From 09b66ccda165095475da73d5af819dd0af8097ac Mon Sep 17 00:00:00 2001
From: JakeBreath <tolozacdcmo@gmail.com>
Date: Fri, 7 Aug 2026 21:19:33 -0500
Subject: [PATCH] implemented working Cartridge for NROm/Mapper 0

---
 Cargo.lock       |  7 ++++
 src/bus.rs       |  0
 src/cartridge.rs | 85 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/cpu.rs       |  0
 src/main.rs      |  2 ++
 5 files changed, 94 insertions(+)
 create mode 100644 Cargo.lock
 create mode 100644 src/bus.rs
 create mode 100644 src/cartridge.rs
 create mode 100644 src/cpu.rs

diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..333a95b
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "rnes"
+version = "0.1.0"
diff --git a/src/bus.rs b/src/bus.rs
new file mode 100644
index 0000000..e69de29
diff --git a/src/cartridge.rs b/src/cartridge.rs
new file mode 100644
index 0000000..76204fc
--- /dev/null
+++ b/src/cartridge.rs
@@ -0,0 +1,85 @@
+
+pub enum Mirroring { Horizontal, Vertical, FourScreen }
+
+pub struct Cartridge {
+    prg_rom: Vec<u8>,
+    chr_rom: Vec<u8>,
+    mirroring: Mirroring,
+    battery: bool,
+    trainer: bool,
+    four_screen: bool,
+    mapper: u8,
+}
+
+impl Cartridge {
+    pub fn from_file(path: &str) -> Result<Cartridge, String> {
+        let data: Vec<u8> = std::fs::read(path).map_err(|e| format!("couldn't read {path}: {e}"))?;
+        if data.len() < 16 {
+            return Err("file is too short to be an iNES ROM".to_string())
+        }
+
+        if &data[0..4] != b"NES\x1a" {
+            return Err("not a valid iNES ROM (bad magic numbers)".to_string())
+        }
+
+        // Extract the bytes we care about
+        let prg_banks: u8 = data[4];
+        let chr_banks: u8 = data[5];
+        let flags6: u8 = data[6];
+        let flags7: u8 = data[7];
+
+        // Decode bits from the bytes on the ROM
+        let mirroring: Mirroring = if flags6 & 0b0001 != 0 { Mirroring::Vertical} else { Mirroring::Horizontal };
+        let battery: bool = flags6 & 0b0010 != 0;
+        let trainer: bool = flags6 & 0b0100 != 0;
+        let four_screen: bool = flags6 & 0b1000 != 0; // this is meant to override mirroring but both will be stored
+        let mapper: u8 = ((flags7 & 0xF0) >> 4) << 4 | ((flags6 & 0xF0) >> 4);
+
+        // Gate for Mapper as we are going NROM-only for now
+
+        if mapper != 0 {
+            return Err(format!("mapper {mapper} not supported yet, this is temporal"));
+        }
+
+        // Compute the ROM offsets for future use
+
+        let prg_start: usize = 16 + if trainer { 512 } else { 0 }; // i'd like to call this padding but on the spec it gets called a "Trainer" so it stays like that
+        let prg_len: usize = (prg_banks as usize) * 0x4000;
+        let chr_start: usize = prg_start + prg_len;
+        let chr_len: usize = (chr_banks as usize) * 0x2000;
+
+        if prg_start + prg_len > data.len() || chr_start + chr_len > data.len() {
+            return Err("ROM data truncated or changed".to_string());
+        }
+
+        // Construct and return a valid Cartridge
+
+        Ok(Cartridge { 
+            prg_rom: data[prg_start..prg_start + prg_len].to_vec(), 
+            chr_rom: if chr_len == 0 { vec![0u8; 0x2000] } else { data[chr_start..chr_start + chr_len].to_vec() }, // Vector of 8KB filled with 0's or the actual CHR ROM data
+            mirroring, 
+            battery, 
+            trainer, 
+            four_screen, 
+            mapper,
+        })
+    }
+
+    // read_* operations are for the CPU to work with, letting it read the ROM chips on the emulated cartridge
+    pub fn read_prg(&self, address: u16) -> u8 {
+        debug_assert!(address >= 0x8000, "PRG read outside of $8000-$FFFF; Address from CPU: {address:#06X}"); // #06X makes the Address readable similar to $1FFF
+        let mut offset: u16 = address - 0x8000;
+        if self.prg_rom.len() == 0x4000 {
+            offset %= 0x4000;
+        }
+        self.prg_rom[offset as usize]
+    }
+    pub fn read_chr(&self, address: u16) -> u8 {
+        debug_assert!(address < 0x2000, "CHR reads outside $0000-$1FFF; Address from CPU: {address:#06X}");
+        self.chr_rom[address as usize]
+    }
+    pub fn write_chr(&mut self, address: u16, value: u8) {
+        debug_assert!(address < 0x2000, "CHR writes outside $0000-$1FFF; Address from CPU: {address:#06X}");
+        self.chr_rom[address as usize] = value
+    }
+}
\ No newline at end of file
diff --git a/src/cpu.rs b/src/cpu.rs
new file mode 100644
index 0000000..e69de29
diff --git a/src/main.rs b/src/main.rs
index e7a11a9..ed3eee0 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,5 @@
+mod cartridge;
+
 fn main() {
     println!("Hello, world!");
 }