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Different CPU types

The Rockwell data booklet 29651N52 (technical information about R65C00 microprocessors, dated October 1984), lists the following differences between NMOS R6502 microprocessor and CMOS R65C00 family:

 1. Indexed addressing across page boundary.
        NMOS: Extra read of invalid address.
        CMOS: Extra read of last instruction byte.

 2. Execution of invalid op codes.
        NMOS: Some terminate only by reset. Results are undefined.
        CMOS: All are NOPs (reserved for future use).

 3. Jump indirect, operand = XXFF.
        NMOS: Page address does not increment.
        CMOS: Page address increments and adds one additional cycle.

 4. Read/modify/write instructions at effective address.
        NMOS: One read and two write cycles.
        CMOS: Two read and one write cycle.

 5. Decimal flag.
        NMOS: Indeterminate after reset.
        CMOS: Initialized to binary mode (D=0) after reset and interrupts.

 6. Flags after decimal operation.
        NMOS: Invalid N, V and Z flags.
        CMOS: Valid flag adds one additional cycle.

 7. Interrupt after fetch of BRK instruction.
        NMOS: Interrupt vector is loaded, BRK vector is ignored.
        CMOS: BRK is executed, then interrupt is executed.
base/different_cpu_types.txt · Last modified: 2015-04-17 04:31 (external edit)