Merge pull request #18 from toots/mbc2

Implement Memory Bank Controller 2 (MBC2).

Author: linoscope

lib/cartridge/detect_cartridge.ml

@@ -8,4 +8,7 @@ let f ~rom_bytes =
   | MBC1
   | MBC1_RAM
   | MBC1_RAM_BATTERY        -> (module Mbc1 : Cartridge_intf.S)
-  | _                       -> assert false
+  | MBC2
+  | MBC2_BATTERY            -> (module Mbc2 : Cartridge_intf.S)
+  | other ->
+    failwith (Printf.sprintf "Unsupported cartridge type: %s" (Cartridge_type.show other))

lib/cartridge/mbc2.ml

@@ -0,0 +1,84 @@
+(* MBC2 - Memory Bank Controller 2
+
+   Features:
+   - Up to 256KB ROM (16 banks)
+   - Built-in 512×4 bits RAM (only lower nibble used)
+   - RAM at 0xA000-0xA1FF (mirrored in 0xA200-0xBFFF)
+
+   Register writes:
+   - 0x0000-0x3FFF with bit 8 = 0: RAM enable (0x0A enables)
+   - 0x0000-0x3FFF with bit 8 = 1: ROM bank number (4 bits, 0→1)
+
+   Reference: https://gbdev.io/pandocs/MBC2.html *)
+
+open Uints
+
+type t = {
+  rom_bytes : Bigstringaf.t;
+  ram_bytes : Bigstringaf.t;
+  rom_bank_count : int;
+  mutable ram_enabled : bool;
+  mutable rom_bank_num : int;
+}
+
+let create ~rom_bytes =
+  let h = Cartridge_header.create ~rom_bytes in
+  let rom_bank_count = Cartridge_header.get_rom_bank_count h in
+  (* MBC2 has built-in 512×4 bit RAM *)
+  let ram_bytes = Bigstringaf.create 512 in
+  {
+    rom_bytes;
+    ram_bytes;
+    rom_bank_count;
+    ram_enabled = false;
+    rom_bank_num = 1;
+  }
+
+let read_byte t addr =
+  let addr = Uint16.to_int addr in
+  match addr with
+  | _ when addr <= 0x3FFF ->
+    Bigstringaf.unsafe_get t.rom_bytes addr
+    |> Uint8.of_char
+  | _ when addr <= 0x7FFF ->
+    let offset = 0x4000 * t.rom_bank_num + (addr - 0x4000) in
+    Bigstringaf.unsafe_get t.rom_bytes offset
+    |> Uint8.of_char
+  | _ when 0xA000 <= addr && addr <= 0xBFFF ->
+    (* RAM - 512 bytes mirrored, only lower 4 bits valid
+       This memory is at A000–A1FF and mirrored at
+       A200–BFFF hence the 0x1FF below. *)
+    if t.ram_enabled then
+      let ram_addr = (addr - 0xA000) land 0x1FF in
+      let value = Bigstringaf.unsafe_get t.ram_bytes ram_addr |> Char.code in
+      Uint8.of_int (value lor 0xF0)  (* Upper 4 bits read as 1 *)
+    else
+      Uint8.of_int 0xFF
+  | _ -> assert false
+
+let write_byte t ~addr ~data =
+  let addr = Uint16.to_int addr in
+  let data = Uint8.to_int data in
+  match addr with
+  | _ when addr <= 0x3FFF ->
+    if addr land 0x100 = 0 then
+      t.ram_enabled <- (data land 0x0F) = 0x0A
+    else begin
+      let bank = data land 0x0F in
+      let bank = if bank = 0 then 1 else bank in
+      t.rom_bank_num <- bank mod t.rom_bank_count
+    end
+  | _ when 0x4000 <= addr && addr <= 0x7FFF ->
+    (* Writes to this area do nothing on MBC2 *)
+    ()
+  | _ when 0xA000 <= addr && addr <= 0xBFFF ->
+    (* RAM - only lower 4 bits written *)
+    if t.ram_enabled then begin
+      let ram_addr = (addr - 0xA000) land 0x1FF in
+      Bigstringaf.unsafe_set t.ram_bytes ram_addr (Char.unsafe_chr (data land 0x0F))
+    end
+  | _ -> assert false
+
+let accepts _ addr =
+  let addr = Uint16.to_int addr in
+  (addr <= 0x7FFF) || (0xA000 <= addr && addr <= 0xBFFF)

lib/cartridge/mbc2.mli

@@ -0,0 +1 @@
+include Cartridge_intf.S