rewrote ROM controller

lib/waterfoul/mbc/rom.rb

@@ -1,29 +1,26 @@
 module Waterfoul
   module MBC
     class ROM
-      EXTERNAL_RAM_START_ADDR = 0xA000
+      EXTERNAL_RAM_SIZE = 0x1FFF
 
       def initialize(program)
-        @external_ram = []
+        @external_ram = Array.new EXTERNAL_RAM_SIZE, 0
         @program = program
       end
 
       def [](i)
-        case i & 0xE000
-        when 0xA000,0xB000
-          @external_ram[i]
+        case i
+        when 0xA000...0xC000
+          @external_ram[i - 0xA000]
         else
           @program[i]
         end
       end
 
       def []=(i,v)
-        case i & 0xE000
-        when 0xA000, 0xB000
-          addr = i - EXTERNAL_RAM_START_ADDR
-          @external_ram[addr] = v
-        else
-          fail 'forbidden write to rom'
+        case i
+        when 0xA000...0xC000
+          @external_ram[i - 0xA000] = v
         end
       end
     end