ack.7

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Even when underscores are enabled,
they still may not start an identifier.
.PP
Constants of type \*(CwLONGINT\fR are integers with a suffix letter \*(CwD\fR
(for instance \*(Cw1987D\fR).
Constants of type \*(CwLONGREAL\fR have suffix \*(CwD\fR if a scale factor is missing,
or have \*(CwD\fR in place of \*(CwE\fR in the scale factor (f.i. \*(Cw1.0D\fR,
\*(Cw0.314D1\fR).
This addition was made,
because there was no way to indicate long constants,
and also because the addition was made in Wirth's newest Modula-2 compiler.
.SS "Declarations and scope rules (section 4)"
.PP
Standard identifiers are predeclared,
and valid in all
parts of a program.
They are called \fIpervasive\fR.
Unfortunately,
the Report does not state how this pervasiveness is accomplished.
However,
page 87 of [1] states: 'Standard identifiers are automatically
imported into all modules'.
Our implementation therefore allows
redeclarations of standard identifiers within procedures,
but not within
modules.
.SS "Constant expressions (section 5)"
.PP
Each operand of a constant expression must be a constant:
a string,
a number,
a set,
an enumeration literal,
a qualifier denoting a
constant expression,
a type transfer with a constant argument,
or one of the standard procedures
\*(CwABS\fR,
\*(CwCAP\fR,
\*(CwCHR\fR,
\*(CwLONG\fR,
\*(CwMAX\fR,
\*(CwMIN\fR,
\*(CwODD\fR,
\*(CwORD\fR,
\*(CwSIZE\fR,
\*(CwSHORT\fR,
\*(CwTSIZE\fR,
or \*(CwVAL\fR,
with constant argument(s);
\*(CwTSIZE\fR and \*(CwSIZE\fR may also have a variable as argument.
.SS "Type declarations (section 6)"
.LP
.ft I
1. Basic types (section 6.1)
.PP
The type \*(CwCHAR\fR includes the ASCII character set as a subset.
Values range from
\*(Cw0C\fR to \*(Cw377C\fR,
not from \*(Cw0C\fR to \*(Cw177C\fR.
.LP
.ft I
2. Enumerations (section 6.2)
.PP
The maximum number of enumeration literals in any one enumeration type
is \*(CwMAX(INTEGER)\fR.
.LP
.ft I
3. Record types (section 6.5)
.PP
The syntax of variant sections in [1] is different from the one in [2].
Our implementation recognizes both,
giving a warning for the older one.
.LP
.ft I
4. Set types (section 6.6)
.PP
The only limitation imposed by the compiler is that the base type of the
set must be a subrange type,
an enumeration type,
\*(CwCHAR\fR,
or \*(CwBOOLEAN\fR.
So,
the lower bound may be negative.
However,
if a negative lower bound is used,
the compiler gives a warning of the \fIrestricted\fR class.
.PP
The standard type \*(CwBITSET\fR is defined as
.XS
.CW
TYPE BITSET = SET OF [0 .. 8*SIZE(INTEGER)-1];
.ft P
.XE
.SS "Expressions (section 8)"
.LP
.ft I
1. Operators (section 8.2)
.LP
.ft I
1.1. Arithmetic operators (section 8.2.1)
.PP
The Report does not specify the priority of the unary
operators \*(Cw+\fR or \*(Cw-\fR:
It does not specify whether
.XS
.CW
- 1 + 1
.ft P
.XE
means
.XS
.CW
- (1 + 1)
.ft P
.XE
or
.XS
.CW
(-1) + 1
.ft P
.XE
The
.MX
Modula-2 compiler implements the second alternative.
.SS "Statements (section 9)"
.LP
.ft I
1. Assignments (section 9.1)
.PP
The Report does not define the evaluation order in an assignment.
Our compiler certainly chooses an evaluation order,
but it is explicitly left undefined.
Therefore,
programs that depend on it may cease to work later.
.PP
The types \*(CwINTEGER\fR and \*(CwCARDINAL\fR are assignment-compatible with
\*(CwLONGINT\fR,
and \*(CwREAL\fR is assignment-compatible with \*(CwLONGREAL\fR.
.LP
.ft I
2. Case statements (section 9.5)
.PP
The size of the type of the case-expression must be less than or equal to
the word-size.
.PP
The Report does not specify what happens if the value of the case-expression
does not occur as a label of any case,
and there is no \*(CwELSE\fR-part.
In our implementation,
this results in a runtime error.
.LP
.ft I
3. For statements (section 9.8)
.PP
The Report does not specify the legal types for a control variable.
Our implementation allows the basic types (except \*(CwREAL\fR),
enumeration types,
and subranges.
A runtime warning is generated when the value of the control variable
is changed by the statement sequence that forms the body of the loop,
unless runtime checking is disabled.
.LP
.ft I
4. Return and exit statements (section 9.11)
.PP
The Report does not specify which result-types are legal.
Our implementation allows any result type.
.SS "Procedure declarations (section 10)"
.PP
Function procedures must exit through a RETURN statement,
or a runtime error occurs.
.LP
.ft I
1. Standard procedures (section 10.2)
.PP
Our implementation supports \*(CwNEW\fR and \*(CwDISPOSE\fR
for backwards compatibility,
but issues warnings for their use.
.PP
Also,
some new standard procedures were added,
similar to the new standard procedures in Wirth's newest compiler:
.IP \-
\*(CwLONG\fR converts an argument of type \*(CwINTEGER\fR or \*(CwREAL\fR to the
types \*(CwLONGINT\fR or \*(CwLONGREAL\fR.
.IP \-
\*(CwSHORT\fR performs the inverse transformation,
without range checks.
.IP \-
\*(CwFLOATD\fR is analogous to \*(CwFLOAT\fR,
but yields a result of type
\*(CwLONGREAL\fR.
.IP \-
\*(CwTRUNCD\fR is analogous to \*(CwTRUNC\fR,
but yields a result of type
\*(CwLONGINT\fR.
.SS "System-dependent facilities (section 12)"
.PP
The type \*(CwBYTE\fR is added to the \*(CwSYSTEM\fR module.
It occupies a storage unit of 8 bits.
\*(CwARRAY OF BYTE\fR has a similar effect to \*(CwARRAY OF WORD\fR,
but is safer.
In some obscure cases the \*(CwARRAY OF WORD\fR mechanism does not quite
work properly.
.PP
The procedure \*(CwIOTRANSFER\fR is not implemented.
.SS "Backwards compatibility"
.PP
Besides recognizing the language as described in [1],
the compiler recognizes most of the language described in [2],
for backwards compatibility.
It warns the user for old-fashioned
constructions (constructions that [1] does not allow).
If the \fI\(en3\fR option is passed to \fIm2\fR,
this backwards compatibility feature is disabled.
.SS "Compile time errors"
.PP
The compile time error messages are intended to be self-explanatory,
and not listed here.
The compiler also sometimes issues warnings,
recognizable by a warning-classification between parentheses.
There are 3 classifications:
.IP "(old-fashioned use)"
.br
These warnings are given on constructions that are not allowed by [1],
but are allowed by [2].
.IP (strict)
.br
These warnings are given on constructions that are supported by the
.MX
Modula-2 compiler,
but might not be supported by others.
Examples: functions returning structured types,
SET types of subranges with
negative lower bound.
.IP (warning)
.br
The other warnings,
such as warnings about variables that are never assigned,
never used,
etc.
.SS "Runtime errors"
.PP
The \fITraps\fR module enables the user to install his own runtime
error handler.
The default one just displays what happened and exits.
Basically,
a trap handler is just a procedure that takes an INTEGER as
parameter.
The INTEGER is the trap number.
This INTEGER can be one of the
EM trap numbers,
listed in [3],
or one of the numbers listed in the
\fITraps\fR definition module.
.PP
The following runtime errors may occur:
.IP "array bound error"
.br
This error is detected if the \fI\(enA\fR option is given to \fIm2\fR.
.IP "range bound error"
.br
Range bound errors are always detected,
unless runtime checks are disabled.
.IP "set bound error"
.IP "cardinal overflow"
.br
This error is detected,
unless runtime checks are disabled.
.IP "cardinal underflow"
.br
This error is detected,
unless runtime checks are disabled.
.IP "divide by 0"
.IP "divide by 0.0"
.IP "conversion error"
.br
This error occurs when assigning a negative value of type INTEGER to a
variable of type CARDINAL,
or when assigning a value of CARDINAL that is > MAX(INTEGER),
to a variable of type INTEGER.
It is detected,
unless runtime checking is disabled.
.IP "heap overflow"
.br
This might happen when ALLOCATE fails.
.IP "case error"
.br
This error occurs when non of the cases in a CASE statement are selected,
and the CASE statement has no ELSE part.
.IP "stack size of process too large"
.br
This is most likely to happen if the reserved space for a coroutine stack
is too small.
In this case,
increase the size of the area given to
\*(CwNEWPROCESS\fR.
It can also happen if the stack needed for the main
process is too large and there are coroutines.
In this case,
the only fix is to reduce the stack size needed by the main process,
f.i. by avoiding local arrays.
.IP "too many nested traps + handlers"
.br
This error can only occur when the user has installed his own trap handler.
It means that during execution of the trap handler another trap has occurred,
and that several times.
In some cases,
this is an error because of overflow of some internal tables.
.IP "no RETURN from function procedure"
.br
This error occurs when a function procedure does not return properly
('falls' through).
.IP "illegal instruction"
.br
This error might occur when you use floating point operations on an
implementation that does not have floating point.
.PP
In addition,
some of the library modules may give error messages.
The \fBTraps\fR-module has a suitable mechanism for this.
.SS "The procedure call interface"
.PP
Parameters are pushed on the stack in reversed order.
For VAR parameters,
its address is passed,
for value parameters its value.
The only exception to this rule is with conformant arrays.
For conformant arrays,
the address is passed,
and an array descriptor is
passed.
The descriptor is an EM array descriptor.
It consists of three
fields: the lower bound (always 0),
upper bound \(mi lower bound,
and the size of the elements.
The descriptor is pushed first.
If the parameter is a value parameter,
the called routine must make sure
that its value is never changed,
for instance by making its own copy
of the array.
.PP
When the size of the return value of a function procedure is larger than
the maximum of \*(CwSIZE(LONGREAL)\fR and twice the pointer-size,
the caller reserves this space on the stack,
above the parameters.
Callee then stores
its result there,
and returns no other value.
.SS "The Modula-2 runtime library"
.PP
The definition modules of the modules available in the
.MX
Modula-2 runtime library reside in the directory \fI/usr/lib/ack/m2\fR.
.SS References
.IP [1]
Niklaus Wirth,
.ft I
Programming in Modula-2, third, corrected edition,
.ft R
Springer-Verlag, Berlin (1985)
.IP [2]
Niklaus Wirth,
.ft I
Programming in Modula-2,
.ft R
Stringer-Verlag, Berlin (1983)
.IP [3]
A.S.Tanenbaum, J.W.Stevenson, Hans van Staveren, E.G.Keizer,
.ft I
Description of a machine architecture for use with block structured languages,
.ft R
Informatica rapport IR-81, Vrije Universiteit, Amsterdam
.SH "THE MINIX PASCAL COMPILER"
.PP
.de IT
.PP
.B BS\ \\$1:
..
.de IS
.PP
.in +.5i
..
.PP
This section refers to the (1982) BSI standard for Pascal [1].
.MX
Pascal complies with the requirements of level 1 of BS 6192: 1982, with
the exceptions as listed in this section.
.PP
The standard requires an accompanying document describing the
implementation-defined and implementation-dependent features,
the reaction on errors and the extensions to standard Pascal.
These four items will be treated in the rest of this section.
.SS "Implementation-defined features"
.PP
For each implementation-defined feature mentioned in the BSI standard
we give the section number, the quotation from that section and the definition.
First we quote the definition of implementation-defined:
.PP
.RS
Possibly differing between processors, but defined for any particular
processor.
.RE
.IT 6.1.7
Each string-character shall denote an implementation-defined value of the
required char-type.
.IS
All 7-bit ASCII characters except linefeed LF (10) are allowed.
.IT 6.4.2.2
The values of type real shall be an implementation-defined subset
of the real numbers denoted as specified by 6.1.5 by the signed-real values.
.IS
The set of real values range from a low of \(mi1.7976931348623157e+308 to
a high of 1.7976931348623157e+308.
.IT 6.4.2.2
The type char shall be the enumeration of a set of implementation-defined