16-bit Hack Computer

Project overview

The following describes in detail how I implemented the Hack Computer from the nand2tetris course, which includes:

• The ALU and supporting logic
• The CPU (registers, program counter, control)
• A Hack assembler that translates .asm → .hack

For full course details and reference material see the nand2tetris site: https://www.nand2tetris.org

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Hack machine — key specifications

Instruction formats

• A-instruction: @value

  • 16 bits -> Low 15 bits hold a decimal constant (or, in the full assembler, a symbol address).

• C-instruction: dest=comp;jump

  • 16 bits: 111 a c1 c2 c3 c4 c5 c6 d1 d2 d3 j1 j2 j3
    • a — selects A (a=0) vs M (a=1) as second ALU operand
    • c1..c6 — computation encoding bits
    • d1..d3 — register destination bits
    • j1..j3 — jump bits

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ALU - Schematic

ALU operations

• Bitwise: !D, !A, !M, D&A, D|A, D&M, D|M
• Arithmetic: D+1, A+1, M+1, D-1, A-1, M-1, D+A, D-A, A-D, D+M, D-M, M-D

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CPU — Schematic

CPU Components

• ALU
• A and D registers
• Program Counter
• Memory Register (read/write)

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Assembler

This is a two-pass assembler written in C++.

• Pass 1: builds a symbol table, mapping labels such as (LOOP) and variables to memory or instruction addresses.
• Pass 2: translates instructions into 16-bit machine code using the symbol table and lookup tables