Your First Computer

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If you're a new player, you might have already built a few circuits already: an adder, some kind of memory, a decoder. But getting to the next level where you have a functioning and turing complete computer, is incredibly daunting. This article will serve as a guide to how you might build your first computer.

Planning The Computer

The structure of the computer in this article

While you might be inclined to jump head first into building, this results in at the very least messy building in Logic World, or a malfunctioning computer. In our computer we are going to first plan and document the capabilities our computer will have.

To start with the planning section, it helps to think about what this computer we are building is for. This could range from wanting to test out an ALU you built, or to perform a specific program you have in mind, e.g. matrix multiplication.

In this guide, our computer will be built as a way to learn each of the different parts of a computer, without any bells or whistles. So, we are going to give the computer the bare minimum.

Data Width

Data width refers to how many bits the binary numbers in our computer will use. This doesn’t have to be universal across an entire computer; for example, the memory might store in 32 bits, but our ALU only 16. In our computer however, we will use the same data width across our entire computer. While you can definitely go smaller, 8 bits is a good number to make it less complicated while being able to do a lot with our computer, so our data width is going to be 8 bits.

The ROM

The read only memory in our computer is where we are going to store all of the instructions for our computer. We aren’t looking for anything too big, so to keep with our data width of 8 bits, we will have 2^8 or 256 addresses in our ROM (this only needs to be addressed with 8 bits). And each address will store 8 bits of data for our instructions.

Instruction Set

This is the most influential part of planning. It’s tough for a beginner to get a feel for how to design an instruction set, and has the most impact on how easy it will be to build your computer. If you wanted to design a more advanced computer, you should design your own instruction set, and see how that influences your computer.

Every instruction has an opcode that tells the computer what instruction you actually want it to perform. There are roughly 4 main types:

• Data management. This involves moving data from one register to another, loading or saving to memory, or loading an arbitrary value into your registers, etc.
• Operations. These instructions use your ALU to transform data in a specific way, e.g adding two registers or using bitwise logic.
• Conditional logic. This is where jumps come into play. These instructions let you test registers against each other, and change the order the computer executes instructions based on tests.
• Interrupts. While you generally wouldn’t find interrupts in a computer in Logic World, these let you talk to computer hardware, or call specific kernel functions (don’t worry if you don’t understand what that means, it isn’t important for this guide).

With that, this is the instruction set we are going to base our computer around:

Name	Opcode	Opperands	Description
Immediate	00xxxxxxxx	x: The immediate value	Puts a value given by x into register 0.
Add	01000___		Adds the values in register 1 and 2, and puts the result in register 3.
Subtract	01001___		Subtracts the value in register 1 from register 2 and puts the result in register 3.
And	01010___		Bit-wise ANDs the values in register 1 and 2, and puts the result in register 3.
Or	01011___		Bit-wise ORs the values in register 1 and 2, and puts the result in register 3.
XOR	01100___		Bit-wise XORs the values in register 1 and 2 and puts the result in register 3.
Not	01101___		Bit-wise NOTs the values in register 1 and 2 and puts the result in register 3.
Move	10xxxyyy	x: The register to move data from y: The register to move data to	Moves the data from register x into register y.
Test	11000___		Tests the values in register 4 and 5 against each other, putting the result into the flags storage
Jump 0	11001___		Jumps to the instruction in register 6, if the 0 flag is ON.
Jump carry	11010___		Jumps to the instruction in register 6, if the carry flag is ON.
Jump negative	11011___		Jumps to the instruction in register 6, if the carry flag is ON.
Jump	11100___		Jumps to the instruction in register 6, regardless of an flags.

TODO: I will expand this article later