The 8 Bit Computer – Instructions and Programming

Find the introduction here.

With the electronics finished and working, the computer now needs to be told what to do. This is realised by implementing instructions that perform small individual tasks on the computer, like moving a value between registers or adding two numbers. These instructions can then be combined to create more complex programs.

Each of these instructions is associated with an opcode (operation code) which is used to identify the instruction in the program in memory. The computer reads an opcode from memory, executes the associated instruction, then reads the next opcode, executes that, and so on. This fetch-execute cycle continues until the program ends, i.e. a special instruction tells the computer to stop executing the program.

To the program, the instructions are atomic, which means they are the smallest steps the program can be divided into. However, in this computer implementation, these instructions are subdivided into even smaller microinstructions (as discussed in the “Modules, Part 5” chapter). Each of these microinstructions is exactly one clock cycle long and defines the hardware control signals that should be set for the execution of the desired behaviour. For example, to read a value from memory, first the target memory address needs to be moved to the memory address register, then the value at that memory location can be read. Some of these microinstructions are also used to implement the fetch-execute cycle (read next opcode and pass it to the control logic).

To manage the programming of these instructions, I wrote an app in C# where you can define the control signals that exist in the computer, use them to program instructions out of microinstructions/steps, and use those instructions to write programs that can run on the computer:

Control signal configuration window
Control signal configuration (not all signals shown here).
Instruction definition window
Instruction definition and microinstruction programming.
Assembly programming window
Editor for writing programs using the defined instructions.

The instructions and programs can then be written to instruction ROM or computer RAM, respectively, using an Arduino (which is controlled from this same app).

With this, you can program the computer to do essentially anything that it’s physically capable of with the provided hardware. The program shown in the third screenshot above prints “Hello World!” to the LCD screen (bottom left). Here is a video of it running:

To avoid waiting too long for the message to appear, the program is running too quickly for the human eye (tens of clock cycles per second), so you can’t really see the individual instructions running – but it is rather spectactular to watch the control signals (white LEDs) going crazy! If you look closely, you can see the “HLT” LED at the right light up pink at the end – this is the “halt” signal that tells the computer to stop running when the program is done.

So with that, we have a working and programmable computer! In the final post for this project, I will discuss the end result and give an outlook of what may come next.

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