Eliac I, an introduction

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The ELIAC I

The Extremely Limited Computer (the A is there for the show)

The ELIAC I is a fictional computer, invented by me, for the sole purpose of helping you understand how a computer works internally.

This computer is, contrary to most real computers, a decimal computer. Most (virtually all) real computers work with base two; they are thus binary computers.

Word length: The so-called word length is two decimal digits in this computer. Most computers today have a word length of 32 bits (about 9 or 10 decimal digits) or 64 bits (about 19 decimal digits). The word length is the most common size of blocks of data that a computer works with. When installing programs on a PC, you will sometimes find two versions of the software depending on the word length of the computer.

Memory:  ELIAC I has 100 memory cells, with addresses from 00 to 99, and each cell can contain values from 00 to 99 inclusive, i.e. each cell contains one word. All the cells do initially have the contents 00.

Registers: The CPU has 6 registers, each of them one word in length. One is the so-called program counter (PC) that points to the cell currently used by the program. Then we have five so-called general-purpose registers, named A, B, C, D and E, that are used for all kinds of calculations during the program execution. The registers correspond somewhat to variables in maths.

Flags: A flag is a register that can hold one of two values only, say true or false, on or off or 0 and 1. This CPU has two flags:

  • Carry, CY, that is set (turned on) if an addition gives a result >99 or a subtraction gives a result <0, else it is reset (turned off).
  • Zero, Z, that is set if the result of an addition or subtraction is 0, else it is reset.

Instructions: The CPU can handle a few different instructions. In the text below r and s stand for general-purpose registers, nn is a number with at most two digits, and aa is an address to a memory cell. You can see the instructions on the next page.

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Next page : The instruction setLast modified: Jul 7, 2025 @ 13:55