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    Flags register

    The flags register contains various bits that control and record the
    state of the microprocessor, as defined below.


               Bit:    15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0
                      +-----------------------------------------------+
    Flags register:   |  |  |  |  |OF|DF|IF|TF|SF|ZF|  |AF|  |PF|  |CF|
                      +-----------------------------------------------+

    DEBUG readout:
    Flag clear (0)                 NV UP DI .. PL NZ    NA    PO    NC
    Flag set   (1)                 OV DN EI .. NG ZR    AC    PE    CY

    ------------------------------- Flags ------------------------------

    Overflow            Set when an arithmetic overflow occurred.  An
                        arithmetic overflow occurs when the size of a
                        computation exceeds the size of the
                        destination.

    Direction           Set for auto-decrement with string
                        instructions, clear for auto-increment.

    Interrupt Enable    Interrupts are enabled as long as this flag is
                        set.  When this flag is cleared, interrupts
                        except for nonmaskable interrupts are disabled.

    Trap                This flag is used by debuggers to single step
                        through programs.  When this flag is set, an
                        INT 3 is generated after every instruction.

    Sign                Set when the high-order bit of the result is 1.
                        In other words, S = 0 for positive numbers and
                        S = 1 for negative numbers.

    Zero                Set whenever the result is 0.

    Auxiliary Carry     Set when the is a carry out of the lower half
                        of an 8 or 16 bit number, or when there is a
                        borrow from the upper to the lower half.  This
                        flag is used mainly by the decimal-arithmetic
                        instructions.

    Parity Flag         Set if there is an even number of 1-bits in the
                        result.  Cleared if there is an odd number of
                        1-bits.

    Carry Flag          Set if there was a carry out of, or a borrow
                        into the high-order bit of the result.  This
                        flag is useful for propagating carries and
                        borrows for multi-word numbers.