cpu.rs

fn ld_mff00u8_a(cpu: &mut Cpu) {
    let byte = cpu.read_u8();
    cpu.mmu.write(0xff00 + byte as u16, cpu.a);
}

fn ld_mff00c_a(cpu: &mut Cpu) {
    cpu.mmu.write(0xff00 + cpu.c as u16, cpu.a);
}

fn ld_mu16_a(cpu: &mut Cpu) {
    let word = cpu.read_u16();
    cpu.mmu.write(word, cpu.a);
}

fn ld_a_mff00u8(cpu: &mut Cpu) {
    let byte = cpu.read_u8();
    cpu.a = cpu.mmu.read(0xff00 + byte as u16);
}

fn cp_a_u8(cpu: &mut Cpu) {
    let byte = cpu.read_u8();
    sub_set_flags(cpu, cpu.a, byte);
}

fn rl_c(cpu: &mut Cpu) {
    cpu.c = rl(cpu, cpu.c);
}

fn bit_7_h(cpu: &mut Cpu) {
    bit_h(cpu, 7);
}

/// Helper function that rotates (shifts) the given
/// byte (probably from a register) and updates the
/// proper flag registers.
fn rl(cpu: &mut Cpu, byte: u8) -> u8 {
    let carry = cpu.get_carry();

    cpu.set_carry(byte & 0x80 == 0x80);

    let result = (byte << 1) | carry as u8;

    cpu.set_sub(false);
    cpu.set_zero(result == 0);
    cpu.set_half_carry(false);

    result
}

/// Helper function to test one bit in a u8.
/// Returns true if bit is 0.
fn bit_zero(val: u8, bit: u8) -> bool {
    (val & (1u8 << (bit as usize))) == 0
}

fn bit_h(cpu: &mut Cpu, bit: u8) {
    cpu.set_sub(false);
    cpu.set_zero(bit_zero(cpu.h, bit));
    cpu.set_half_carry(true);
}

fn sub_set_flags(cpu: &mut Cpu, x: u8, y: u8) -> u8 {
    // checks for borrow using 32bit arithmetics
    let x = x as u32;
    let y = y as u32;

    let value = x.wrapping_sub(y);
    let value_b = value as u8;

    cpu.set_sub(true);
    cpu.set_carry(value & 0x100 == 0x100);
    cpu.set_zero(value_b == 0);
    cpu.set_half_carry((x ^ y ^ value) & 0x10 == 0x10);

    value_b
}

fn add_u16_u16(cpu: &mut Cpu, first: u16, second: u16) -> u16 {
    let first = first as u32;
    let second = second as u32;
    let value = first.wrapping_add(second);

    cpu.set_sub(false);
    cpu.set_carry(value & 0x10000 == 0x10000);
    cpu.set_half_carry((first ^ second ^ value) & 0x1000 == 0x1000);

    value as u16
}