THE IMP INTERMEDIATE CODE



   The IMP Intermediate Code may be considered the
order-code of a stack-oriented machine which manipulates
objects exclusively through descriptors.  The code is made
up from ASCII characters (See Appendix 2 for a complete list
of valid characters).  Note that the symbols 'SPACE' and
'NEWLINE' are not valid characters; their inclusion in the
examples is purely for ease of reading, they do not occur in
the actual code.

The workspace for this machine comprises three areas:

    1.   Descriptor Area     This is used to hold all
                             descriptors which are currently
                             defined.

    2.   Parameter Area      This is used to hold the
                             descriptors for procedure
                             (formal) parameters and record
                             format elements.

    3.   Stack               This is used to hold copies of
                             descriptors which are being
                             processed.
                             Note that in general these
                             copies are removed whenever
                             they are accessed.
   The code is input by a program which "simulates" the
operations specified and thereby generates code for the
target machine.
   The code is not intended to be interpreted.
   It is important to note that the intermediate code is a
set of commands to a code-generator and that the operations
described are compile-time operations.  This means that, for
example, the operation '@' (q.v.) cannot be used to stack a
descriptor as the result of a condition.  Hence the ALGOL
statement:
                 A := if B=C then D else E
could not be encoded as:
               @a @b @c ? #1 @d F2 :1 @e :2 S


                         NOTATION

    [V]       -    The descriptor for the entity with name
                   V.
    @V        -    Stack the descriptor for V.

   Unless otherwise stated all numerical constants in the
intermediate code are in octal.




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