Structs and impl Blocks

Rust has no classes. The equivalent is a struct for data and an impl block for methods — kept deliberately separate.

The struct

A struct is a named group of fields:

#![allow(unused)]
fn main() {
pub struct Cpu {
    pub v:     [u8; 16],    // general-purpose registers V0–VF
    pub i:     u16,         // index register
    pub pc:    u16,         // program counter
    pub sp:    u8,          // stack pointer
    pub stack: [u16; 16],   // return address stack
}
}

Each field has a name and a type. pub makes a field visible outside the module — without it, the field is private by default.

The impl block

Methods live in a separate impl block:

#![allow(unused)]
fn main() {
impl Cpu {
    pub fn new() -> Self {
        Cpu {
            v:     [0; 16],
            i:     0,
            pc:    0x200,   // programs start at 0x200
            sp:    0,
            stack: [0; 16],
        }
    }

    pub fn step(&mut self) {
        // fetch, decode, execute one instruction
    }
}
}

new() is a static constructor — it takes no self. step() takes &mut self because it modifies the CPU state.

&self vs &mut self

This distinction is enforced at compile time:

• &self — read-only access to the struct's fields
• &mut self — read and write access

If a method with &self tries to modify a field, the compiler rejects it. This prevents accidental mutations and is part of Rust's core ownership system.

OOP comparison

Why separate data from behavior?

It makes each concern independently visible. Looking at a struct, you see exactly what data it holds. Looking at an impl, you see exactly what it can do. You can have multiple impl blocks — useful for putting tests in a separate file — and you can implement traits in their own blocks.

In the emulator: Cpu, Memory, and Display are all structs with their own impl blocks. Cpu owns a Memory and a Display as fields. The CHIP-8 Registers chapter describes what each field of Cpu represents in hardware terms. The Constructors chapter explains why new() is needed for Cpu specifically.