diff --git a/lib/waterfoul/cpu.rb b/lib/waterfoul/cpu.rb
index 06c32cf..749f88e 100644
--- a/lib/waterfoul/cpu.rb
+++ b/lib/waterfoul/cpu.rb
@@ -65,6 +65,7 @@ module Waterfoul
       @a = @b = @c = @d = @e = @f = @h = @l = @f = 0x00
       @m = 0
       @timer = Timer.new
+      @ime = false
     end
 
     # This method emulates the CPU cycle process. Each instruction is
@@ -72,13 +73,9 @@ module Waterfoul
     # This processes repeats infinitly until the process is closed
     def step
       reset_tick
-      if halted?
-        halt_step
-      else
-        serve_interrupt if @ime
-        instruction_byte = fetch_instruction
-        perform_instruction instruction_byte
-      end
+      serve_interrupt if @ime
+      instruction_byte = fetch_instruction
+      perform_instruction instruction_byte
       @timer.tick @m
     end
 
@@ -107,7 +104,7 @@ module Waterfoul
       raise 'instruction not found' if operation.nil?
       # perform the instruction
       self.public_send operation
-      @m = instruction_cycle_time instruction
+      @m = instruction_cycle_time(instruction) * 4
     end
 
     # fetch the next byte to be executed from memory and increment the program
@@ -123,7 +120,9 @@ module Waterfoul
     # get the number of cycles a instruction takes to execute. The times
     # can be found in the instruction opcode table
     def instruction_cycle_time(instruction)
-      if @branched
+      if @prefix_cb
+        CB_OPCODE_TIMINGS[@prefix_cb]
+      elsif @branched
         OPCODE_CONDITIONAL_TIMINGS[instruction]
       else
         OPCODE_TIMINGS[instruction]
@@ -137,24 +136,30 @@ module Waterfoul
       # master disable interrupts
       @ime = false
       push_onto_stack @pc
-      @m = 20
-      # point to instruction which handles appropiate interrupt
+      if_reg = $mmu.read_byte 0xFF0F
       case interrupt
       when Interrupt::INTERRUPT_VBLANK
         @pc = 0x40
+        $mmu.write_byte(0xFF0F, if_reg & 0xFE)
       when Interrupt::INTERRUPT_LCDSTAT
         @pc = 0x48
+        $mmu.write_byte(0xFF0F, if_reg & 0xFD)
       when Interrupt::INTERRUPT_TIMER
         @pc = 0x50
+        $mmu.write_byte(0xFF0F, if_reg & 0xFB)
       when Interrupt::INTERRUPT_SERIAL
         @pc = 0x58
+        $mmu.write_byte(0xFF0F, if_reg & 0xF7)
       when Interrupt::INTERRUPT_JOYPAD
         @pc = 0x60
+        $mmu.write_byte(0xFF0F, if_reg & 0xEF)
       end
+      @m = 20
     end
 
     # reset variables that are set on every instruction
     def reset_tick
+      @prefix_cb = false
       @branched = false
       @m = 0
     end
diff --git a/lib/waterfoul/instructions/misc.rb b/lib/waterfoul/instructions/misc.rb
index 29f8264..8143d02 100644
--- a/lib/waterfoul/instructions/misc.rb
+++ b/lib/waterfoul/instructions/misc.rb
@@ -34,10 +34,10 @@ module Waterfoul
       # @flags - - - -
       def prefix_cb
         ins = $mmu.read_byte @pc
+        @prefix_cb = ins
         @pc += 1
 
         opcode = Waterfoul::CPU::CB_OPCODE[ins]
-        #p "cb opcode: #{opcode}"
         self.public_send opcode
       end
 
diff --git a/lib/waterfoul/timer.rb b/lib/waterfoul/timer.rb
index 6540d6e..dff3ab9 100644
--- a/lib/waterfoul/timer.rb
+++ b/lib/waterfoul/timer.rb
@@ -1,6 +1,6 @@
 module Waterfoul
   class Timer
-    DIV_INC_TIME = 255 # cycles
+    DIV_INC_TIME = 256 # cycles
 
     def initialize
       @div_cycles = 0
@@ -9,8 +9,6 @@ module Waterfoul
     end
 
     def tick(cycles = 0)
-      # increment TIMA and DIV register
-      @tima_cycles += cycles
       @div_cycles += cycles
       # incremnt DIV register if its time to
       inc_div_register if @div_cycles >= DIV_INC_TIME
@@ -18,6 +16,8 @@ module Waterfoul
       @tima.update
       # dont bother if TIMA is not running
       if @tima.running?
+        # increment TIMA and DIV register
+        @tima_cycles += cycles
         frequency = @tima.frequency
         if @tima_cycles >= frequency
           inc_tima_register
@@ -32,7 +32,7 @@ module Waterfoul
         tima = $mmu.read_byte 0xFF06
         Interrupt.request_interrupt(Interrupt::INTERRUPT_TIMER)
       else
-        tima = (tima + 1) & 0xFF
+        tima += 1
       end
 
       $mmu.write_byte 0xFF05, tima, hardware_operation: true