Implement timers

src/memory.ml

@@ -1,8 +1,14 @@
 open Bytes
 open Printf
 
-(** http://bgb.bircd.org/pandocs.htm#memorymap
+(** @see http://bgb.bircd.org/pandocs.htm#memorymap
-    http://imrannazar.com/GameBoy-Emulation-in-JavaScript:-Memory *)
+    @see http://imrannazar.com/GameBoy-Emulation-in-JavaScript:-Memory *)
+
+(** Common addresses *)
+let gDIV = 0xFF04 (** divider register *)
+let gTIMA = 0xFF05 (** timer counter *)
+let gTMA = 0xFF06 (** timer modulo *)
+let gTAC = 0xFF07 (** timer control *)
 
 type map = {
   rom_bank_00 : bytes; (* cartridge, 16KB *)
@@ -15,8 +21,14 @@ type map = {
   interrupt   : bytes; (* Interrupt Enable Register *)
 }
 
+type t = {
+  map       : map;
+  timer_div : Timer.t;
+  tima      : Timer.t;
+}
+
 let init (cartridge: Cartridge.t) =
-  {
+  let map = {
     rom_bank_00 = sub cartridge.full_rom 0 0x4000;
     rom_bank_01 = create 0x4000;
     vram = create 0x2000;
@@ -25,7 +37,19 @@ let init (cartridge: Cartridge.t) =
     io = create 0x0080;
     hram = create 0x2000;
     interrupt = create 1
-  }
+  } in
+
+  (** Init register values
+      @see http://bgb.bircd.org/pandocs.htm#powerupsequence *)
+  let zero = char_of_int 0 in
+  set map.io 0x05 zero; (* TIMA, 0xFF05 *)
+  set map.io 0x06 zero; (* TMA,  0xFF06 *)
+  set map.io 0x07 zero; (* TAC,  0xFF07 *)
+
+  let timer_div = Timer.create 16384 true in
+  let tima = Timer.create_tima 0 in
+
+  { map; timer_div; tima; }
 
 let get_mem_bank mem addr =
   match addr with
@@ -50,3 +74,28 @@ let get mem addr =
 let set mem addr c =
   let m, x = get_mem_bank mem addr in
   set m x c
+
+(** Increment byte in memory, wrap value in case of overflow *)
+let inc mem addr = 
+  let m, x = get_mem_bank mem addr in
+  let value = Bytes.get m x |> int_of_char in
+  let inc_value = value + 1 in
+  let overflow = inc_value > 0xFF in
+  let c = inc_value mod 0x01FF |> char_of_int in
+  Bytes.set m x c;
+  overflow
+
+
+let update_timers mem cycles =
+  let should_inc_div = Timer.update mem.timer_div cycles in
+  if should_inc_div then ignore (inc mem.map gDIV);
+
+  let should_inc_tima = Timer.update mem.tima cycles in
+  if should_inc_tima then begin
+    let overflow = inc mem.map gTIMA in
+    if overflow then begin
+      let tma = get mem.map gTMA in
+      set mem.map gTIMA tma
+      (* TODO: INT 50 - Timer interupt *)
+    end
+  end

src/oboy.ml

@@ -3,16 +3,19 @@ open Printf
 let fps = 60
 let cycles_per_frame = Cpu.frequence / fps
 
-let rec run (cpu: Cpu.t) (mem: Memory.map) (screen: Screen.t) =
+let rec run (cpu: Cpu.t) (mem: Memory.t) (screen: Screen.t) =
-  printf "\n";
   let start = Unix.gettimeofday () in
   printf "start %f\n" start;
 
-  let rec run_for cpu mem cycles_remaining =
+  let rec run_for cpu (mem: Memory.t) cycles_remaining =
     if cycles_remaining > 0 then
     begin
-      let inst, cycles = Cpu.run cpu mem in
+      printf "\n";
+      let inst, cycles = Cpu.run cpu mem.map in
       printf "[Instruction] %s\n" inst;
+
+      Memory.update_timers mem cycles;
+      
       run_for cpu mem (cycles_remaining - cycles)
     end
   in

src/timer.ml

@@ -0,0 +1,38 @@
+
+let frequence = 4194304
+
+type t = {
+  clock_rate          : int;
+  period              : int;
+  mutable enabled     : bool;
+  mutable cycles_left : int;
+}
+
+let create clock_rate enabled =
+  let period = frequence / clock_rate in
+  {
+    clock_rate;
+    period;
+    enabled;
+    cycles_left = period;
+  }
+
+(** TIMA - Timer counter *)
+let create_tima tac =
+  let enabled = Bit.is_set tac 2 in 
+  let clock_select = tac land 0b00000011 in
+  let clock_rate = match clock_select with
+    | 0 -> 4096
+    | 1 -> 262144
+    | 2 -> 65536
+    | 3 -> 16384
+    | _ -> failwith "Unreachable clock rate code."
+  in
+  create clock_rate enabled
+
+let update timer cycles =
+  let remain = timer.cycles_left - cycles in
+  let should_inc = remain <= 0 in
+  if should_inc then
+    timer.cycles_left <- remain + timer.period;
+  should_inc