IsDigit PROC

   Determines whether the character in AL is a valid decimal digit.

Call args:  AL = character

Return arg: ZF=1 valid digit
            ZF=0 not a valid digit
Example:

.data
achar BYTE ?

.code
      mov  al,achar
      call IsDigit
      jnz  notDigit
      ...           ;process digit
notDigit: