$a invert=0; cap=0; capsh=0; und=0; undsh=0
$a line=80; page=0; top=0; bottom=0
$l
APM IMP Compiler  --  Version 2
$b
The IMP compiler translates programs written in the high-level
language IMP to M68000 native code.
$b
The operation of the compiler is very similar to the APM
Pascal compiler -- for example, the available options are identical.
$b$l0
!<ALERT information

.  This information relates to the Version 2 Compiler
   Modules compiled with this compiler are incompatible with
   (ie cannot be linked with) modules compiled by the V1 compiler.

.  The checking of unassigned variables and nil references is not
   100 per cent.

$b2$l
!>
!<Calling the compiler
$b
$l0
                         Example commands

 IMP MYPROG                  Compile MYPROG with default options
                                  producing object file MYPROG.MOB

 IMP PARSER-LIST             Compile PARSER producing object file PARSER.MOB 
                                  and listing file PARSER.LIS
 
 IMP NEW.IMP-NOCHECK-NODIAG  Compile NEW.IMP to NEW.MOB
                                  without checks and diagnostic hooks

 IMP PROG2-NOEDIT            Compile PROG2 to PROG2.MOB
                                  without calling the editor to correct faults
                                  detected in the program
$b$l1
!<Parameters
$b
There is only one obligatory parameter for the IMP command
and that is the name of the IMP source program to be compiled.
There is no default file-name extension for the source file-name.
As a matter
of personal preference, programmers may or may not choose to adopt the
extension .IMP for IMP programs.
$b
There are two output files which may be
generated by the Compiler: the object
file containing the compiled code and the listing file.
By default, the names for these files are derived from the source file-name.
The default names may be overridden by specifying alternative names,
either positionally or by keyword. For example, the generation of an
object file may be suppressed in either of the following forms:
$l0
           IMP PROG2/:N
    or     IMP PROG2-OFILE=:N
$b
Similarly a listing could be directed to a designated file by:
$l0
           IMP MYPROG/,PUB:TEMP         {note the comma
    or     IMP MYPROG-LFILE=PUB:TEMP
$l2
!>
!<Object file
$b
By default, the Compiler generates an object file provided that no
errors are detected during compilation.
$b
If no name is provided explicitly, the name is derived from the
source file-name (without the extension .IMP if present).
The extension .MOB (for Motorola object) is applied to the object file-name.
$b
The compiled code
file is eligible for execution by citing the name (without extension)
as a command verb at APM command level.
There is no linking stage after compilation; the object file is
immediately executable.
$b
For example:
$l3
      } IMP MYPROG
      MYPROG compiled: 115 statements (+ 38 comments) to 1220 bytes (+ 24)
      } MYPROG
$b$l1
!<Object file control
$b
$a tab=2,6,25,40
$a indent=3
$t- -OFILE=<filename> $t+ produce object file with specified name
$b0 (the extension .MOB is added unless present already)
$b$t- -OFILE=:N $t+ do not produce object file
$b$t- -FORCE $t+ produce object file even if program faulty
$a indent=0
$l3
!>
!>
!<Listing file
$b
By default the Compiler does not produce a listing. If one is required,
it is requested either by including the keyword -LIST in the command
or by specifying a file-name as the second output parameter or as the
value for the keyword -LFILE.
$b
If no name is provided explicitly, the name is derived from the
source file-name (without the extension .IMP if present).
The extension .LIS is applied to the listing file-name.
$b
The listing file consists
of the text of the source file with added line numbers, and any
fault or warning messages produced during compilation. Line numbering
runs from 1 at the start of the file and 
includes blank lines and
comment lines. Lines in included files are numbered independently,
with an ampersand as an indicator.
$b$l
!<Listing control
$a indent=3
$b
$t- -LIST $t+ produce source file listing
$b$t- -LFILE=<filename> $t+ send listing to specified file
$b0 (the extension .LIS is added unless already present)
$b$t- -TT $t+ send listing to terminal
$b
$t- -MAP $t+ output information at end of each procedure,
indicating size of code, etc.
$b1$t- -LOG $t+ print statistics at end of compilation
indicating number of statements,
atoms per statement, identifiers per statement,
time taken, etc.
$l1
!PAGE
$t1 {The following may not be available in the standard release}
$b$t- -CODE $t+ print hybrid assembly language interpretation
of code generated for each statement.  This is
before address fix-up and branch shortening,
and so does not fully reflect the final code
$b$t- -DICT $t+ print dictionary entries for identifiers
$a indent=0
$b$l3
!>
!>
!<Options
$l1
Run-time checks:
$a indent=1
$b
To disable any individual check, use the indicated keyword
prefixed by NO, for example -NOSTRASS
$a indent=3
$b$t- -ASS $t+ include unassigned check on full integers
$b$t- -STRASS $t+ include unassigned check on strings
$b$t- -SASS $t+ include unassigned check on 16-bit values
$b$t- -BASS $t+ include unassigned check on 8-bit values
$b$t1 Defaults: -ASS -STRASS -NOSASS -NOBASS
$a indent=1
$b
The unassigned check is implemented by standardising all
newly declared dynamic variables to a fixed pattern which
is an improbable integer value. The check is defaulted off
for variables occupying less than 32 bits
because there is a greater possibility
of the special pattern occurring as a genuine value.
$l1
!PAGE
$b
Note that almost all examples of the violations listed below which involve
only literal operands are detected and rejected at compile-time rather than
run-time.
$a indent=3
$b$t- -ARR $t+ include array bound checking
$b$t- -OVER $t+ include overflow check on addition/subtraction
$b$t- -CAP $t+ include capacity checking on assignment
$b$t- -STACK $t+ include stack over-run check
$b$t1 Defaults: -ARR -CAP -STACK
$b$t- -NOCHECK $t+ suppress all checks apart from stack over-run
$b$t- -NOCHECK-NOSTACK $t+ suppress all checks
$l1
!PAGE
$l1
Run-time diagnostic control:
$b$t- -LINE $t+ include line-number updating for diagnostics
$b$t- -TRACE $t+ generate code which allows the program to
be executed one line at a time under the
control of the Software Front Panel
$b$t1 Default: -LINE -NOTRACE
$b2$l1
Compiler limits:
$b$t- -IDENTS=n $t+ allow for n user idents (default 1000)
$b0 $t+ (also controls the total lengths allowed for)
$b$t- -KBYTES=n $t+ allow for codesize nK (default 64k, max 128k)
$l1
!PAGE
$l1
Compile-time control:
$b$t- -NONS $t+ enable warning-free use of non-standard features
$b$t- -STRICT $t+ exclude use of non-standard features
$b$t- -LOW $t+ enable use of low-level features
$b$t- -VOL $t+ assume functions and for-loop increments and end-values
$b0$t+ are volatile
$b$t- -WARN $t+ request soft warnings
$b$t- -EDIT $t+ automatically transfer control to the standard editor
(VECCE) on detection of an error in the program. Compilation
resumes when the edit is closed (just hit CPY if you do not
want to correct the error at this stage).
$b$t1 Default: -NONONS -NOSTRICT -NOLOW -VOL -WARN -EDIT
$a indent=0
$b$l3
!>
!>
!<Language facilities
$b
The IMP language as implemented on the APM is substantially the same
as the Vax/VMS version
as defined in the EUCSD Report "The IMP77 Language"
(3rd Edition).
Differences and special options are described in
the following sections.
$a tab=4,8,12; indent=2
$b
$l1
!<Exclusions
$l0
$b
$i- .$t+ full diagnostics and %monitor are not yet available
$b
$i- .$t+ %string(*)%array%name variables are not implemented
$b
$i- .$t+ procedures and functions passed as parameters must be in scope to the
procedure to which they are passed
$b
$i- .$t+ there is no built-in function TYPEOF
$b
$i- .$t+ untyped %name parameters are usable only to supply an address,
not type or length information; the built-in function SIZEOF
is not applicable to such parameters
$b
$i- .$t+ %name %function as variant for %map is not accepted
$b
$i- .$t+ %on %event * is not permitted as a way of trapping all events
$b$l
!PAGE
$b
$i- .$t+ line-breaks after %and and %or are not ignored
$b
$i- .$t+ non-decimal constants expressed in IBM style (X'...') are accepted,
but with a Non-standard warning
$b
$i- .$t+ the Atlas Autocode style of %for loop (without %for) is accepted,
but with a Non-standard warning
$b
$i- .$t+ loops with a control clause after as well as before (ie an %until
as well as a %while or %for) are accepted, but with a Non-standard
warning
$a indent=0
$l2
!>
!<Modified features
$a indent=2
$b
$i- .$t+ the effect of %continue is to pass control to the head of
the containing loop, where any %while or %for control clause is
tested; any %until clause at the end of the loop is ignored.
$b
$i- .$t+ if no initial value is specified for an own variable, this means
that it is initially unassigned, NOT zero.
$i- .$t+ the preferred way of initialising %name variables of all kinds
is in the form "==NIL". The earlier form "==0" is also accepted, with a warning.
$b
$i- .$t+ event 9 is used exclusively for end-of-input; file system errors
are signalled as event 3, and non-numeric input for READ as event 4.
$b
$i- .$t+ the Compiler is stricter about the ordering of statements
than the Vax IMP Compiler.  The normal ordering in any block
should be declarations, then event trap if any, then instructions.
It is acceptable for static declarations (eg procedures and %own
variable declarations) to be interspersed among instructions, but the Compiler
queries any dynamic variable declarations which appear after
instructions, and hard-faults any which appear in a loop or after a
sequence change.
$l
!PAGE
$b
$i- .$t+ In the declaration of an array name variable, as in
$l0i
        %INTEGER %ARRAY %NAME A(lo1:hi1,...,loN:hiN)
each of the lower and upper bounds may be:
$a indent=3
$i- (a) $t+ a literal or literal expression, for example:
$l2i
        %integer%array%name a(1:1000)
        %real%array%name delta(10:20,1:30)
In this case, the corresponding bound of every actual parameter
must have the identical literal value.
$i- (b) $t+ a variable declaration, for example:
$l2i
        %byte%array%name used(1:%integer xdim,1:%integer ydim)
        %short%array%name count(%byte lo:%byte hi)
In this case, the corresponding bound of the actual parameter is
assigned to the stated variable at the time of procedure entry.
This case, which is similar to the Pascal conformant array concept,
is confined to %array %names as procedure parameters.  It cannot
be used for %array %names declared as ordinary variables.
$i- (c) $t+ an asterisk, for example:
$l1i
        %integer%array%name cases(1:*)
The use of an asterisk should be confined to the UPPER bound of
the FIRST (outer) dimension only, and when it appears, all other
bounds should be literal.  It implies an unspecified number of
elements in the array (and the absence of bound-checking even
with checks on).
$b
$a indent=2
For the time being, the Vax/VMS form ... %array(n)%name ... is also
accepted, but note that the '(n)' part is obligatory even when n is one.
$a indent=0
$l2
!>
!<New warning messages
$b
Warnings are issued in a number of cases where it may be useful to
have an indication of possible hazards at compile-time rather than
run-time.
$a indent=2
$l0
$b
$i- .$t+ All unused switch index values are reported
$b0 [add a switch label with index (*) to eliminate]
$b
$i- .$t+ A report is made if the destination variable(s) in a string
resolution are not manifestly capacious enough to receive whatever
might be assigned to them.
$b0 [increase the length (to 255 if necessary) to eliminate]
$a indent=0
$l2
!>
!<Data types
$b$l0
%integer      range: -2147483648 (-2^31) to 2147483647 (2^31 - 1)
              storage: 32 bits
%long%integer  not accepted

%short        range: -32768 to 32767         storage: 16 bits
%half         range: 0 to 65535              storage: 16 bits
%byte         range: 0 to 255                storage: 8 bits
%mite         range: -128 to 127             storage: 8 bits

%real         reals are limited to single-precision (32-bits) and the
              implementation is by software
%long%real    treated as %real, apart from type-checking

%record, %array, %string -- standard
$b$l2
!>
!<%OPTION
$b
Compiler options are generally established at the outset in the command
used to call the compiler. However, most of them may also be switched off and on
within the program text by means of the directive
$b$l0
                       %OPTION "...."

         for example   %OPTION "-NOWARN-NOLIST"
$b
The quoted string must consist of a sequence of Option keywords each preceded by
a dash (hyphen).
$b$l2
    Caution: indiscriminate variation of checking options within a
             program can create considerable confusion for debugging.
$b
The effect of options is localised to the current procedure or block.

$b$l2
!>
!<%INCLUDE
$b
Other files may be included in the source by means of the directive
$b$l0
                       %INCLUDE "...."

         for example   %INCLUDE "GRAPHICS.IMP-NOCHECK"
$b
The quoted string
specifies the name of the file together with any Options
to be applied to it. These Options, and any which are specified
within the included file itself, are localised to this file.
$b
Include files may be nested to a maximum of three.
$b$l2
!>
!<Record initialisation
$b
A record type identifier may be used as a constructor for that
record type, from components values specified in parentheses
following the record type identifier. At present only literal
component values are permitted. This form is particularly useful
for specifying initial values for records, but it is not restricted
to that context.
$b
The component values may be
presented either positionally or
using the record component names, preceded by the sub character ('$_'),
as keywords.
$b$l1
!<Examples of use of record constructors
$l0

%record%format f1(%integer lo,hi, %string(7) code,
                 %byte%name where, %short weight)

%own%record(f1) r1=0
  {Meaning: numeric values = 0, strings = "", names == NIL}

%own%record(f1) r2=f1(-100,100,"FACTOR",nil,1)
%own%record(f1) r3=f1(0,,"DUMMY")
  {By enumeration, any omitted values unassigned}

%own%record(f1) r4=f1(_code="UNDEF",_weight=99)
%own%record(f1) r5=f1(_hi=99999,_code="MAX")
  {By selection using field names, any omitted values unassigned}
  { field names must appear in order of occurrence in declaration}
    
$b$l3
!>
!>
!<Declaration attributes
$b
The keyword %volatile may be prefixed to a function declaration to indicate
that the result of the function is not a pure function of its arguments.
This stops the compiler optimising repeated calls.
$b2
The prefix '@'<machine-address> may be placed in front of a variable
or procedure declaration to indicate that the corresponding entity
is to be addressed at the location indicated. The <machine-address>
may be either an absolute value or a register-relative value.
For example:
$l0

    @16_041234 %byte DEVICE STATUS
    @16_1BC8   %routine ABANDON   (formally equivalent to an %external %spec)
    @4(A5)     %integer USERNO
$b
Caution: this is a Low-level facility
$b$l2
!>
!<Name-relative addressing
$b
When a number of elements of a given type are stored consecutively,
for example, in an array or mapped file, it is sometimes convenient
to use a %name variable as a base point for addressing different
elements.
The name-relative addressing facility allows this to be done.
This takes the form of a %name variable followed by an index value enclosed in
square
brackets. Such an expression denotes a reference to the variable
located that number of elements away from the element identified
by the current assignment to the name variable.
$b
Caution: this is a Low-level facility
$b There is no
check on the validity of the references. Use of the facility is, however,
preferable to
the use of store-mapping functions in a similar role, because it preserves
the type identity of the objects denoted.
$b$l
!<Example
$b$l0
With the following definitions and assignment:

          %recordformat       PAIR(%real x,y)
          %record(pair)%array A(1:100)
          %record(pair)%name  P
          P == A(7)

the following would hold:

          P[2]  would denote A(9)
          P[-6] would denote A(1)
          P == P[1] would advance P to denote A(8)
$b$l4
!>
!>
!>
!<Standard libraries
$b$l
!<Pre-declared procedures
$b
The procedures listed below are available without specification.
$b
$l0
%integer%map     INTEGER(%integer a)
%real%map        REAL(%integer a)
%string(*)%map   STRING(%integer a)
%record(*)%map   RECORD(%integer a)
%byte%map        BYTEINTEGER(%integer a)
%short%map       SHORTINTEGER(%integer a)
%byte%map        LENGTH(%string(*)%name s)
%byte%map        CHARNO(%string(*)%name s, %integer n)
%integer%fn      ADDR(%name n)
!PAGE
%string(1)%fn    TOSTRING(%integer k)
%string(255)%fn  SUBSTRING(%string(255) s, %integer from,to)

%integer%fn      REM(%integer a,b)

%integer%fn      INTPT(%real x)
%integer%fn      INT(%real x)
%real%fn         FRACPT(%real x)
%real%fn         SQRT(%real x)

%integer%fn      CPUTIME  
                 !in milliseconds (since process creation)

%name            NIL
%record%format   EVENTFM(%byte event,sub, %short line, %integer extra,
                 %string(255) message,%integerarray r(0:15))
                 !LINE and R are APM-specific extensions
%record(eventfm) EVENT
!PAGE
%const%integer   NL
%const%string    SNL
%integer%fn      NEXTSYMBOL
%routine         READSYMBOL(%name n)
%routine         PRINTSYMBOL(%integer k)
%routine         SKIPSYMBOL
%routine         PRINTSTRING(%string(255) s)
%routine         READ(%name n)
%routine         WRITE(%integer m, n)
%routine         PRINT(%real x, %integer n,m)
%routine         PRINTFL(%real x, %integer n)
%routine         NEWLINE
%routine         NEWLINES(%integer i)
%routine         SPACE
%routine         SPACES(%integer i)
%routine         SELECT INPUT(%integer n)
%routine         SELECT OUTPUT(%integer n)
%routine         CLOSE INPUT
%routine         CLOSE OUTPUT
!PAGE
%routine         SET INPUT(%integer pos)
                 !establish new active position for current input stream
                 ! set input(0) resets to the beginning of a file
                 !!!  temporary restriction: only POS=0 accepted ***
%routine         SET OUTPUT(%integer pos)
                 !establish new active position for current output stream
                 ! set output(0) resets to the beginning of a file
                 !!!  temporary restriction: only POS=0 accepted ***
%routine         RESET INPUT    {equivalent to SET INPUT(0)
%routine         RESET OUTPUT   {equivalent to SET OUTPUT(0)
%integer%fn      INSTREAM
%integer%fn      OUTSTREAM
%routine         OPEN INPUT(%integer n, %string(255) S)
                 !*selects N at present*
%routine         OPEN OUTPUT(%integer n, %string(255) S)
                 !*selects N at present*
%string(255)     CLIPARAM
                 ! returns the parameter string supplied with the command
                 ! invoking the program
%routine         PROMPT(%string(255) S)
$b$l2
!>
!<General-purpose Libraries
$b
There are a number of libraries available on the system which provide
groups of related procedures for use in writing programs. In order to
make use of these, it is necessary to include the necessary declarations
in the source program. Also, when running the program, it may be necessary to
INSTALL the related object file (see the general system information on
Libraries).
$b
Short files containing the relevant declarations for the main system
libraries are available for general use in the directory INC. These
files may be 'included' in IMP programs by means of the $%INCLUDE
statement, for example, %INCLUDE 'INC:UTIL.IMP'.
$b
Library interface files currently available are:
$l0
          INC:UTIL.IMP  --  sundry utility procedures
          INC:FS.IMP    --  file system procedures

        [others under preparation]

!<Example of use of UTIL

                  {Simple file copy program}
    %begin
    %include "INC:UTIL.IMP"
    %string(255) inname,outname
    %on %event 9 %start;  !end-of-file
      %stop
    %finish
      define param("Input file:",inname,PamNodefault+PamInfile)
      define param("Output file:",outname,PamNewGroup+PamNoDefault+PamOutfile)
      process parameters(cliparam)
      print line("Copying ".inname." to ".outname)
      select input(1);  select output(1)
      %cycle
        read symbol(ch);  print symbol(ch)
      %repeat
    %endofprogram
$b$l3
!>
!>
!<External linkage
$b
The pre-prepared libraries make use of the general external linkage
mechanism implemented for IMP. This allows external REFERENCES in one
program to be matched up with external DEFINITIONS in another program
or module. The linkage takes place automatically when the program is run.
$b
In IMP, an external reference starts with the keyword %external and
includes the keyword %spec after the type. For example:
$l2
    %external %routine %spec INITIALISE(%integer case)
    %external %integer %spec CURRENT DEVICE
$b
In APM IMP, external references must appear at the outermost textual
level, that is, before the first %begin or in the main program block.
An external spec for a procedure may be satisfied by a procedure
occurring later in the same module, but the same is not true for variables.
$l
!PAGE
$b
External definitions start with the keyword %external. For example:
$l0
    %external %routine INITIALISE(%integer case)
       ......
    %end

    %external %integer CURRENT DEVICE=2
$b0 These must appear in the source file before the main block (if any).
$b2
A program module containing external procedures or variables, and
terminated by %endoffile, is compiled in the ordinary way, eg IMP MYLIB.
Before the externals it contains can
be accessed, the compiled module must be installed by the INSTALL
command. The INSTALL command takes as parameter the name of a
program file (extension MOB assumed) -- or list of names
separated by commas. For example INSTALL MYLIB.
The effect of installing a program file is to add all the external names
it contains to the external symbol table; the
code of the module is not loaded at this stage.
Thereafter (until logoff) these names are
available for external linking, which is done automatically when
a program referencing any of the names is loaded.
It is only necessary to re-install a program file if a change is made
to the external names it contains.
$b
An external name is the identifier declared as external (standardised
to space-free lower-case form) unless this is over-ridden by an alias, in
which case the alias name is used instead (exactly as presented).
Note that an alias over-rides the declared identifier, rather than
providing an alternative to it.
$b$l2
!>
!<Cross-calling
$b
In general, data formats and the mechanisms for passing parameters
and function results
are completely compatible between IMP and
Pascal. This applies to full integers, reals, varying strings, records,
and arrays.
$b
IMP %name parameters are compatible with Pascal VAR parameters.
$b
IMP predicates are compatible with Pascal Boolean functions .
$b
The IMP %byte is storage compatible with the Pascal Char.
$b
Integer subrange correspondence is as follows:
$l0

             IMP                     Pascal

           %short                -32768..32767
           %half                      0..65535
           %mite                   -128..127
           %byte                      0..255
$b$l3
!>
!>
!<Compiler Error Reports
$b
Error reports are kept short to economise on screen space.
For reports which relate to a particular component of a statement,
the culprit is identified by a marker at the start of the component.
$b
$l0
 REPORT                     MEANING

Faulty form         statement part indicated is syntactically faulty
Unknown atom        lexical atom indicated is mis-spelt or unknown
Non-starter         atom at the start of the statement is not a
                      possible statement introducer
Unknown name        identifier indicated has not been declared
Duplicate           identifier indicated has already been declared
Mismatch            parameters in body of procedure do not match spec
Not variable        operand in assignment context is not a variable
Not reference       operand in pointer context is not a reference
Wrong type          expression is of wrong type for context
Wrong class         category of identifier is wrong for context
Not literal         expression in literal context is not literal
Inside out          upper bound is less than lower
Endless loop        literal %for loop cannot terminate
Out of range        operand value is out of range
Too few args        too few arguments are given for procedure call
Too many args       too many arguments are given for procedure call
Not in loop         %exit %or %continue is not within a loop construct
Not in routine      %return is not within a routine
Not in fn/map       %result is not within a function or map
Not in pred         %true/%false is not within a predicate
%CYCLE missing      %repeat encountered with no matching %cycle
%REPEAT missing     %end reached with unmatched %cycle
%START missing      %finish or %else encountered with no matching %start
%FINISH missing     %end reached with unmatched %start
Extra %ELSE         %else encountered matching earlier unconditional %else
%BEGIN missing      %end encountered with no matching %begin or
                      procedure header
%END missing        %end %of %program or %file reached with unmatched
                      %begin or procedure header
%RESULT missing     there is apparently a path to the end of a function
                      or map which does not specify a result
Not accessible      instruction apparently cannot be executed (warning only)
Out of order        statement appears in incorrect order in block
                      (warning if benign)
Nonstandard         non-standard language feature (warning only)
<ident> void        identifier is used before a value is assigned to it
                      (warning only)
<ident> missing     forward label or specced identifier does not appear
                      in block
<ident>(?) missing  switch label to which there is an explicit jump
                      has not been specified
<n> extra values for <ident>
                    too many values for %const or %own array
Faulty operand      incorrect operand type for machine instruction
Wrong size          operand is of incorrect size for machine instruction
!PAGE

Too complex!          compiler limitation exceeded (eg no free register)

Internal error <n>!   compiler fault

Out of reach!         %const or literal string, record or array is not
                      within the range of the machine addressing capability
<ident> Out of reach! call to procedure specified is not within the
                      range of the machine addressing capability

In the case of programs which exceed 32k bytes in size, the compiler
uses a number of techniques to avoid creating within-program references
which breach the M68000 PC-relative addressing limitation of +-32k.
There is a possibility in some cases that these do not succeed, leading
to an 'out-of-reach' report.
If a name is given in the report, it is that of a procedure which is too
distant from one of its calls.
If no name is given, the problem is access to a constant.
$l2
!PAGE
Disastrous errors
$b
These reports relate to compiler limits being exceeded. They all
cause compilation to be abandoned.
$l0

** Program too big          Total size of the compiled code and constants
                            exceeds the maximum allowed for (see Option -KBYTES)
** Code space exhausted     Size of the code for currently open blocks
                            exceeds the maximum allowed for
** Identifiers too big      Total length of all current identifiers exceeds
                            the maximum allowed for (see Option -IDENTS)
** Too many identifiers     Number of current identifiers exceeds the maximum
                            allowed for (see Option -IDENTS)
** Too many levels          Depth of textual nesting of blocks exceeds
                            the maximum permitted (8)
** Too many literals        Internal storage space for literals is exhausted
** Input ended              There is no %endofprogram or %endoffile statement
** Too many nested includes There are more than three levels of nesting
$b$l2
!>
!<Run-time diagnostics
$b
Errors detected at run-time are reported through the system's exception
signalling mechanism (also used for IMP 'events').
If an exception is not trapped, the occurrence of the exception is reported
in the form of an error code and text message.
In general, the line number at which the failure occurred is provided,
but full monitoring of program variables is not at present provided.
$b
For most errors, detection at run-time depends on whether particular
checking options were selected at compile-time. The availability of a
relevant line number depends on having compiled with the option -LINE.
The default compile-time options include line numbering and most checks,
but not unassigned checks on variables occupying less than 32 bits nor
overflow checks for integer addition or subtraction.
$b$l
!<Run-time errors
$l0
         ERROR                            CODE          REQUISITE
                                                         OPTION
Variable value undefined (unassigned)      8 1            -ASS, -BASS, etc
  [sometimes detected as "Void" at compile-time]

Array index out of range                   6 2 <culprit>  -ARR

Integer arithmetic overflow                1 1            -OVER
Real arithmetic overflow                   1 2            -OVER
Division by zero (integer or real)         1 4            none
Trig function error                       10              none

Assignment value out-of-range              6 1            -CAP
Value parameter out-of-range               6 1            -CAP

Switch label not catered for             taken as at %end
    [warning issued at compile-time]

Name variable NIL                          8 1            -ASS
Name variable undefined                    8 1            -ASS

File system error                          3 x            none

Non-numeric character for numeric READ     4 1 <culprit>

End-of-input                               9              none
$b$l3
!>
!>
!<Efficiency
$l0

 . In the absence of floating-point hardware, all floating-point
   operations are performed by software.

 . Integer multiply and divide are also performed by software, although
   a number of special cases are optimised.

 . Full checking is expensive in both time and space. Developed programs
   should be compiled with checks and diagnostics disabled.
   Fullest optimisation is obtained by specifying:
               -NOCHECK-NODIAG-NOSTACK-NOVOL

 . Procedure and function calls are implemented with a minimum of overhead
   so that they may be freely used without significant penalty in all but
   the most time-critical contexts. The space-saving from making full use
   of procedures and functions can be substantial.
   
$l2
!>
!<Low-level facilities
$b
As a system implementation language IMP provides a number of facilities
which permit access to parts of the system other languages do not reach.
The store-mapping functions (INTEGER, BYTEINTEGER, etc) allow for direct
addressing using machine addresses; the name-relative indexing facility
(as in CUR[1] or POS[-2]) extends the capability of %name variables;
the fixed-address declaration facility (as in @16_200C %byte TIMER, KBIN,
KBOUT) allows efficient source-level access to specific areas of store.
$b
The use of these facilities involves some loss of the protection
normally conferred by a high-level language and a potential (particularly
on a processor without hardware protection) for uncontrolled malfunction.
They should, therefore, be used with restraint and care.
$b
$l1
!<Assembler in IMP
$b
Sequences of assembly language instructions may be incorporated in an IMP
program at any point. At present no enabling directive is required.
Apart from the usual hazards of machine level programming, there
is the danger of corrupting the environment pre-supposed by the
high-level facilities. For this reason, it is sensible to keep
the use of these instructions to a minimum and to take
particular care in programming these sections.
$b
Assembly language instructions are distinguished from IMP
statements by being prefaced by an asterisk (eg *MOVE D0,D1).
They appear within IMP blocks and procedures and are executed
according to the normal flow of control, subject to any
control transfers invoked by the instructions themselves.
IMP conventions for statement termination, labelling and commenting
apply, but otherwise the form of instruction follows closely that
defined under the heading of Assembler Syntax for each individual instruction
in the Motorola M68000 manual.
$l
!PAGE
$b
Machine-level operands are specified using the mnemonics D0-D7
and A0-A7 (or SP), and the standard syntax for <ea> modes.
Immediate and Address register variants of op-codes are selected automatically.
By comparison with full Assembler, most of the directives do not apply;
assembly-time expression evaluation is restricted;
size specification in cases where the size is implicit in the
op-code is not in general allowed;
the indexed PC-relative mode is not supported, nor is the 'current
location counter'.
$b
The error report 'Faulty operand' indicates use of an invalid
operand for the context, but
the compiler does not fully check the validity of <ea> modes for the
particular instruction.
$b
NB the default 'size' applied is Long (32-bits)
rather than Word (16-bits) which is the manufacturer's default.
Programmers may choose to make a practice of including the size suffix
explicitly for all relevant instructions.
$l
!PAGE
$b
The special mnemonics USP (User Stack
Pointer), SR (Status Register) and CCR (Condition Code Register)
are NOT supported but the relevant effects can be achieved by
use of the additional op-codes:
$l0
  MTCCR <ea>          for      MOVE <ea>,CCR
  MTSR <ea>           for      MOVE <ea>,SR
  MFSR <ea>           for      MOVE SR,<ea>
  MTUSP An            for      MOVE An,USP
  MFUSP An            for      MOVE USP,An
  ATCCR #<data>       for      AND #<data>,CCR
  ATSR #<data>        for      AND #<data>,SR
  ETCCR #<data>       for      EOR #<data>,CCR
  ETSR #<data>        for      EOR #<data>,SR
  OTCCR #<data>       for      OR #<data>,CCR
  OTSR #<data>        for      OR #<data>,SR
$l
!PAGE
$b
IMP identifiers may also be used as operands for assembly language
instructions.
The detailed implications of this over the whole range of data types
are fairly complex and subject to change. The following
cases are reasonably straightforward and stable.
Scalar variables or elements of scalar records
may be used as <ea> operands provided that they
are:
$l4
       declared as own
   or  declared at the outermost level
   or  declared at the current level
   or  declared by means of a fixed-address declaration
Variables declared at intermediate levels should not be
accessed since they are not necessarily directly addressable.
In the case of a %name variable the effective operand is
the pointer value (32-bit address) rather than the referenced
object.
$l
!PAGE
$b
Labels and procedure names may be referenced in the
Branch group of instructions, including BSR. The only alternative
form of operand for these instructions is immediate (eg *BLT #-4),
the value specified being the machine-level displacement.
Short and long branches are handled automatically (though the
Compiler's CODE listing does not show this).
To access a forward label in an assembly language instruction,
it is neccesary for the label to have been declared by means
of the IMP declaration $%LABEL <lab>.
$b
The registers D0-D3 and A0-A3 may be freely used within
assembly sections, but no assumptions can be made about
their contents after execution of most IMP statements.
Other registers may be used only on the basis that their values
are restored before reverting to IMP. In particular this applies
to SP. In addition, NOTE WELL that the accessing of local variables depends on
SP; if SP is changed, access to such variables in machine instructions
becomes a nonsense (though no error report can be made).
Similarly, any modification of A4-A6 rules out access to non-local
variables.
$b
Note that any declaration of IMP identifiers which co-incide with
the register mnemonics takes precedence.
$l2
!>
!>