!TITLE Constants

a)  An integer constant of  any  integer  base  from  2  to  36  may  be
    specified.  The form is

          base_constant

    where  base  is a decimal value and constant is an integer expressed
    with respect to the base.  The letters A, B, ..., Y, Z are  used  to
    represent the digits 10, 11, ...., 34, 35 in the integer.

    Examples:
          2_1010         ten in binary
          8_12           ten in octal
          16_A            ten in hexadecimal
!PAGE
    An  alternative  form  is provided for binary, octal and hexadecimal
    constants:

          B'1010'     ten in binary
          K'12'       ten in octal
          X'A'        ten in hexadecimal

b)  Variables of all types can be given the attribute  %constant.   This
    can  be  considered a special form of %own variable, which cannot be
    changed from its initial value.  However it is  probably  better  to
    consider  such variables as "named constants", since 1) this accords
    with their intended use, i.e. for  replacing  arithmetic  or  string
    constants  within  code by meaningful names; and 2) they do not have
    addresses, unlike other variables (but like constants).

    Wherever a constant is permitted in an IMP80  program,  a  "constant
    expression" can be used instead.  A constant expression is one which
    can be evaluated at compile-time, i.e. its operands are constants or
    named constants.
!PAGE
    Example:
          %string(73) DELIVERY

    can be replaced by

          %constant %integer MAXNAME=20, MAXADDRESS=52
          %string(MAXNAME+1{for the newline}+MAXADDRESS) DELIVERY

    Example:
          %constant %integer NO=0, YES=1,
                             INPUT=1, CALCULATION=2,
                             OUTPUT=3
          %switch PHASE(INPUT:OUTPUT)
          :
          :
          ->PHASE(OUTPUT) %if DONE=YES
          :
          :
          PHASE(OUTPUT): ! Now print the results