proper MBC1 implementation

lib/waterfoul/mbc/mbc1.rb

@@ -3,27 +3,67 @@ module Waterfoul
     class MBC1 < ROM
       EXTERNAL_RAM_SIZE = 0x2000
 
-      def initialize(program, rom_bank = 1)
-        @rom_bank = rom_bank
-        @cartridge_program = program
-        @rom_offset = 0x4000
-        @ram_bank = Array.new EXTERNAL_RAM_SIZE, 0
+      attr_accessor :ram_enabled, :mode, :rom_bank, :ram_bank
+
+      def initialize(program)
+        @rom_bank = 1
+        @ram_bank = 1
+        @mode = 0
+        @ram_enabled = false
+        @game_program = program
+        @ram = Array.new EXTERNAL_RAM_SIZE, 0
       end
 
       def [](i)
-        case i & 0xE000
-        when 0x4000, 0x6000
-          relative_addr = (i - 0x4000) + @rom_offset
-          @cartridge_program[relative_addr]
-        when 0xA000
-          byebug
-        else
-          @cartridge_program[i]
+        case i
+        when 0x0...0x4000 # ROM Bank 0
+          @game_program[i]
+        when 0x4000...0x8000 # ROM Bank n
+          addr = i - 0x4000
+          offset = @rom_bank * 0x4000
+          @game_program[offset + addr]
+        when 0xA000...0xC000
+          fail 'trying to read invalid ram' unless @ram_enabled
+          addr = i - 0xA000
+          if @mode == 0
+            @ram[addr]
+          else
+            offset = @rom_ram_bank_number * 0x8000
+            @ram[offset + addr]
+          end
         end
       end
 
       def []=(i,v)
-        byebug
+        case i
+        when 0x0...0x2000
+          @ram_enabled = (v & 0xA == 0xA ? true : false)
+        when 0x2000...0x4000
+          if @mode == 1
+            @rom_bank = (v & 0x1F) | (@rom_bank & 0xE0)
+          else
+            @rom_bank = v & 0x1F
+          end
+        when 0x4000...0x6000
+          v = v & 0x3
+          if @mode == 1
+            @ram_bank = v
+          else
+            @rom_bank = (@rom_bank & 0x1F) | (v << 5)
+            @rom_bank += 1 if [0x0, 0x20, 0x40, 0x60].include? @rom_bank
+          end
+        when 0x6000...0x8000
+          @mode = v & 0x1
+        when 0xA000...0xC000
+          fail 'trying to write invalid ram' unless @ram_enabled
+          addr = i - 0xA000
+          if @mode == 0
+            @ram[addr] = v
+          else
+            offset = @ram_bank * 0x8000
+            @ram[offset + addr] = v
+          end
+        end
       end
     end
   end

spec/mbc/mbc1_spec.rb

@@ -1,13 +1,80 @@
 require 'spec_helper'
 
 describe Waterfoul::MBC::MBC1 do
+  let(:program) { double :program }
+  subject { Waterfoul::MBC::MBC1.new program }
+
   describe '#[]' do
-    context 'read from addr 0x2000' do
-      it 'reads bytes from bank 0' do
+    context 'when reading at 0x0001' do
+      it 'reads byte from bank 0' do
+        expect(program).to receive(:[]).with(0x0001)
+        subject[0x0001]
+      end
+    end
+
+    context 'when reading at 0x4001' do
+      it 'reads from bank 1' do
+        expect(program).to receive(:[]).with(0x4001)
+        subject[0x4001]
       end
     end
   end
 
   describe '#[]=' do
+    context 'when writing to 0x1' do
+      it 'enables the external ram' do
+        subject[0x1] = 0xA
+        expect(subject.ram_enabled).to eq true
+      end
+    end
+
+    context 'when writing to 4001' do
+      it 'sets rom bank from 5 bits' do
+        subject[0x2000] = 0xFF
+        expect(subject.rom_bank).to eq 0x1F
+      end
+    end
+
+    context 'when writing to 0x4000' do
+      context 'with mode set to 1' do
+        before { subject[0x6000] = 0x1 }
+        it 'sets ram bank' do
+          subject[0x4000] = 0x3
+          expect(subject.ram_bank).to eq 0x3
+        end
+      end
+
+      context 'with mode set to 0' do
+        before { subject[0x6000] = 0x0 }
+        before { subject[0x2000] = 0x1F }
+        it 'sets upper 3 bits of rom bank' do
+          subject[0x4000] = 0x3
+          expect(subject.rom_bank).to eq 0x7F
+        end
+      end
+    end
+
+    context 'when writing to 0xA001' do
+      context 'when RAM is enabled' do
+        before { subject[0x1] = 0xA }
+        it 'saves the value to external memory' do
+          subject[0xA001] = 0x1
+          expect(subject[0xA001]).to eq 0x1
+        end
+      end
+
+      context 'when ram is disabled' do
+        it 'raises an error' do
+          expect { subject[0xA001] = 0x1 }.to raise_error RuntimeError
+        end
+      end
+    end
+
+    context 'writing to 0x6001' do
+      it 'sets the ROM mode to 1' do
+        subject[0x6001] = 0x1
+        expect(subject.mode).to eq 1
+      end
+    end
   end
 end