92 lines
2.1 KiB
Zig
92 lines
2.1 KiB
Zig
const types = @import("types.zig");
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const Mirroring = types.Mirroring;
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const Mapper76 = @This();
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bank_select: u8 = 0,
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bank_registers: [8]u8 = [_]u8{0} ** 8,
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prg_bank_count: usize,
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chr_bank_count: usize,
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fixed_mirroring: Mirroring,
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pub fn init(
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prg_size: usize,
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chr_size: usize,
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mirroring_mode: Mirroring,
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) Mapper76 {
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return .{
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.prg_bank_count = prg_size / 0x2000,
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.chr_bank_count = if (chr_size > 0) chr_size / 0x0800 else 1,
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.fixed_mirroring = mirroring_mode,
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};
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}
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pub fn cpuMapRead(
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self: *const Mapper76,
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address: u16,
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) ?usize {
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if (address < 0x8000) return null;
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const offset = @as(usize, address & 0x1fff);
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const last_bank = self.prg_bank_count - 1;
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const second_last_bank = self.prg_bank_count - 2;
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const r6 = @as(usize, self.bank_registers[6]) % self.prg_bank_count;
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const r7 = @as(usize, self.bank_registers[7]) % self.prg_bank_count;
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switch ((address - 0x8000) / 0x2000) {
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0 => return r6 * 0x2000 + offset,
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1 => return r7 * 0x2000 + offset,
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2 => return second_last_bank * 0x2000 + offset,
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3 => return last_bank * 0x2000 + offset,
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else => unreachable,
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}
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}
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pub fn cpuWrite(
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self: *Mapper76,
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address: u16,
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value: u8,
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) void {
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if (address < 0x8000) return;
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if ((address & 1) == 0) {
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self.bank_select = value & 0x07;
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} else {
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self.bank_registers[self.bank_select] = value;
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}
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}
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pub fn ppuMapRead(
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self: *const Mapper76,
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address: u16,
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) ?usize {
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if (address >= 0x2000) return null;
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const offset = @as(usize, address & 0x07ff);
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const slot = address / 0x0800;
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const reg_idx = @as(usize, 2 + slot);
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const bank = (@as(usize, self.bank_registers[reg_idx])) % self.chr_bank_count;
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return bank * 0x0800 + offset;
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}
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pub fn ppuMapWrite(
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self: *const Mapper76,
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address: u16,
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) ?usize {
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return self.ppuMapRead(address);
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}
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pub fn mirroring(
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self: *const Mapper76,
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) Mirroring {
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return self.fixed_mirroring;
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}
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pub fn irqAsserted(
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_: *const Mapper76,
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) bool {
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return false;
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}
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