rom bank mapping

Author: seanburkes

src/domain/cartridge.rs

@@ -1,6 +1,11 @@
 use super::rom::{RomHeader, RomHeaderError};
 
 const ROM_BANK_SIZE: usize = 0x4000;
+const ROM_FIXED_START: usize = 0x0000;
+const ROM_FIXED_END: usize = 0x3FFF;
+const ROM_SWITCH_START: usize = 0x4000;
+const ROM_SWITCH_END: usize = 0x7FFF;
+const OPEN_BUS: u8 = 0xFF;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Cartridge {
@@ -18,6 +23,14 @@ impl Cartridge {
         RomBankView::new(&self.bytes)
     }
 
+    pub fn rom_mapping(&self) -> RomBankMapping<'_> {
+        RomBankMapping::new(&self.bytes)
+    }
+
+    pub fn read_rom(&self, addr: u16, switchable_bank: usize) -> u8 {
+        RomBankMapping::with_switchable_bank(&self.bytes, switchable_bank).read(addr)
+    }
+
     pub fn declared_bank_count(&self) -> Option<usize> {
         self.header.rom_size.bank_count()
     }
@@ -55,6 +68,64 @@ impl<'a> RomBankView<'a> {
     }
 }
 
+#[derive(Debug, Clone, Copy)]
+pub struct RomBankMapping<'a> {
+    bytes: &'a [u8],
+    switchable_bank: usize,
+}
+
+impl<'a> RomBankMapping<'a> {
+    pub fn new(bytes: &'a [u8]) -> Self {
+        Self {
+            bytes,
+            switchable_bank: 1,
+        }
+    }
+
+    pub fn with_switchable_bank(bytes: &'a [u8], switchable_bank: usize) -> Self {
+        Self {
+            bytes,
+            switchable_bank,
+        }
+    }
+
+    pub fn switchable_bank(&self) -> usize {
+        self.switchable_bank
+    }
+
+    pub fn set_switchable_bank(&mut self, bank: usize) {
+        self.switchable_bank = bank;
+    }
+
+    pub fn read(&self, addr: u16) -> u8 {
+        let addr = addr as usize;
+        match addr {
+            ROM_FIXED_START..=ROM_FIXED_END => self.read_at(addr),
+            ROM_SWITCH_START..=ROM_SWITCH_END => {
+                let offset = addr - ROM_SWITCH_START;
+                self.read_bank(self.switchable_bank, offset)
+            }
+            _ => OPEN_BUS,
+        }
+    }
+
+    fn read_bank(&self, bank: usize, offset: usize) -> u8 {
+        let base = match bank.checked_mul(ROM_BANK_SIZE) {
+            Some(base) => base,
+            None => return OPEN_BUS,
+        };
+        let index = match base.checked_add(offset) {
+            Some(index) => index,
+            None => return OPEN_BUS,
+        };
+        self.read_at(index)
+    }
+
+    fn read_at(&self, index: usize) -> u8 {
+        self.bytes.get(index).copied().unwrap_or(OPEN_BUS)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::{Cartridge, ROM_BANK_SIZE};
@@ -81,4 +152,45 @@ mod tests {
         assert_eq!(banks.bank(0).expect("bank 0").len(), ROM_BANK_SIZE);
         assert_eq!(banks.bank(1).expect("bank 1").len(), 1);
     }
+
+    #[test]
+    fn rom_mapping_reads_fixed_and_switchable_banks() {
+        let mut bytes = vec![0; ROM_BANK_SIZE * 2];
+        bytes[..ROM_BANK_SIZE].fill(0xAA);
+        bytes[ROM_BANK_SIZE..].fill(0xBB);
+
+        let cart = Cartridge::from_bytes(bytes).expect("cartridge");
+        let mapping = cart.rom_mapping();
+
+        assert_eq!(mapping.read(0x0000), 0xAA);
+        assert_eq!(mapping.read(0x3FFF), 0xAA);
+        assert_eq!(mapping.read(0x4000), 0xBB);
+        assert_eq!(mapping.read(0x7FFF), 0xBB);
+    }
+
+    #[test]
+    fn rom_mapping_switches_banks() {
+        let mut bytes = vec![0; ROM_BANK_SIZE * 3];
+        bytes[..ROM_BANK_SIZE].fill(0x11);
+        bytes[ROM_BANK_SIZE..ROM_BANK_SIZE * 2].fill(0x22);
+        bytes[ROM_BANK_SIZE * 2..].fill(0x33);
+
+        let cart = Cartridge::from_bytes(bytes).expect("cartridge");
+        let mut mapping = cart.rom_mapping();
+
+        mapping.set_switchable_bank(2);
+        assert_eq!(mapping.read(0x4000), 0x33);
+
+        mapping.set_switchable_bank(0);
+        assert_eq!(mapping.read(0x4000), 0x11);
+    }
+
+    #[test]
+    fn rom_mapping_returns_open_bus_for_unmapped_addrs() {
+        let bytes = vec![0; ROM_BANK_SIZE];
+        let cart = Cartridge::from_bytes(bytes).expect("cartridge");
+        let mapping = cart.rom_mapping();
+
+        assert_eq!(mapping.read(0x8000), 0xFF);
+    }
 }

src/domain/mod.rs

@@ -2,7 +2,7 @@ pub mod cartridge;
 pub mod emulator;
 pub mod rom;
 
-pub use cartridge::{Cartridge, RomBankView};
+pub use cartridge::{Cartridge, RomBankMapping, RomBankView};
 pub use emulator::Emulator;
 pub use rom::{
     CartridgeType, CgbFlag, Destination, Licensee, RamSize, RomHeader, RomHeaderError, RomSize,