treated as an address from which a variable is fetched, or to which a
   variable is stored, according to the context of the call. Mapping
   functions thus enable symbolic names to be given to areas of storage
   outside the normal stack area allocated to the program. The
   pre-declared mapping functions:

   integer (I) real (I) string (I) shortinteger (I) longreal (I) record
   (I) byteinteger (I)

   are used to access a variable of the corresponding type whose address
   is given by the integer expression I. These functions are often used in
   conjunction with pointer variables.

   e.g. On the ICL 4-75 the word whose address is 72 is the Channel
   Address Word. To access this it is necessary to code:

   integername caw caw == integer (72) Hereafter, any reference to caw
   accesses the Channel Address Word. The function: addr (v) can be used
   to obtain the address of an IMP variable and the special mapping
   function: array can be used to map arrays on to data files. The mapping
   of arrays and records is a powerful and widely used facility which is
   very similar in effect to PL/I's 'BASED STRUCTURE'.

   Machine code It is possible to write assembly code at any point within
   an IMP program although, naturally, this is strongly discouraged. All
   the instructions are available and IMP variables and labels may be
   used. This seemed preferable to providing special functions to permit
   Supervisor to use the privileged instructions. The presence of assembly
   code served also to reassure those who are certain that a high level
   language is too 'inefficient' for a supervisor program.

   Punching conventions To facilitate punching on terminals or on cards, %
   character is reserved as a shift character to indicate that the
   following word is underlined. Statements are terminated by a semi-colon
   or by a newline. Consequently, if a statement is to occupy more than
   one line, all lines except the last are terminated by the continuation
   symbol e. Spaces and superfluous terminating characters are ignored
   (except within string constants). The " (double quotes) character is
   not used within the language and is used by EMAS Director (Rees, 1973)
   as a delete character when accepting input from a terminal. Comments
   may be inserted by means of: comment [text] or ! [text] The Atlas
   Autocode fault statement: fault [list of error conditions] → label has
   been retained. Its effect is to intercept non-catastrophic errors and
   to restart the program from the specified label. If a fault occurs
   which has not been trapped, execution of the program ceases and a stack
   post mortem is output. An example of such a termination is given in
   Appendix I. The unconditional instruction monitor can be placed at any
   point in the program. Its effect is to obtain a post mortem print
   without otherwise disturbing execution of the program.

   Language facilities withdrawn It may be of interest to comment on three
   features of the original IMP specification which have been discontinued
    1. Arrays of pointer variables e.g. Integer name array na (1:50). Name
       arrays hold the addresses of variables assigned using the address
       assignment operator ==. These inoffensive variables proved of
       little use and were dropped.
    2. Routine variables These were provided in an attempt to extend the
       routine parameter mechanism. Declarations took the form routine
       name r routine name array ra (1:m) Routines could be assigned to
       routine variables e.g. ra(1) == select input ra(2) == select output

   and finally a statement of the form ra(i) (k) calls the routine that
   was last assigned to ra(i) passing k as a parameter. To enable the call
   to be compiled, it was necessary to restrict the routines assigned to
   routine variables to ones having the same parameter structure. It was
   further necessary to ensure that all routines assigned were global to
   the routine variable declarations—otherwise calls could be made on
   routines when their global variables were not present on the stack.
   These restrictions emasculated what appeared to be an interesting
   facility.
    3. Dynamic formats The early IMP compilers allowed arrays in record
       formats to have dynamic bounds. This meant that format statements
       had run time significance and that a dope-vector was required with
       each format. The price in execution time was judged to be too high
       for the advantages provided.

   Compiler restrictions The current EMAS compiler imposes two
   restrictions on the language described. (a) The static depth of nested
   blocks must not exceed eleven levels of which not more than five may be
   routine ... end groupings. (b) Own arrays, switches, and arrays within
   record formats are restricted to one dimension only.

   Compiler diagnostic facilities The compiler can operate in checking or
   optimising mode, the former being the default. In checking mode,
   additional instructions are planted to ensure that: (a) No variable is
   used before a value has been assigned to it. (b) All references to
   array elements are within the declared bounds. (c) No truncation takes
   place when assigning to variables of type byte integer, short integer
   or string. (d) Overflow is tested at every stage of every arithmetic
   operation. (e) Any cycle of the form cycle i = p, q, r will terminate.
   (f) Every switch label is set. (g) The source line which corresponds to
   the object code currently being executed is known. (h) Pointers are
   maintained to ensure a useful post-mortem can be produced in source
   language terms.