+/-), affected by the result in the accumulator.

Memory consists of the other half of the cardboard cutout. There are 100 cells. Cell 0 is “ROM”, always containing a numeric "1"; cells 1 to 98 are “RAM”; available for instructions and data; and cell 99 can best be described as “EEPROM”.

Memory cells hold signed decimal numbers from 0 to ±999 and are written with a pencil. Cells are erased with an eraser. A “bug” is provided to act as a program counter, and is placed in a hole beside the current memory cell.

Programming

CARDIAC has a 10 instruction machine language. An instruction is three decimal digits (the sign is ignored) in the form OAA. The first digit is the op code (O); the second and third digits are an address (AA). Addressing is one of accumulator to memory absolute, absolute memory to accumulator, input to absolute memory and absolute memory to output.

High level languages have never been developed for CARDIAC as they would defeat one of the purposes of the device: to introduce concepts of assembly language programming.

Programs are hand assembled and then are penciled into the appropriate memory cells.

Instruction Set

CARDIAC Instruction Set
Opcode	Mnemonic	Instruction	Description
0	INP	Input	take a number from the input card and put it in a specified memory cell.
1	CLA	Clear and add	clear the accumulator and add the contents of a memory cell to the accumulator.
2	ADD	Add	add the contents of a memory cell to the accumulator.
3	TAC	Test accumulator contents	performs a sign test on the contents of the accumulator; if minus, jump to a specified memory cell.
4	SFT	Shift	shifts the accumulator x places left, then y places right, where x is the upper address digit and y is the lower.
5	OUT	Output	take a number from the specified memory cell and write it on the output card.
6	STO	Store	copy the contents of the accumulator into a specified memory cell.
7	SUB	Subtract	subtract the contents of a specified memory cell from the accumulator.
8	JMP	Jump	jump to a specified memory cell. The current cell number is written in cell 99. This allows for one level of subroutines by having the return be the instruction at cell 99 (which had '8' hardcoded as the first digit.
9	HRS	Halt and reset	move bug to the specified cell, then stop program execution.

Operation

Programs are run by first sliding three slides so that the number in the instruction register equals the number in the memory cell the bug is sitting in. Once that is done the bug is moved to the next memory cell. The user then follows an arrow which will then tell them what to do next. This continues for all of program execution.

External links

a discussion of the CARDIAC with examples and a simulator in JavaScript
an interpreter and interactive debugger on the .NET platform on GitHub
cinc/jcinc – a simulator for the Java platform with both command-line and GUI interfaces on SourceForge
Software Emulator for the CARDIAC – hosted on Macintosh, Windows (untested), and Linux (untested); provides editor, simple assembler, and examples; opcodes 0 and 9 are interchanged as described on the page; source code appears to be proprietary; free downloads
Quick Tour of the Built CARDIAC computer on YouTube
CPU Design on Paper – Al Williams' spreadsheet version of CARDIAC on the Dr. Dobb's site
Paper to FPGA – Al Williams' FPGA implementation of CARDIAC on an FPGA board (final article of a series on Dr. Dobb's web site)
Emulator written in RUST - This is a RUST implementation of an emulator for the CARDIAC