oboy/src/memory.ml

(**
 * Copyright (c) 2015, Fabien Freling
 * All rights reserved.
 *
 * This source code is licensed under the BSD 2-clause license found in the
 * LICENSE file at the top level directory of this source tree.
 *)

open Bytes
open Printf

(** @see http://bgb.bircd.org/pandocs.htm#memorymap
    @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 *)
  rom_bank_01 : bytes; (* additional bank, 16KB *)
  vram        : bytes; (* Video RAM, 8KB *)
  wram_bank_0 : bytes; (* work RAM, 4KB *)
  wram_bank_1 : bytes; (* work RAM, 4KB *)
  io          : bytes; (* I/O ports *)
  hram        : bytes; (* High RAM, 8KB *)
  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;
    wram_bank_0 = create 0x1000;
    wram_bank_1 = create 0x1000;
    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
  | x when x < 0x4000 -> mem.rom_bank_00, x
  | x when x < 0x8000 -> mem.rom_bank_01, (x - 0x4000)
  | x when x < 0xA000 -> mem.vram,  (x - 0x8000)
  | x when x < 0xC000 -> failwith "Unimplemented memory range."
  | x when x < 0xD000 -> mem.wram_bank_0,  (x - 0xC000)
  | x when x < 0xE000 -> mem.wram_bank_1,  (x - 0xD000)
  | x when x < 0xFF00 -> failwith "Unimplemented memory range."
  | x when x < 0xFF80 -> mem.io, x - 0xFF00
  | x when x < 0xFFFF -> mem.hram, x - 0xFF80
  | 0xFFFF -> mem.interrupt, 0
  | x -> eprintf "Memory access 0x%06X\n" x;
         failwith "Invalid memory range."


let get mem addr =
  let m, x = get_mem_bank mem addr in
  get m x

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