Routine:        Vischk
Function:       Check that the viscosity is within the user defined range
Called by:      Pb2, Pb3.
Calls:          Error, Sub
Entry:          Viscty holds th viscosity value to be checked, Vismin and Vismax hold the preset values
Exit:           Z Flag set if viscosity is within the range
Preserved:      None


VISCHK  LD      BC,VISCTY
        LD      DE,VISMIN
        LD      HL,NUM1
        CALL    SUB
; Subtract Vismin from the viscosity, and signal error 8 if the result is negative.
        PUSH    HL
        POP     IX
        BIT     7,(IX+3)
        JR      Z,VISCH1
        LD      A,8
        CALL    ERROR
        INC     A
; Inc A to reset the Z flag, signifying that an error occurred.
        RET
VISCH1  LD      BC,VISMAX
        LD      DE,VISCTY
        LD      HL,NUM1
        CALL    SUB
; Subtract Vismax from the viscosity, and signal error 9 if the result is negative.
        BIT     7,(IX+3)
        RET     Z
        LD      A,9
        CALL    ERROR
        INC     A
; Inc A to reset the Z flag, signifying that an error occurred.
        RET