Routine:        Mark
Function:       Counts "Mark" of the Mark/Space ratio
Called by:      Read
Calls:          None
Entry:          HL points to the location mark will be stored at
Exit:           Mark stored at HL
Preserved:      None


MARK    IN      A,(3)
        LD      E,A
        IN      A,(4)
        LD      D,A
        IN      A,(5)
        LD      C,A
; Input the counter values through ports 3, 4 and 5.
        LD      B,24
        XOR     A
MARK1   BIT     7,C
        JR      NZ,MARK2
; Normalise by shifting left until the leftermost bit is a 1.
        SLA     E
        RL      D
        RL      C
        DEC     A
        DJNZ    MARK1
MARK2   RES     7,C
; Store the result at HL.
        LD      (HL),0
        INC     HL
        LD      (HL),E
        INC     HL
        LD      (HL),D
        INC     HL
        LD      (HL),C
        INC     HL
        LD      (HL),A
        RET