Emulate the boot ROM by chaining it in front of the real cartridge

lib/waterfoul/boot_rom.rb

@@ -49,10 +49,19 @@ module Waterfoul
       0x3E, 0x01, 0xE0, 0x50
     ].freeze
 
+    attr_reader :cartridge
+
+    def initialize(cartridge)
+      @cartridge = cartridge
+    end
 
     # Read bootstrap instruction given an index (memory location)
-    def self.[](i)
-      ROM[i]
+    def [](i)
+      ROM[i] || @cartridge[i]
+    end
+
+    def []=(i, v)
+      @cartridge[i] = v
     end
   end
 end

lib/waterfoul/cartridge.rb

@@ -54,5 +54,9 @@ module Waterfoul
         MBC::MBC5
       end
     end
+
+    def self.empty
+      Array.new(0x8000, 0)
+    end
   end
 end

lib/waterfoul/emulator.rb

@@ -6,8 +6,8 @@ module Waterfoul
       # read the given file as binary and break it down into an array of bytes
       rom = File.binread(rom_filename).bytes
       # initialize emulated CPU, GPU & Scren components
-      $mmu = MMU.new
       @cartridge = Cartridge.new rom
+      $mmu = MMU.new(@cartridge)
       @cpu = CPU.new
       @cpu = SkipBoot.set_state(@cpu) if options.has_key?('skip_boot')
       @ppu = PPU.new
@@ -15,7 +15,6 @@ module Waterfoul
     end
 
     def run
-      $mmu.cartridge = @cartridge
       loop do
         @cpu.step
         @ppu.step @cpu.m

lib/waterfoul/mmu.rb

@@ -11,8 +11,6 @@ module Waterfoul
     MEMORY_SIZE = 65536 # bytes
     # unmap boot rom register address
     UNMAP_BOOT_ROM_MEM_LOC = 0xFF50
-    # location in memory where the boot rom ends
-    BOOT_ROM_END_MEM_LOC = 0xFF
     # DMA register function address
     DMA_TRANSFER_MEM_LOC = 0xFF46
     # DIV register memory location
@@ -21,10 +19,9 @@ module Waterfoul
     attr_reader :memory
     attr_accessor :cartridge
 
-    def initialize
-      @cartridge = Array.new 0x8000, 0
+    def initialize(cartridge = Cartridge.empty)
       # map the boot rom when the device starts
-      @map_boot_rom = true
+      @cartridge = BootROM.new(cartridge)
       # storage for usable memory (zero filled)
       @memory = Array.new MEMORY_SIZE, 0
     end
@@ -37,12 +34,7 @@ module Waterfoul
       when 0xFF00
         Input.read_keyboard @memory[i]
       when 0x0000...0x8000 # ROM Bank 0 + n
-        # if the boot rom is enabled and the address is < 0x100
-        if @map_boot_rom && i <= BOOT_ROM_END_MEM_LOC
-          BootROM[i]
-        else
-          @cartridge[i]
-        end
+        @cartridge[i]
       when 0x8000...0xA000 # Video RAM
         @memory[i]
       when 0xA000...0xC000 # RAM Bank
@@ -65,7 +57,9 @@ module Waterfoul
       unless options[:hardware_operation]
         case i
         when UNMAP_BOOT_ROM_MEM_LOC # unmap the boot rom when 0xFF50 is wrtiten to in memory
-          @map_boot_rom = false if v == 0x1 && @map_boot_rom
+          if v == 0x1 && @cartridge.is_a?(BootROM)
+            @cartridge = @cartridge.cartridge
+          end
         when 0xFF00
           @memory[i] = v | 0xF
         when 0xFF46 # DMA transfer