Trait Variant

Source

pub trait Variant {
    // Required methods
    fn decode(opcode: u8) -> Option<(Instruction, AddressingMode)>;
    fn adc_binary(
        accumulator: u8,
        value: u8,
        carry_set: bool,
    ) -> ArithmeticOutput;
    fn adc_decimal(
        accumulator: u8,
        value: u8,
        carry_set: bool,
    ) -> ArithmeticOutput;
    fn sbc_binary(
        accumulator: u8,
        value: u8,
        carry_set: bool,
    ) -> ArithmeticOutput;
    fn sbc_decimal(
        accumulator: u8,
        value: u8,
        carry_set: bool,
    ) -> ArithmeticOutput;

    // Provided methods
    fn penalty_cycles_for_decimal_mode() -> u8 { ... }
    fn penalty_cycles_for_indirect_jmp() -> u8 { ... }
}

Expand description

Trait for different 6502 CPU variants with their historical differences.

The 6502 family evolved over decades with various manufacturers creating specialized versions for different applications:

Revision A (1975): Very early 6502 variant missing the ROR instruction entirely. Found in early KIM-1 systems and some Apple-1 computers. The ROR instruction was intentionally unimplemented due to design constraints, not a “bug” as commonly believed. Production ended around June 1976.
NMOS 6502 (1976): Complete MOS Technology processor with working ROR instruction, used in Apple II, Commodore 64, Atari 2600. Has unreliable decimal mode flags but full BCD support. This became the standard reference implementation.
65C02 (1983): WDC’s CMOS version with bug fixes, additional instructions, and reliable decimal mode flags. Development began in 1981 with samples released in early 1983. Used in Apple IIc/IIe and many embedded systems.
Ricoh 2A03 (1983): Nintendo’s cost-reduced variant for NES/Famicom (released July 15, 1983). Removed decimal mode entirely to avoid patent issues. Used as a core in their custom ASIC which also included sound generation and other features.

Choose the variant that matches your target system for accurate emulation. Note that software written for later variants may not run on earlier ones due to missing instructions (particularly ROR on Revision A).