From a867c30f165a8351a94f99b18480c1afaaa4fa9f Mon Sep 17 00:00:00 2001
From: Lea <me@janderedev.xyz>
Date: Wed, 25 Jan 2023 15:24:22 +0100
Subject: [PATCH] add emulator, implement ADD and NOP instruction

---
 Cargo.toml                              |   4 +
 src/emulator/instructions/arithmetic.rs |  15 +++
 src/emulator/instructions/mod.rs        |   1 +
 src/emulator/main.rs                    | 134 ++++++++++++++++++++++++
 4 files changed, 154 insertions(+)
 create mode 100644 src/emulator/instructions/arithmetic.rs
 create mode 100644 src/emulator/instructions/mod.rs
 create mode 100644 src/emulator/main.rs

diff --git a/Cargo.toml b/Cargo.toml
index bbc199a..f590fae 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -7,6 +7,10 @@ edition = "2021"
 
 [dependencies]
 
+[[bin]]
+name = "emulator"
+path = "src/emulator/main.rs"
+
 [[bin]]
 name = "disassembler"
 path = "src/disassembler/main.rs"
diff --git a/src/emulator/instructions/arithmetic.rs b/src/emulator/instructions/arithmetic.rs
new file mode 100644
index 0000000..d8dc7d6
--- /dev/null
+++ b/src/emulator/instructions/arithmetic.rs
@@ -0,0 +1,15 @@
+use crate::{EmulatorState, Register, get_register};
+
+/// Sets the condition code flags according to `result`
+fn set_cc(state: &mut EmulatorState, result: u16) {
+    state.cc.z = (result & 0xff) == 0;
+    state.cc.s = (result & 0x80) == 1;
+    state.cc.c = result > 0xff;
+    state.cc.p = (result & 0xff).count_ones() % 2 == 0;
+}
+
+pub fn add(register: Register, state: &mut EmulatorState) {
+    let result = get_register(register, state) + state.a as u16;
+    set_cc(state, result);
+    state.a = (result & 0xff) as u8;
+}
diff --git a/src/emulator/instructions/mod.rs b/src/emulator/instructions/mod.rs
new file mode 100644
index 0000000..965db62
--- /dev/null
+++ b/src/emulator/instructions/mod.rs
@@ -0,0 +1 @@
+pub mod arithmetic;
diff --git a/src/emulator/main.rs b/src/emulator/main.rs
new file mode 100644
index 0000000..75da473
--- /dev/null
+++ b/src/emulator/main.rs
@@ -0,0 +1,134 @@
+use std::{fs, env};
+use instructions::arithmetic::add;
+
+mod instructions;
+
+pub struct EmulatorState {
+    a: u8,
+    b: u8,
+    c: u8,
+    d: u8,
+    e: u8,
+    h: u8,
+    l: u8,
+
+    cc: ConditionCodes,
+
+    /// Stack pointer
+    sp: u16,
+    /// Memory pointer
+    pc: u16,
+    memory: [u8; 8000],
+}
+
+pub struct ConditionCodes {
+    /// Zero (Z), set if the result is zero.
+    z: bool,
+    /// Sign (S), set if the result is negative.
+    s: bool,
+    /// Parity (P), set if the number of 1 bits in the result is even.
+    p: bool,
+    /// Carry (C), set if the last addition operation resulted in a carry or if the last subtraction operation required a borrow.
+    c: bool,
+    /// Auxiliary carry (AC or H), used for binary-coded decimal arithmetic (BCD).
+    ac: bool,
+}
+
+pub enum Register {
+    B, C, D, E,
+    H, L, M, A,
+}
+
+/// Returns a Register enum based on the input number 0..7 in the order B,C,D,E,H,L,M,A
+fn register_from_num(b: u8) -> Register {
+    match b {
+        0 => Register::B,
+        1 => Register::C,
+        2 => Register::D,
+        3 => Register::E,
+        4 => Register::H,
+        5 => Register::L,
+        6 => Register::M,
+        7 => Register::A,
+        _ => panic!("'{b}' cannot be converted to register enum"),
+    }
+}
+
+fn get_register(register: Register, state: &EmulatorState) -> u16 {
+    match register {
+        Register::B => state.b as u16,
+        Register::C => state.c as u16,
+        Register::D => state.d as u16,
+        Register::E => state.e as u16,
+        Register::H => state.h as u16,
+        Register::L => state.l as u16,
+        Register::A => state.a as u16,
+        Register::M => (state.memory[(state.h as usize)] as u16) << 8 | (state.memory[state.l as usize] as u16),
+    }
+}
+
+fn set_register(register: Register, value: u8, state: &mut EmulatorState) {
+    match register {
+        Register::B => state.b = value,
+        Register::C => state.c = value,
+        Register::D => state.d = value,
+        Register::E => state.e = value,
+        Register::H => state.h = value,
+        Register::L => state.l = value,
+        Register::A => state.a = value,
+        Register::M => panic!("Cannot set pseudoregister 'M'"),
+    };
+}
+
+fn main() {
+    let mut state = EmulatorState {
+        a: 0,
+        b: 0,
+        c: 0,
+        d: 0,
+        e: 0,
+        h: 0,
+        l: 0,
+        sp: 0,
+        cc: ConditionCodes { z: true, s: true, p: true, c: true, ac: true },
+        pc: 0,
+        memory: [0; 8000],
+    };
+
+    // Load the ROM into memory
+    let mut args = env::args();
+    let filename = args
+        .nth(1)
+        .expect("Provide a path to a ROM file to emulate as an argument");
+    let file = fs::read(filename).expect("where file");
+
+    for i in 0..8000 {
+        state.memory[i] = file[i];
+    }
+
+    while state.pc < 8000 {
+        tick(&mut state);
+    }
+}
+
+fn tick(state: &mut EmulatorState) {
+    let instruction = state.memory[state.pc as usize];
+
+    let mut next_byte = || {
+        state.pc += 1;
+        return state.memory[state.pc as usize];
+    };
+
+    match instruction {
+        0x0  => {} // NOP
+        0x80..=0x87 => add(register_from_num(instruction & 0xf), state), // ADD
+        _ => not_implemented(state),
+    }
+
+    state.pc += 1;
+}
+
+fn not_implemented(state: &EmulatorState) {
+    let instruction = state.memory[state.pc as usize];
+    panic!("Unimplemented instruction {:#02X} at {:#04X}", instruction, state.pc);
+}