thumb halfword data transfer immediate, pram read/write

src/crab/arm/arm.cr

@@ -38,6 +38,7 @@ module ARM
         lut[idx] = ->arm_single_data_transfer(Word)
       elsif idx & 0b111001001001 == 0b000001001001
         # halfword data transfer immediate offset
+        lut[idx] = ->arm_halfword_data_transfer_immediate(Word)
       elsif idx & 0b111001001001 == 0b000000001001
         # halfword data transfer register offset
       elsif idx & 0b111111111111 == 0b000100100001

src/crab/arm/halfword_data_transfer_imm.cr

@@ -0,0 +1,49 @@
+module ARM
+  def arm_halfword_data_transfer_immediate(instr : Word) : Nil
+    pre_index = bit?(instr, 24)
+    add = bit?(instr, 23)
+    write_back = bit?(instr, 21)
+    load = bit?(instr, 20)
+    rn = bits(instr, 16..19)
+    rd = bits(instr, 12..15)
+    offset_high = bits(instr, 8..11)
+    sh = bits(instr, 5..6)
+    offset_low = bits(instr, 0..3)
+
+    address = @r[rn]
+    offset = offset_high << 4 | offset_low
+
+    if pre_index
+      if add
+        address &+= offset
+      else
+        address &-= offset
+      end
+    end
+
+    case sh
+    when 0b00 # swp, no docs on this?
+    when 0b01 # ldrh/strh
+      if load
+        @r[rd] = @gba.bus.read_half address
+      else
+        @gba.bus[address] = @r[rd]
+      end
+    when 0b10 # ldrsb
+      @r[rd] = @gba.bus[address].to_i8!.to_u32
+    when 0b11 # ldrsh
+      @r[rd] = @gba.bus.read_half(address).to_i16!.to_u32
+    else raise "Invalid halfword data transfer imm op: #{sh}"
+    end
+
+    if !pre_index
+      if add
+        @r[rn] &+= offset
+      else
+        @r[rn] &-= offset
+      end
+    elsif write_back
+      @r[rn] = address
+    end
+  end
+end

src/crab/bus.cr

@@ -17,6 +17,7 @@ class Bus
       else
         raise "Unmapped i/o read: #{hex_str index.to_u32}"
       end
+    when 0x5 then @gba.ppu.pram[index]
     when 0x6
       address = 0x1FFFF_u32 & index
       address &= ~0x8000 if address > 0x17FFF
@@ -51,6 +52,7 @@ class Bus
       else
         raise "Unmapped i/o write: #{hex_str index.to_u32}"
       end
+    when 0x5 then @gba.ppu.pram[index & 0x3FF] = value
     when 0x6
       address = 0x1FFFF_u32 & index
       address &= ~0x8000 if address > 0x17FFF

src/crab/ppu.cr

@@ -25,6 +25,7 @@ class PPU
     num bg_mode, 3                  # (0-5=Video Mode 0-5, 6-7=Prohibited)
   end
 
+  getter pram = Bytes.new 0x400
   getter vram = Bytes.new 0x18000
 
   @dispcnt : DISPCNT = DISPCNT.new 0