Daisy I: Logic gates

All computer chips are made from the same building blocks: elementary logic gates. A logic gate is a physical device that implements a mathematical logic (Boolean) function. The simplest example of a logic gate is the NOT gate. It takes a single input and outputs its inverse. If the signal is ON, the inverter outputs OFF and vice versa. These gates can be constructed in many different ways but their logic behaviour remains consistent across all computers.

For this project, I started with the Nand gate and then built all other gates from it. These standard set of gates were then used to construct the computer’s processing and storage chips.

Nand: the special one

The Nand gate is designed to compute the following Boolean function:

| Input A | Input B | Nand(A, B) |
| ------- | ------- | ---------- |
| 0       | 0       | 1          |
| 0       | 1       | 1          |
| 1       | 0       | 1          |
| 1       | 1       | 0          |

The Nand function has an interesting property. Each one of the operations And, Or and Not can be constructed from it, and it alone.

E.g. x Or y = (x Nand x) Nand (y Nand y)

Since every Boolean function can be constructed from And, Or and Not operations using the canonical representation method, it follows that every Boolean function can be constructed from Nand function alone. This is why Nand gate was chosen as the primitive gate and the foundational element of all other gates in the project.

Elementary gates

  • Nand (primitive)
  • Not
  • And
  • Or
  • Xor
  • Mux
  • Dmux
  • Not16, a 2-input bitwise NOT, where each input is 16 bits wide
  • And16, a 2-input bitwise AND, where each input is 16 bits wide
  • Or16, a 2-input bitwise OR, where each input is 16 bits wide
  • Mux16, a 2-input, 16-bit multiplexor
  • Or8Way, a 8-input OR gate
  • Mux4Way16, a 4-input, 16-bit multiplexor
  • Mux8Way16, a 8-input, 16-bit multiplexor
  • DMux4Way, a 4-output demultiplexor
  • DMux8Way, a 8-output demultiplexor

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