6soz/src/cpu/m6502/addressing.zig

const Instruction = @import("instruction.zig");
const AddressingMode = Instruction.AddressingMode;

const utils = @import("utils.zig");

// addressing mode operations monomorphized for Cpu
pub const Addressing = @This();

pub const addRelative = utils.addRelative;
pub const pageCrossed = utils.pageCrossed;
pub const bitNumber = utils.bitNumber;

pub fn readOperand(cpu: anytype, mode: AddressingMode) u8 {
    return switch (mode) {
        .immediate => cpu.fetchByte(),

        .zero_page => blk: {
            const address = cpu.fetchByte();
            break :blk cpu.readCycle(address);
        },

        .zero_page_x => blk: {
            const base = cpu.fetchByte();
            _ = cpu.readCycle(base);
            break :blk cpu.readCycle(base +% cpu.registers.x);
        },

        .zero_page_y => blk: {
            const base = cpu.fetchByte();
            _ = cpu.readCycle(base);
            break :blk cpu.readCycle(base +% cpu.registers.y);
        },

        .zero_page_indirect => blk: {
            const pointer = cpu.fetchByte();
            const low = cpu.readCycle(pointer);
            const high = cpu.readCycle(pointer +% 1);
            break :blk cpu.readCycle(utils.makeWord(low, high));
        },

        .absolute => blk: {
            const address = cpu.fetchWord();
            break :blk cpu.readCycle(address);
        },

        .absolute_x => readAbsoluteIndexed(cpu, cpu.registers.x),
        .absolute_y => readAbsoluteIndexed(cpu, cpu.registers.y),
        .indexed_indirect_x => readIndexedIndirectX(cpu),
        .indirect_indexed_y => readIndirectIndexedY(cpu),

        else => unreachable,
    };
}

pub fn readAbsoluteIndexed(cpu: anytype, index: u8) u8 {
    const low = cpu.fetchByte();
    const high = cpu.fetchByte();
    const base = utils.makeWord(low, high);
    const address = base +% index;

    if (pageCrossed(base, address)) {
        if (@TypeOf(cpu.*).features.cmos_instructions) {
            _ = cpu.readCycle(cpu.registers.pc -% 1);
        } else {
            const dummy = (base & 0xff00) | (address & 0x00ff);
            _ = cpu.readCycle(dummy);
        }
    }

    return cpu.readCycle(address);
}

pub fn readIndexedIndirectX(cpu: anytype) u8 {
    const operand = cpu.fetchByte();
    _ = cpu.readCycle(operand);
    const pointer = operand +% cpu.registers.x;

    const low = cpu.readCycle(pointer);
    const high = cpu.readCycle(pointer +% 1);
    return cpu.readCycle(utils.makeWord(low, high));
}

pub fn readIndirectIndexedY(cpu: anytype) u8 {
    const pointer = cpu.fetchByte();
    const low = cpu.readCycle(pointer);
    const high = cpu.readCycle(pointer +% 1);

    const base = utils.makeWord(low, high);
    const address = base +% cpu.registers.y;

    if (pageCrossed(base, address)) {
        if (@TypeOf(cpu.*).features.cmos_instructions) {
            _ = cpu.readCycle(cpu.registers.pc -% 1);
        } else {
            const dummy = (base & 0xff00) | (address & 0x00ff);
            _ = cpu.readCycle(dummy);
        }
    }

    return cpu.readCycle(address);
}

pub fn writeOperand(cpu: anytype, mode: AddressingMode, value: u8) void {
    switch (mode) {
        .zero_page => {
            const address = cpu.fetchByte();
            cpu.writeCycle(address, value);
        },

        .zero_page_x => {
            const base = cpu.fetchByte();
            _ = cpu.readCycle(base);
            cpu.writeCycle(base +% cpu.registers.x, value);
        },

        .zero_page_y => {
            const base = cpu.fetchByte();
            _ = cpu.readCycle(base);
            cpu.writeCycle(base +% cpu.registers.y, value);
        },

        .zero_page_indirect => {
            const pointer = cpu.fetchByte();
            const low = cpu.readCycle(pointer);
            const high = cpu.readCycle(pointer +% 1);
            cpu.writeCycle(utils.makeWord(low, high), value);
        },

        .absolute => {
            const address = cpu.fetchWord();
            cpu.writeCycle(address, value);
        },

        .absolute_x => writeAbsoluteIndexed(cpu, cpu.registers.x, value),
        .absolute_y => writeAbsoluteIndexed(cpu, cpu.registers.y, value),
        .indexed_indirect_x => writeIndexedIndirectX(cpu, value),
        .indirect_indexed_y => writeIndirectIndexedY(cpu, value),

        else => unreachable,
    }
}

pub fn writeAbsoluteIndexed(cpu: anytype, index: u8, value: u8) void {
    const low = cpu.fetchByte();
    const high = cpu.fetchByte();
    const base = utils.makeWord(low, high);
    const address = base +% index;

    const dummy = (base & 0xff00) | (address & 0x00ff);
    _ = cpu.readCycle(dummy);

    cpu.writeCycle(address, value);
}

pub fn writeIndexedIndirectX(cpu: anytype, value: u8) void {
    const operand = cpu.fetchByte();
    _ = cpu.readCycle(operand);
    const pointer = operand +% cpu.registers.x;

    const low = cpu.readCycle(pointer);
    const high = cpu.readCycle(pointer +% 1);
    cpu.writeCycle(utils.makeWord(low, high), value);
}

pub fn writeIndirectIndexedY(cpu: anytype, value: u8) void {
    const pointer = cpu.fetchByte();
    const low = cpu.readCycle(pointer);
    const high = cpu.readCycle(pointer +% 1);

    const base = utils.makeWord(low, high);
    const address = base +% cpu.registers.y;

    const dummy = (base & 0xff00) | (address & 0x00ff);
    _ = cpu.readCycle(dummy);

    cpu.writeCycle(address, value);
}

pub fn modifyAddress(
    cpu: anytype,
    address: u16,
    comptime operation: fn (*@TypeOf(cpu.*), u8) u8,
) void {
    const old = cpu.readCycle(address);

    if (@TypeOf(cpu.*).features.cmos_instructions) {
        _ = cpu.readCycle(address);
    } else {
        cpu.writeCycle(address, old);
    }

    const new = operation(cpu, old);
    cpu.writeCycle(address, new);
}

pub fn modifyAbsoluteIndexed(
    cpu: anytype,
    index: u8,
    comptime operation: fn (*@TypeOf(cpu.*), u8) u8,
) void {
    const low = cpu.fetchByte();
    const high = cpu.fetchByte();
    const base = utils.makeWord(low, high);
    const address = base +% index;

    if (@TypeOf(cpu.*).features.cmos_instructions) {
        if (pageCrossed(base, address)) {
            _ = cpu.readCycle(cpu.registers.pc -% 1);
        }
    } else {
        const dummy = (base & 0xff00) | (address & 0x00ff);
        _ = cpu.readCycle(dummy);
    }

    modifyAddress(cpu, address, operation);
}

pub fn modifyIndexedIndirectX(
    cpu: anytype,
    comptime operation: fn (*@TypeOf(cpu.*), u8) u8,
) void {
    const operand = cpu.fetchByte();
    _ = cpu.readCycle(operand);
    const pointer = operand +% cpu.registers.x;

    const low = cpu.readCycle(pointer);
    const high = cpu.readCycle(pointer +% 1);
    modifyAddress(cpu, utils.makeWord(low, high), operation);
}

pub fn modifyIndirectIndexedY(
    cpu: anytype,
    comptime operation: fn (*@TypeOf(cpu.*), u8) u8,
) void {
    const pointer = cpu.fetchByte();
    const low = cpu.readCycle(pointer);
    const high = cpu.readCycle(pointer +% 1);

    const base = utils.makeWord(low, high);
    const address = base +% cpu.registers.y;

    const dummy = (base & 0xff00) | (address & 0x00ff);
    _ = cpu.readCycle(dummy);

    modifyAddress(cpu, address, operation);
}