thumb load/store halfword, add halfword type, add halfword write

src/crab/bus.cr

@@ -54,6 +54,12 @@ class Bus
     end
   end
 
+  def []=(index : Int, value : HalfWord) : Nil
+    log "write #{hex_str index.to_u32} -> #{hex_str value}"
+    self[index + 1] = 0xFF_u8 & (value >> 8)
+    self[index] = 0xFF_u8 & value
+  end
+
   def []=(index : Int, value : Word) : Nil
     log "write #{hex_str index.to_u32} -> #{hex_str value}"
     self[index + 3] = 0xFF_u8 & (value >> 24)

src/crab/thumb/load_store_halfword.cr

@@ -0,0 +1,14 @@
+module THUMB
+  def thumb_load_store_halfword(instr : Word) : Nil
+    load = bit?(instr, 11)
+    offset = bits(instr, 6..10)
+    rb = bits(instr, 3..5)
+    rd = bits(instr, 0..2)
+    address = @r[rb] + (offset << 1)
+    if load
+      @r[rd] = @gba.bus.read_half(address)
+    else
+      @gba.bus[address] = @r[rd].to_u16!
+    end
+  end
+end

src/crab/thumb/thumb.cr

@@ -25,7 +25,7 @@ module THUMB
       elsif idx & 0b11110000 == 0b10010000
         # sp-relative load/store
       elsif idx & 0b11110000 == 0b10000000
-        # load/store halfword
+        lut[idx] = -> thumb_load_store_halfword(Word)
       elsif idx & 0b11100000 == 0b01100000
         lut[idx] = ->thumb_load_store_immediate_offset(Word)
       elsif idx & 0b11110010 == 0b01010010

src/crab/types.cr

@@ -1,3 +1,4 @@
 alias Byte = UInt8
+alias HalfWord = UInt16
 alias Word = UInt32
 alias Words = Slice(UInt32)