How to Build an 8-Bit Computer

Step 8: Program Memory and RAM

The program memory of your computer will store the instructions to be executed. It will also act as RAM that can store values during the operation of the computer. The program memory consists of three main parts: the memory, the memory address register (MAR) and the multiplexer. The memory is a chip that has 16 bytes of storage. There is a four bit address that is fed into the memory that tells it what byte it should read or write. The MAR stores the current address for the byte to be read or written from the memory. It is constantly feeding into the memory chip unless the computer is in its programming state. A multiplexer allows you to choose between two inputs and output the given input. The multiplexer used in my computer allows you to select from two four bit inputs (the MAR and a manual input). When the computer is in its programming state the manual address is fed into the memory and allows you to program bytes into the computers memory at the address that you define.