nes6502 0.1.4

An emulated NES version of the 6502.
Documentation 
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use super::{handle_invalid_addressing_mode, pack_bytes, pack_bytes_wrapped, unpack_bytes};
use super::{AddressingMode, Cpu};
use crate::Interrupts;
use crate::Mapper;

impl<M: Mapper, I: Interrupts> Cpu<M, I> {
    pub(crate) fn instruction_jmp(
        &mut self,

        addressing_mode: AddressingMode,
        low_byte: Option<u8>,
        high_byte: Option<u8>,
    ) -> u8 {
        match addressing_mode {
            AddressingMode::Absolute => {
                self.program_counter = pack_bytes_wrapped(low_byte, high_byte);
                3
            }
            AddressingMode::Indirect => {
                // We do an indrect read here. We do not have a general function
                // as JMP is the only instruction that uses it
                let base_address = pack_bytes_wrapped(low_byte, high_byte);

                // check for the bug referenced here https://www.nesdev.org/obelisk-6502-guide/reference.html#JMP
                self.program_counter = match (base_address & 0xFF) == 0xFF {
                    true => {
                        let lsb = self.read(base_address);
                        let msb = self.read(base_address - 0xFF);
                        pack_bytes(lsb, msb)
                    }
                    false => pack_bytes(self.read(base_address), self.read(base_address + 1)),
                };

                5
            }
            _ => handle_invalid_addressing_mode(),
        }
    }

    pub(crate) fn instruction_jsr(&mut self, low_byte: Option<u8>, high_byte: Option<u8>) -> u8 {
        let subroutine_address = pack_bytes_wrapped(low_byte, high_byte);

        let (pc_low, pc_high) = unpack_bytes(self.program_counter - 1);

        self.push(pc_high);
        self.push(pc_low);

        self.program_counter = subroutine_address;

        6
    }

    pub(crate) fn instruction_rts(&mut self) -> u8 {
        let pc_low = self.pop();
        let pc_high: u8 = self.pop();

        self.program_counter = pack_bytes(pc_low, pc_high) + 1;

        6
    }
}