path: root/src/cpu.rs

// SPDX-License-Identifier: AGPL-3.0-or-later

//! Implementation of the MOS 6502 CPU.

use bitflags::bitflags;

use crate::{
	instruction::{get_operation, AddressingMode, Instruction},
	memory::Memory,
};

/// Implementation of the Central Processing Unit of the 6502.
pub struct Cpu {
	/// The 8 bit accumulator is used all arithmetic and logical operations
	/// (with the exception of increments and decrements). The contents of the
	/// accumulator can be stored and retrieved either from memory or the stack.
	///
	/// Most complex operations will need to use the accumulator for arithmetic
	/// and efficient optimisation of its use is a key feature of time critical
	/// routines.
	accumulator: u8,

	/// The 8 bit index register is most commonly used to hold counters or
	/// offsets for accessing memory. The value of the X register can be loaded
	/// and saved in memory, compared with values held in memory or incremented
	/// and decremented.
	///
	/// The X register has one special function. It can be used to get a copy of
	/// the stack pointer or change its value.
	index_register_x: u8,

	/// The Y register is similar to the X register in that it is available for
	/// holding counter or offsets memory access and supports the same set of
	/// memory load, save and compare operations as wells as increments and
	/// decrements. It has no special functions.
	index_register_y: u8,

	/// Processor flags. See documentation for [`ProcessorStatus`].
	processor_status: ProcessorStatus,

	/// The program counter is a 16 bit register which points to the next
	/// instruction to be executed. The value of program counter is modified
	/// automatically as instructions are executed.
	///
	/// The value of the program counter can be modified by executing a jump, a
	/// relative branch or a subroutine call to another memory address or by
	/// returning from a subroutine or interrupt.
	program_counter: u16,

	/// The processor supports a 256 byte stack located between $0100 and $01FF.
	/// The stack pointer is an 8 bit register and holds the low 8 bits of the
	/// next free location on the stack. The location of the stack is fixed and
	/// cannot be moved.
	///
	/// Pushing bytes to the stack causes the stack pointer to
	/// be decremented. Conversely pulling bytes causes it to be incremented.
	///
	/// The CPU does not detect if the stack is overflowed by excessive pushing
	/// or pulling operations and will most likely result in the program
	/// crashing.
	stack_pointer: u8,
}

impl Cpu {
	/// Creates a new instance of the [`Cpu`].
	pub const fn new() -> Self {
		// TODO: Ensure these are the correct values for init

		Self {
			accumulator: 0,
			index_register_x: 0,
			index_register_y: 0,
			program_counter: 0xFFFC,
			stack_pointer: 0x0000,
			processor_status: ProcessorStatus::empty(),
		}
	}

	/// Fetches the next byte, pointed to by the program counter.
	fn fetch_byte(&mut self, cycles: &mut u32, memory: &Memory) -> u8 {
		let data = *memory
			.data
			.get(usize::from(self.program_counter))
			.expect("program_counter indexed outside of memory");

		self.program_counter = self.program_counter.wrapping_add(1);
		*cycles -= 1;

		data
	}

	/// Sets the LDA status after executing an LDA operation.
	fn lda_set_status(&mut self) {
		self.processor_status
			.set(ProcessorStatus::ZERO, self.accumulator == 0);
		self.processor_status.set(
			ProcessorStatus::NEGATIVE,
			(self.accumulator & 0b1000_0000) > 0,
		);
	}

	/// Reads the next byte, pointed to by the address.
	fn read_byte(cycles: &mut u32, address: u8, memory: &Memory) -> u8 {
		let data = *memory
			.data
			.get(usize::from(address))
			.expect("address indexed outside of memory");

		*cycles -= 1;

		data
	}

	/// Executes instructions on the [`Cpu`] from memory. Runs the specified
	/// number of cycles.
	pub fn execute(&mut self, cycles: u32, memory: &Memory) {
		// Shadow cycles with a mutable copy
		let mut cycles = cycles;

		while cycles > 0 {
			let opcode: u8 = self.fetch_byte(&mut cycles, memory);

			let operation = get_operation(opcode);

			if let Some(operation) = operation {
				match (operation.instruction, operation.addressing_mode) {
					(Instruction::Lda, AddressingMode::Immediate) => {
						let value = self.fetch_byte(&mut cycles, memory);

						self.accumulator = value;
						self.lda_set_status();
					}
					(Instruction::Lda, AddressingMode::ZeroPage) => {
						let zero_page_address = self.fetch_byte(&mut cycles, memory);

						self.accumulator = Self::read_byte(&mut cycles, zero_page_address, memory);
						self.lda_set_status();
					}
					(Instruction::Lda, AddressingMode::ZeroPageX) => {
						let mut zero_page_address = self.fetch_byte(&mut cycles, memory);

						zero_page_address += self.index_register_x;
						cycles -= 1;

						self.accumulator = Self::read_byte(&mut cycles, zero_page_address, memory);
						self.lda_set_status();
					}
				}
			}
		}
	}

	/// Resets the [`Cpu`]. This will put the Cpu back into the state it was at
	/// on boot.
	pub fn reset(&mut self, memory: &mut Memory) {
		// TODO: Incomplete. Use correct values from later on in tutorial

		self.program_counter = 0xFFFC;
		self.stack_pointer = 0x0000;

		// Set all flags to 0
		self.processor_status = ProcessorStatus::empty();

		self.accumulator = 0;
		self.index_register_x = 0;
		self.index_register_y = 0;

		// Zero out memory
		memory.reset();
	}
}

bitflags! {
	/// As instructions are executed a set of processor flags are set or clear to
	/// record the results of the operation. This flags and some additional control
	/// flags are held in a special status register. Each flag has a single bit
	/// within the register.
	pub struct ProcessorStatus: u8 {
		/// The negative flag is set if the result of the last operation had bit 7
		/// set to a one.
		const NEGATIVE = 0b1000_0000;

		/// The overflow flag is set during arithmetic operations if the result has
		/// yielded an invalid 2's complement result (e.g. adding to positive numbers
		/// and ending up with a negative result: 64 + 64 => -128). It is determined by
		/// looking at the carry between bits 6 and 7 and between bit 7 and the carry
		/// flag.
		const OVERFLOW = 0b0100_0000;

		// No instruction for 0b0010_0000

		/// The break command bit is set when a BRK instruction has been executed and an
		/// interrupt has been generated to process it.
		const BREAK_COMMAND = 0b0001_0000;

		/// While the decimal mode flag is set the processor will obey the rules of
		/// Binary Coded Decimal (BCD) arithmetic during addition and subtraction. The
		/// flag can be explicitly set using 'Set Decimal Flag' (SED) and cleared with
		/// 'Clear Decimal Flag' (CLD).
		const DECIMAL_MODE = 0b0000_1000;

		/// The interrupt disable flag is set if the program has executed a 'Set
		/// Interrupt Disable' (SEI) instruction. While this flag is set the processor
		/// will not respond to interrupts from devices until it is cleared by a 'Clear
		/// Interrupt Disable' (CLI) instruction.
		const INTERRUPT_DISABLE = 0b0000_0100;

		/// The zero flag is set if the result of the last operation as was zero.
		const ZERO = 0b0000_0010;

		/// The carry flag is set if the last operation caused an overflow from bit 7 of
		/// the result or an underflow from bit 0. This condition is set during
		/// arithmetic, comparison and during logical shifts. It can be explicitly set
		/// using the 'Set Carry Flag' (SEC) instruction and cleared with 'Clear Carry
		/// Flag' (CLC).
		const CARRY = 0b0000_0001;
	}
}