read cartridges

1 files changed, 65 insertions, 0 deletions

diff --git a/src/nes/cartridge.cc b/src/nes/cartridge.cc
new file mode 100644
index 0000000..5970d2a
--- /dev/null
+++ b/src/nes/cartridge.cc
@@ -0,0 +1,65 @@
+#include "nes/cartridge.hh"
+#include <fstream>
+
+namespace
+{
+	constexpr auto prg_rom_bank_size = std::size_t{ 16 * 1024 };
+	constexpr auto chr_rom_bank_size = std::size_t{ 8 * 1024 };
+	constexpr auto ram_bank_size = std::size_t{ 8 * 1024 };
+} // namespace
+
+namespace nes
+{
+	auto cartridge::from_data(void const* data, std::size_t const length) -> cartridge
+	{
+		auto const bytes = static_cast<std::uint8_t const*>(data);
+		auto res = cartridge{};
+		res.is_valid_ = true;
+
+		// Parse header.
+		if (length < 16) { return cartridge{}; }
+		if (bytes[0] != 'N' || bytes[1] != 'E' || bytes[2] != 'S' || bytes[3] != 0x1A) { return cartridge{}; }
+		auto const prg_rom_banks = bytes[4];
+		auto const chr_rom_banks = bytes[5];
+		auto const control_1 = bytes[6];
+		auto const control_2 = bytes[7];
+		auto const ram_banks = (bytes[8] != 0) ? bytes[8] : 1;
+		auto const has_trainer = (control_1 & 0b00000100) != 0;
+
+		res.mapper_number_ =
+			(control_1 & 0b11110000) >> 4 |
+			(control_2 & 0b11110000) >> 0;
+
+		// Load program data.
+		auto offset = 16;
+		if (has_trainer) { offset += 512; }
+
+		auto const prg_rom_size = prg_rom_banks * prg_rom_bank_size;
+		if (length < offset + prg_rom_size) { return cartridge{}; }
+		res.prg_rom_ = std::vector(&bytes[offset], &bytes[offset + prg_rom_size]);
+
+		offset += prg_rom_size;
+
+		auto const chr_rom_size = chr_rom_banks * chr_rom_bank_size;
+		if (length < offset + chr_rom_size) { return cartridge{}; }
+		res.chr_rom_ = std::vector(&bytes[offset], &bytes[offset + chr_rom_size]);
+
+		res.ram_ = std::vector(ram_banks * ram_bank_size, std::uint8_t{ 0 });
+
+		return res;
+	}
+
+	auto cartridge::from_file(std::string_view const path) -> cartridge
+	{
+		auto stream = std::ifstream{ path, std::ios::binary };
+		if (!stream.good()) { return cartridge{}; }
+
+		stream.seekg(0, std::ios::end);
+		auto const length = stream.tellg();
+		stream.seekg(0, std::ios::beg);
+		auto data = std::vector<std::uint8_t>(length);
+		stream.read(reinterpret_cast<char*>(data.data()), data.size());
+
+		return from_data(data.data(), data.size());
+	}
+} // namespace nes
\ No newline at end of file