thumb move/compare/add/subtract, fix thumb conditional branch bug

src/crab/thumb/conditional_branch.cr

@@ -1,7 +1,7 @@
 module THUMB
   def thumb_conditional_branch(instr : Word) : Nil
     cond = bits(instr, 8..11)
-    offset = bits(instr, 0..8).to_i8!
+    offset = bits(instr, 0..7).to_i8!
     if cond
       @r[15] &+= (offset * 2)
       clear_pipeline

src/crab/thumb/move_compare_add_subtract.cr

@@ -0,0 +1,17 @@
+module THUMB
+  def thumb_move_compare_add_subtract(instr : Word) : Nil
+    op = bits(instr, 11..12)
+    rd = bits(instr, 8..10)
+    offset = bits(instr, 0..7)
+    # todo handle carry flag on all ops
+    case op
+    when 0b00 then res = @r[rd] = offset
+    when 0b01 then res = @r[rd] &- offset
+    when 0b10 then res = @r[rd] &+= offset
+    when 0b11 then res = @r[rd] &-= offset
+    else           raise "Invalid move/compare/add/subtract op: #{op}"
+    end
+    @cpsr.zero = res == 0
+    @cpsr.negative = bit?(res, 31)
+  end
+end

src/crab/thumb/thumb.cr

@@ -39,7 +39,7 @@ module THUMB
       elsif idx & 0b11111100 == 0b01000000
         # alu operations
       elsif idx & 0b11100000 == 0b00100000
-        # move/compare/add/subtract immediate
+        lut[idx] = ->thumb_move_compare_add_subtract(Word)
       elsif idx & 0b11111100 == 0b00011000
         # add/subtract
       elsif idx & 0b11100000 == 0b00000000