custom/minimal pipeline implementation in favor of Deque

also removed constants from Bus class, which seems to have improved
performance for some reason as well

src/crab/bus.cr

@@ -1,21 +1,6 @@
 class Bus
-  BIOS         = 0x00000000..0x00003FFF
-  WRAM_BOARD   = 0x02000000..0x0203FFFF
-  WRAM_CHIP    = 0x03000000..0x03007FFF
-  PPU_IO       = 0x04000000..0x0400005F
-  SOUND_IO     = 0x04000060..0x040000AF
-  DMA_IO       = 0x040000B0..0x040000FF
-  TIMER_IO     = 0x04000100..0x0400011F
-  SERIAL_IO_1  = 0x04000120..0x0400012F
-  KEYPAD_IO    = 0x04000130..0x04000133
-  SERIAL_IO_2  = 0x04000134..0x040001FF
-  INTERRUPT_IO = 0x04000200..0x0400FFFF
-  PPU          = 0x05000000..0x07FFFFFF
-  CARTRIDGE    = 0x08000000..0x0FFFFFFF
-  UNUSED       = 0x10000000..0xFFFFFFFF
-
-  @wram_board = Bytes.new Bus::WRAM_BOARD.size
-  @wram_chip = Bytes.new Bus::WRAM_CHIP.size
+  @wram_board = Bytes.new 0x40000
+  @wram_chip = Bytes.new 0x08000
 
   def initialize(@gba : GBA)
   end

src/crab/cpu.cr

@@ -1,5 +1,6 @@
 require "./arm/*"
 require "./thumb/*"
+require "./pipeline"
 
 class CPU
   include ARM
@@ -19,7 +20,7 @@ class CPU
 
   @r = Slice(Word).new 16
   @cpsr : PSR
-  @pipeline = Deque(Word).new 2
+  @pipeline = Pipeline.new
   getter lut : Slice(Proc(Word, Nil)) { fill_lut }
   getter thumb_lut : Slice(Proc(Word, Nil)) { fill_thumb_lut }
 
@@ -35,11 +36,11 @@ class CPU
     while @pipeline.size < 2
       if @cpsr.thumb
         log "Fetch pc: #{hex_str @r[15]}, instr: #{hex_str @gba.bus.read_half(@r[15]).to_u16}"
-        @pipeline << @gba.bus.read_half @r[15]
+        @pipeline.push @gba.bus.read_half @r[15]
         @r[15] &+= 2
       else
         log "Fetch pc: #{hex_str @r[15]}, instr: #{hex_str @gba.bus.read_word @r[15]}"
-        @pipeline << @gba.bus.read_word @r[15]
+        @pipeline.push @gba.bus.read_word @r[15]
         @r[15] &+= 4
       end
     end

src/crab/pipeline.cr

@@ -0,0 +1,29 @@
+# A super minimalistic FIFO queue implementation optimized for
+# use as an ARM instruction pipeline.
+class Pipeline
+  @buffer = Slice(Word).new 2
+  @pos = 0
+  @size = 0
+
+  def push(instr : Word) : Nil
+    raise "Pushing to full pipeline" if @size == 2
+    index = (@pos + @size) & 1
+    @buffer[index] = instr
+    @size += 1
+  end
+
+  def shift : Word
+    @size -= 1
+    val = @buffer[@pos]
+    @pos = (@pos + 1) & 1
+    val
+  end
+
+  def clear : Nil
+    @size = 0
+  end
+
+  def size : Int32
+    @size
+  end
+end