ack/doc/pcref.doc

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.               DATE
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Wiskundig Seminarium
Vrije Universiteit
De Boelelaan 1081
Amsterdam
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.               IMPLEMENTATION 1
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.TL "Amsterdam Compiler Kit-Pascal reference manual"
.AU "Johan W. Stevenson"
.DA "January 4, 1983"
.VU
.CH "Introduction"
This document refers to the (March 1980) ISO standard proposal for Pascal [1].
Ack-Pascal complies with the requirements of this proposal almost completely.
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 document,
each in a separate chapter.
The other chapters describe the deviations from the standard and
the list of options recognized by the compiler.
.PP
The Ack-Pascal compiler produces code for an EM machine as defined in [2].
It is up to the implementor of the EM machine to decide whether errors like
integer overflow, undefined operand and range bound error are recognized or not.
For these errors the reaction of some known implementations is given.
.PP
There does not (yet) exist a hardware EM machine.
Therefore, EM programs must be interpreted, or translated into
instructions for a target machine.
For the following implementations the behavior is documented:
.I1
an interpreter running on a PDP-11.
Normally the interpreter performs some tests to detect undefined
integers, integer overflow, range errors, etc.
However, an option of the interpreter is to skip these tests.
Another option is to perform some extra tests
to check for instance the number of actual parameter
words against the number expected by
the called procedure.
We will refer to these modes of operation as 'test all', 'test on' and 'test off'.
.I2
a translator into PDP-11 instructions.
.IE
.CH "Implementation-defined features"
For each implementation-defined feature mentioned in the ISO standard
we give the section number, the quotation from that section and the definition.
First we quote the definition of implementation-defined:
.DS
Those parts of the language which may differ between processors, but which
will be defined for any particular processor.
.DE
.IT 6.1.7
Each string-character shall denote an implementation-defined value of char-type.
.IS
All 7-bits ASCII characters except linefeed LF (10) are allowed.
Note that an apostrophe ' must be doubled within a string.
.IE
.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.
.IS
The format of reals is not defined in EM.
Even the size of reals depends on the implementation.
The compiler can be instructed, by the f-option, to use a different
size for real values.
The size of reals is preset by the calling program \fIack\fP
[4] to
the proper size.
For each implementation of EM the following constants must be defined:
     epbase: the base for the exponent part
     epprec: the precision of the fraction
     epemin: the minimum exponent
     epemax: the maximum exponent
.br
These constants must be chosen so that zero and all numbers with
exponent e in the range
.EQ
     epemin <= e <= epemax
.EN
and fraction-parts of the form
.EQ
     f = +_ f\d1\u.b\u-1\d + ... + f\depprec\u.b\u-epprec\d
.EN
where
.EQ
     f\di\u = 0,...,epbase-1 and f\d1\u <> 0
.EN
are possible values for reals.
All other values of type real are considered illegal.
(See [3] for more information about these constants).
.br
For the known EM implementations these constants are:
.I1
epbase = 2
.br
epprec = 24
.br
epemin = -127
.br
epemax = +127
.I2
ditto
.IE
.IT 6.4.2.2
The type char shall be the enumeration of a set of implementation-defined
characters, some possibly without graphic representations.
.IS
The 7-bits ASCII character set is used, where LF (10) denotes the
end-of-line marker on text-files.
.IT 6.4.2.2
The ordinal numbers of the character values shall be values of integer-type,
that are implementation-defined, and that are determined by mapping
the character values on to consecutive non-negative integer values
starting at zero.
.IS
The normal ASCII ordering is used: ord('0')=48, ord('A')=65, ord('a')=97, etc.
.IE
.IT 6.4.3.4
The largest and smallest values of integer-type
permitted as numbers of a value
of a set-type shall be implementation-defined.
.IS
The smallest value is 0. The largest value is default 15, but can be
changed by using the i-option of the compiler up to a maximum
of 32767.
The compiler allocates as many bits for set-type variables as are necessary
to store all possible values of the host-type of the base-type of the set,
rounded up to the nearest multiple of 16.
If 8 bits are sufficient then only
8 bits are used if part of a packed structure.
Thus, the variable s, declared by
.EQ
     var s: set of '0'..'9';
.EN
will contain 128 bits, not 10 or 16.
These 128 bits are stored in 16 bytes, both for packed and unpacked sets.
If the host-type of the base-type is integer, then the
number of bits depends on the i-option.
The programmer may specify how many bits to allocate for these sets.
The default is 16, the maximum is 32767.
The effective number of bits is rounded up to the next multiple of 16, or up
to 8 if the number of bits is less than or equal to 8.
Note that the use of set-constructors for sets with more than 256 elements
is far less efficient than for smaller sets.
.IT 6.7.2.2
The predefined constant maxint shall be of integer-type and shall denote
an implementation-defined value, that satisfies the following conditions:
.sp 1
.in +5
.ti -4
(a)~All integral values in the closed interval from -maxint to +maxint
shall be values in the integer-type.
.ti -4
(b)~Any monadic operation performed on an integer value in this interval
shall be correctly performed according to the mathematical rules for
integer arithmetic.
.ti -4
(c)~Any dyadic integer operation on two integer values in this same interval
shall be correctly performed according to the mathematical rules for
integer arithmetic, provided that the result is also in this interval.
.ti -4
(d)~Any relational operation on two integer values in this same interval
shall be correctly performed according to the mathematical rules for
integer arithmetic.
.in -5
.IS
The representation of integers in EM is a \fIn\fP*8-bit word using
two's complement arithmetic.
Where \fIn\fP is called wordsize.
The compiler can only generate code for EM with wordsize 2.
Thus always:
.EQ
     maxint = 32767
.EN
Because the number -32768 may be used to indicate 'undefined', the
range of available integers depends on the EM implementation:
.I1
\*(ON-32767..+32767.
.br
\*(OF-32768..+32767.
.I2
-32768..+32767.
.IE
.IT 6.9.4.2
The default TotalWidth values for integer, Boolean and real types
shall be implementation-defined.
.IS
The defaults are:
     integer    6
     Boolean    5
     real      13
.IT 6.9.4.5.1
ExpDigits, the number of digits written in an exponent part of a real,
shall be implementation-defined.
.IS
ExpDigits is defined as
.EQ
     ceil(log10(log10(2 ** epemax)))
.EN
For the current implementations this evaluates to 2.
.IT 6.9.4.5.1
The character written as part of the representation of
a real to indicate the beginning of the exponent part shall be
implementation-defined, either 'E' or 'e'.
.IS
The exponent part starts with 'e'.
.IT 6.9.4.6
The case of the characters written as representation of the
Boolean values shall be implementation-defined.
.IS
The representations of true and false are 'true' and 'false'.
.IT 6.9.6
The effect caused by the standard procedure page
on a text file shall be implementation-defined.
.IS
The ASCII character form feed FF (12) is written.
.IT 6.10
The binding of the variables denoted by the program-parameters
to entities external to the program shall be implementation-defined if
the variable is of a file-type.
.IS
The program parameters must be files and all, except input and output,
must be declared as such in the program block.
.PP
The program parameters input and output, if specified, will correspond
with the UNIX streams 'standard input' and 'standard output'.
.PP
The other program parameters will be mapped to the argument strings
provided by the caller of this program.
The argument strings are supposed to be path names of the files to be
opened or created.
The order of the program parameters determines the mapping:
the first parameter is mapped onto the first argument string etc.
Note that input and output are ignored in this mapping.
.PP
The mapping is recalculated each time a program parameter
is opened for reading or writing by a call to the standard procedures
reset or rewrite.
This gives the programmer the opportunity to manipulate the list
of string arguments using the external procedures argc, argv and argshift
available in libpc [7].
.IT 6.10
The effect of an explicit use of reset or rewrite
on the standard textfiles input or output shall be implementation-defined.
.IS
The procedures reset and rewrite are no-ops
if applied to input or output.
.CH "Implementation-dependent features"
For each implementation-dependent feature mentioned in the ISO standard draft,
we give the section number, the quotation from that section and the way
this feature is treated by the Ack-Pascal system.
First we quote the definition of 'implementation-dependent':
.DS
Those parts of the language which may differ between processors,
and for which there need not be a definition for a particular processor.
.DE
.IT 5.1.1
The method for reporting errors or warnings shall be implementation-dependent.
.IS
The error handling is treated in a following chapter.
.IE
.IT 6.1.4
Other implementation-dependent directives may be defined.
.IS
Except for the required directive 'forward' the Ack-Pascal compiler recognizes
only one directive: 'extern'.
This directive tells the compiler that the procedure block of this
procedure will not be present in the current program.
The code for the body of this procedure must be included at a later
stage of the compilation process.
.PP
This feature allows one to build libraries containing often used routines.
These routines do not have to be included in all the programs using them.
Maintenance is much simpler if there is only one library module to be
changed instead of many Pascal programs.
.PP
Another advantage is that these library modules may be written in a different
language, for instance C or the EM assembly language.
This is useful if you want to use some specific EM instructions not generated
by the Pascal compiler. Examples are the system call routines and some
floating point conversion routines.
Another motive could be the optimization of some time-critical program parts.
.PP
The use of external routines, however, is dangerous.
The compiler normally checks for the correct number and type of parameters
when a procedure is called and for the result type of functions.
If an external routine is called these checks are not sufficient,
because the compiler can not check whether the procedure heading of the
external routine as given in the Pascal program matches the actual routine
implementation.
It should be the loader's task to check this.
However, the current loaders are not that smart.
Another solution is to check at run time, at least the number of words
for parameters. Some EM implementations check this:
.I1
\*(ALthe number of words passed as parameters is checked, but this will not catch all faulty cases.
.br
\*(ONnot checked.
.I2
not checked.
.IE
.PP
For those who wish the use the interface between C and Pascal we
give an incomplete list of corresponding formal parameters in C and Pascal.
.sp 1
.ta 8 37
.nf
	Pascal	C
	a:integer	int a
	a:char	int a
	a:boolean	int a
	a:real	double a
	a:^type	type *a
	var a:type	type *a
	procedure a(pars)	struct {
		     void (*a)() ;
		     char *static_link ;
		}
	function a(pars):type	struct {
		     type (*a)() ;
		     char *static_link ;
		}
.fi
The Pascal runtime system uses the following algorithm when calling
function/procedures passed as parameters.
.nf
.ta 8 16
	if ( static_link ) (*a)(static_link,pars) ;
	else               (*a)(pars) ;
.fi
.IT 6.7.2.1
The order of evaluation of the operands of a dyadic operator
shall be implementation-dependent.
.IS
Operands are always evaluated, so the program part
.EQ
     if (p<>nil) and (p^.value<>0) then
.EN
is probably incorrect.
.PP
The left-hand operand of a dyadic operator is almost always evaluated
before the right-hand side.
Some peculiar evaluations exist for the following cases:
.IS
.ti -3
1.~\
the modulo operation is performed by a library routine to
check for negative values of the right operand.
.IE
.sp
.ti -3
2.~\
the expression
.EQ
     set1 <= set2
.EN
where set1 and set2 are compatible set types is evaluated in the
following steps:
.IS
.CS
evaluate set2
.CS
evaluate set1
.CS
compute set2+set1
.CS
test set2 and set2+set1 for equality
.IE
This is the only case where the right-hand side is computed first.
.sp
.ti -3
3.~\
the expression
.EQ
     set1 >= set2
.EN
where set1 and set2 are compatible set types is evaluated in the following steps:
.IS
.CS
evaluate set1
.CS
evaluate set2
.CS
compute set1+set2
.CS
test set1 and set1+set2 for equality
.IE
.IT 6.7.3
The order of evaluation, accessing and binding
of the actual-parameters for functions
shall be implementation-dependent.
.IS
The order of evaluation is from right to left.
.IT 6.8.2.2
If access to the variable in an assignment-statement involves the indexing of an array
and/or a reference to a field within a variant of a record
and/or the de-referencing of a pointer-variable
and/or a reference to a buffer-variable,
the decision whether these actions precede or follow the evaluation
of the expression shall be implementation-dependent.
.IS
The expression is evaluated first.
.IT 6.8.2.3
The order of evaluation and binding of the actual-parameters for procedures
shall be implementation-dependent.
.IS
The same as for functions.
.IT 6.9.6
The effect of inspecting a text file to which the page
procedure was applied during generation is
implementation-dependent.
.IS
The formfeed character written by page is
treated like a normal character, with ordinal value 12.
.IT 6.10
The binding of the variables denoted by the program-parameters
to entities external to the program shall be implementation-dependent unless
the variable is of a file-type.
.IS
Only variables of a file-type are allowed as program parameters.
.IE
.CH "Error handling"
There are three classes of errors to be distinguished.
In the first class are the error messages generated by the compiler.
The second class consists of the occasional errors generated by the other
programs involved in the compilation process.
Errors of the third class are the errors as defined in the standard by:
.DS
An error is a violation by a program of the requirements of this standard
such that detection normally requires execution of the program.
.DE
.SH "Compiler errors"
The error messages (and the listing) are not generated by the compiler itself.
The compiler only detects errors and writes the errors in condensed form on
an intermediate file.
Each error in condensed form contains:
.IS
.CS
an optional error message parameter (identifier or number).
.CS
an error number
.CS
a line number
.CS
a column number.
.IE
Every time the compiler detects an error that does not have influence
on the code produced by the compiler or on the syntax decisions, a warning
messages is given.
If only warnings are generated, compilation proceeds and probably results
in a correctly compiled program.
.PP
The intermediate error file is read by the interface program
\fIack\fP [4],
that produces the error messages.
It uses an other file, the error message file,
to find an error script line.
Whenever this error script line contains the character '%', the error messages
parameter is substituted.
For negative error numbers the message constructed is prepended with 'Warning: '.
.PP
Sometimes the compiler produces several errors for the same file position
(line number, column number).
Only the first of these messages is given, because the others are probably
directly caused by the first one.
If the first one is a warning while one of its successors for that position
is a fatal message, then the warning is promoted to a fatal one.
However, parameterized messages are always given.
.PP
The error messages and listing come in three flavors, selected by flags
given to \fIack\fP [4]:
.in +10
.sp
.ti -8
default:no listing, one line per error giving the file name
of the Pascal source file, the line number and the error messages.
.sp
.ti -8
-e:~~~~~for each erroneous line a listing of the line and its predecessor.
The next line contains one or more characters '^' pointing to the
places where an error is detected.
For each error on that line a message follows.
.sp
.ti -8
-E:~~~~~same as for '-e', except that all source lines are listed,
even if the program is perfect.
.IE
.IE
.SH "Other errors detected at compilation time"
Two main categories: file system problems and table overflow.
Problems with the file system may be caused by protection (you may not read
or create files) or by space problems (no space left on device; out of inodes;
too many processes).
Table overflow problems are often caused by peculiar source programs:
very long procedures or functions, a lot of strings.
Table overflow problems can sometimes be cured
by giving a flag (-sl when producing e.out files) to \fIack\fP [4].
.PP
Extensive treatment of these errors is outside the scope of this manual.
.SH "Runtime errors"
Errors detected at run time cause an error message to be generated on the
diagnostic output stream (UNIX file descriptor 2).
The message consists of the name of the program followed by a message
describing the error, possibly followed by the source line number.
Unless the l-option is turned off, the compiler generates code to keep track
of which source line causes which EM instructions to be generated.
It depends on the EM implementation whether these LIN instructions
are skipped or executed:
.I1
LIN instructions are always executed. The old line number is saved and
restored whenever a procedure or function is called.
All error messages contain this line number, except when the l-option
was turned off.
.I2
same as above, but line numbers are not saved when procedures and functions
are called.
.IE
For each error mentioned in the standard we give the section number,
the quotation from that section and the way it is processed by the
Pascal-compiler or runtime system.
.PP
For detected errors the corresponding message
and trap number are given.
Trap numbers are useful for exception-handling routines.
Normally, each error causes the program to terminate.
By using exception-handling routines one can
ignore errors or perform alternate actions.
Only some of the errors can be ignored
by restarting the failing instruction.
These errors are marked as non-fatal,
all others as fatal.
A list of errors with trap number between 0 and 63
(EM errors) can be found in [2].
Errors with trap number between 64 and 127 (Pascal errors) are listed in [8].
.IT 6.4.3.3
It shall be an error if any field-identifier defined within a variant
is used in a field-designator unless the value of the tag-field
is associated with that variant.
.IS
This error is not detected.
Sometimes this feature is used to achieve easy type conversion.
However, using record variants this way is dangerous, error prone and not portable.
.IT 6.4.6
It shall be an error if a value of type T2 must be
assignment-compatible with type T1, while
T1 and T2 are compatible ordinal-types and the value of
type T2 is not in the closed interval specified by T1.
.IS
The compiler distinguishes between array-index expressions and the other
places where assignment-compatibility is required.
.PP
Array subscripting is done using the EM array instructions.
These instructions have three arguments: the array base address,
the index and the address of the array descriptor.
An array descriptor describes one dimension by three values:
the element size, the lower bound on the index and the number of elements
minus one.
It depends on the EM implementation whether these bounds are checked:
.I1
\*(ONchecked (array bound error, trap 0, non-fatal).
.br
\*(OFnot checked
.I2
not checked.
.IE
The other places where assignment-compatibility is required are:
.IS
.CS
assignment
.CS
value parameters
.CS
procedures read and readln
.CS
the final value of the for-statement
.IE
For these places the compiler generates an EM range check instruction, except
when the r-option is turned off, or when the range of values of T2
is enclosed in the range of T1.
If the expression consists of a single variable and if that variable
is of a subrange type,
then the subrange type itself is taken as T2, not its host-type.
Therefore, a range instruction is only generated if T1 is a subrange type
and if the expression is a constant, an expression with two or more
operands, or a single variable with a type not enclosed in T1.
If a constant is assigned, then the EM optimizer removes the range check
instruction, except when the value is out of bounds.
.PP
It depends on the EM implementation whether the range check instruction
is executed or skipped:
.I1
\*(ONchecked (range bound error, trap 1, non-fatal).
.br
\*(OFskipped
.I2
skipped
.IE
.IT 6.4.6
It shall be an error if a value of type T2 must be
assignment-compatible with type T1, while T1 and T2 are compatible
set-types and any member of the value of type T2
is not in the closed interval specified by the base-type
of the type T1.
.IS
This error is not detected.
.IT 6.5.4
It shall be an error if
the pointer-variable has a nil-value or is undefined at the time
it is de-referenced.
.IS
The EM definition does not specify the binary representation of pointer
values, so that it is not possible to choose an otherwise illegal
binary representation for the pointer value NIL.
Rather arbitrary the compiler uses the integer value zero to represent NIL.
For all current implementations this does not cause problems.
.PP
The size of pointers depends on the implementation and is
preset in the compiler by \fIack\fP [4].
The compiler can be instructed, by the p-option, to use
any size for pointer objects.
NIL is represented here by the appropriate number of zero words.
.PP
It depends on the EM implementation whether de-referencing of a pointer
with value NIL causes an error:
.I1
\*(ONfor every de-reference the pointer value is checked to be legal.
The value NIL is always illegal.
Objects addressed by a NIL pointer always cause an error, except
when they are part of some extraordinary sized structure
(bad pointer, trap 22, fatal).
.br
\*(OFde-referencing for fetching will not cause
an error to occur.
However, if the pointer value is used for a store operation,
a segmentation violation probably results (memory fault, trap 21, fatal).
(Note: this is only true if the interpreter is executed with coinciding
address spaces and protected text part. The interpreter must therefore
be loaded with the '-n' option of the UNIX loader [5]).
.I2
de-referencing for a fetch operation will not cause an error.
A store operation probably causes an error if the '-n' flag is
specified to \fIack\fP [4] or ld [5] while loading your program.
.IE
Some implementations of EM initialize all memory cells for newly
created variables with a constant that probably causes an error if that variable
is not initialized with a value of its own type before use.
For each implementation we give whether memory cells are initialized,
with what value, and whether this value causes an error if de-referenced.
.I1
each memory word is initialized with the bit representation 1000000000000000,
representing -32768 in 2's complement notation.
For most small and medium sized programs this value will cause a segmentation
violation (memory fault, trap 21, fatal).
.I2
no initialization.
Whenever a pointer is de-referenced, without being properly initialized,
a segmentation violation (memory fault, trap 21, fatal)
or 'bus error' are possible.
.IE
.IT 6.5.5
It shall be an error if the value of a file-variable f is altered
while the buffer-variable is an actual variable parameter, or
an element of the record-variable-list of a with-statement, or both.
.IS
This error is not detected
.IT 6.5.5
It shall be an error if the value of a file-variable f is altered
by an assignment-statement which contains the buffer-variable f^ in
its left-hand side.
.IS
This error is not detected.
.IT 6.6.5.2
It shall be an error if
the stated pre-assertion does not hold immediately
prior to any use of the file handling procedures
rewrite, put, reset and get.
.IS
For each of these four operations the pre-assertions
can be reformulated as:
.sp
rewrite(f):~no pre-assertion.
.br
put(f):~~~~~f is opened for writing and f^ is not undefined.
.br
reset(f):~~~f exists.
.br
get(f):~~~~~f is opened for reading and eof(f) is false.
.sp
The following errors are detected for these operations:
.sp
rewrite(f):
.in +10
.ti -5
more args expected, trap 64, fatal:
.br
f is a program-parameter and the corresponding
file name is not supplied by the caller of the program.
.ti -5
rewrite error, trap 101, fatal:
.br
the caller of the program lacks the necessary
access rights to create the file in the file system
or operating system problems like table overflow
prevent creation of the file.
.in -10
.sp
put(f):
.in +10
.ti -5
file not yet open, trap 72, fatal:
.br
reset or rewrite are never applied to the file.
The checks performed by the run time system are not foolproof.
.ti -5
not writable, trap 96, fatal:
.br
f is opened for reading.
.ti -5
write error, trap 104, fatal:
.br
probably caused by file system problems.
For instance, the file storage is exhausted.
Because IO is buffered to improve performance,
it might happen that this error occurs if the
file is closed.
Files are closed whenever they are rewritten or reset, or on
program termination.
.in -10
.sp
reset(f):
.in +10
.ti -5
more args expected, trap 64, fatal:
.br
same as for rewrite(f).
.ti -5
reset error, trap 100, fatal:
.br
f does not exist, or the caller has insufficient access rights, or
operating system tables are exhausted.
.in -10
.sp
get(f):
.in +10
.ti -5
file not yet open, trap 72, fatal:
.br
as for put(f).
.ti -5
not readable, trap 97, fatal:
.br
f is opened for writing.
.ti -5
end of file, trap 98, fatal:
.br
eof(f) is true just before the call to get(f).
.ti -5
read error, trap 103, fatal:
.br
unlikely to happen. Probably caused by hardware problems
or by errors elsewhere in your program that destroyed
the file information maintained by  the run time system.
.ti -5
truncated, trap 99, fatal:
.br
the file is not properly formed by an integer
number of file elements.
For instance, the size of a file of integer is odd.
.ti -5
non-ASCII char read, trap 106, non-fatal:
.br
the character value of the next character-type
file element is out of range (0..127).
Only for text files.
.in -10
.IT 6.6.5.3
It shall be an error to change any variant-part of a variable
allocated by the form new(p,c1,...,cn) from the variant specified.
.IS
This error is not detected.
.IT 6.6.5.3
It shall be an error if a variable to be disposed had been allocated
using the form new(p,c1,...,cn) with more variants specified than
specified to dispose.
.IS
This error can cause more memory to be freed then was allocated.
Dispose causes a fatal trap 73 when memory already on the free
list is freed again.
.IT 6.6.5.3
It shall be an error if the variants of a variable to be disposed
are different from those specified by the case-constants to dispose.
.IS
This error is not detected.
.IT 6.6.5.3
It shall be an error if the value of the pointer parameter of dispose has
nil-value or is undefined.
.IS
The same comments apply as for de-referencing NIL or undefined pointers.
.IT 6.6.5.3
It shall be an error if a variable that is identified by the pointer parameter
of dispose (or a component thereof) is currently either an actual
variable parameter, or an element of the record-variable-list of a
with-statement, or both.
.IS
This error is not detected.
.IT 6.6.5.3
It shall be an error if a referenced-variable created using the second form
of new is used in its entirety
as an operand in an expression, or as the variable in an assignment-statement
or as an actual-parameter.
.IS
This error is not detected.
.IT 6.6.6.2
It shall be an error if the mathematical defined result of an
arithmetic function would fall outside the set of values
of the indicated result.
.IS
Except for the errors for undefined arguments,
the following errors may occur for the arithmetic functions:
.in +16
.ti -11
abs(x):~~~~none.
.ti -11
sqr(x):~~~~real underflow, trap 5, non-fatal;
.br
real overflow, trap 4, non-fatal
.ti -11
sin(x):~~~~real underflow, trap 5, non-fatal
.ti -11
cos(x):~~~~real underflow, trap 5, non-fatal
.ti -11
exp(x):~~~~error in exp, trap 65, non-fatal (if x>10000);
.br
real underflow, trap 5, non-fatal;
.br
real overflow, trap 4, non-fatal
.ti -11
ln(x):~~~~~error in ln, trap 66, non-fatal ( if x<=0)
.ti -11
sqrt(x):~~~error in sqrt, trap 67, non-fatal (if x<0)
.ti -11
arctan(x):~real underflow, trap 5, non-fatal;
.br
real overflow, trap 4, non-fatal
.in -16
.IE
.IT 6.6.6.2
It shall be an error if x in ln(x) is not greater than zero.
.IS
See above.
.IT 6.6.6.2
It shall be an error if x in sqrt(x) is negative.
.IS
See above.
.IT 6.6.6.2
It shall be an error if
the integer value of trunc(x) does not exist.
.IS
This error is detected (conversion error, trap 10, non-fatal).
.IT 6.6.6.2
It shall be an error if
the integer value of round(x) does not exist.
.IS
This error is detected (conversion error, trap 10, non-fatal).
.IT 6.6.6.2
It shall be an error if
the integer value of ord(x) does not exist.
.IS
This error can not occur, because the compiler will not allow
such ordinal types.
.IT 6.6.6.2
It shall be an error if
the character value of chr(x) does not exist.
.IS
Except when the r-option is turned off, the compiler generates an EM
range check instruction. The effect of this instruction depends on the
EM implementation as described before.
.IT 6.6.6.2
It shall be an error if the value of succ(x) does not exist.
.IS
Same comments as for chr(x).
.IT 6.6.6.2
It shall be an error if the value of pred(x) does not exist.
.IS
Same comments as for chr(x).
.IT 6.6.6.5
It shall be an error if
f in eof(f) is undefined.
.IS
This error is detected (file not yet open, trap 72, fatal).
.IT 6.6.6.5
It shall be an error if
f in eoln(f) is undefined, or if eof(f) is true at that time.
.IS
The following errors may occur:
.IS
file not yet open, trap 72, fatal;
.br
not readable, trap 97, fatal;
.br
end of file, trap 98, fatal.
.IE
.IT 6.7.1
It shall be an error if any variable or function used as an operand in an expression is
undefined at the time of its use.
.IS
Detection of undefined operands is only possible if there is at least one bit
representation that is not allowed as legal value.
The set of legal values depends on the type of the operand.
To detect undefined operands, all newly created variables must be assigned
a value illegal for the type of the created variable.
The compiler itself does not generate code to initialize newly created variables.
Instead, the compiler generates code to allocate some new memory cells.
It is up to the EM implementation to initialize these memory cells.
However, the EM machine does not know the types of the variables for which
memory cells are allocated.
Therefore, the best an EM implementation can do is to initialize with a value
that is illegal for the most common types of operands.
.PP
For all current EM implementations we will describe whether memory cells
are initialized, which value is used to initialize, for each operand type
whether that value is illegal, and for all operations on all operand
types whether that value is detected as undefined.
.I1
\*(ONnew memory words are initialized with -32768.
Assignment of this value is always allowed. Errors may occur
whenever undefined operands are used in operations.
.br
.ul
integer:
-32768 is illegal. All arithmetic operations (except unary +) cause
an error (undefined integer, trap 8, non-fatal).
Relational operations do not, except for IN when the left operand is undefined.
Printing of -32768 using write is allowed.
.br
.ul
real:
the bit representation of a real, caused by initializing the constituent
memory words with -32768, is illegal.
All arithmetic and relational operations (except unary +) cause an error
(real undefined, trap 9, non-fatal).
Printing causes the same error.
.br
.ul
char:
the value -32768 is illegal. For objects of type 'packed array[] of char'
half the characters will have the value chr(0), which is legal, and the
others will have the value chr(128), outside the valid ASCII range.
The relational operators, however, do not cause an error.
.br
.ul
Boolean:
the value -32768 is illegal. For objects of type 'packed array[] of boolean'
half the booleans will have the value false, while the others have the value v,
where ord(v) = 128, naturally illegal.
However, the Boolean and relational operations do not cause an error.
.br
.ul
set:
undefined operands of type set can not be distinguished from
properly initialized ones.
The set and relational operations, therefore, can never cause an error.
However, if one forgets to initialize a set of character, then spurious
characters like '/', '?', 'O', '_' and 'o' appear.
.sp
\*(OFnew memory cells are initialized with -32768.
The only cases where this value causes an error are when
an undefined operand of type real is used in an arithmetic or relational
operation (except unary +) or when an undefined real is used as an
argument to a standard function.
.I2
Newly created memory cells are not initialized and therefore
they have a random value.
.IT 6.7.1
It shall be an error if
the value of any member denoted by any member-designator of the
set-constructor is outside the implementation-defined limits.
.IS
This error is detected (set bound error, trap 2, non-fatal).
.IT 6.7.1
It shall be an error if
the possible types of an set-constructor do not permit it
to assume a suitable type.
.IS
The compiler allocates as many bits as are necessary to store all
elements of the host-type of the base-type of the set, not the
base-type itself.
Therefore, all possible errors can be detected at compile time.
.IT 6.7.2.2
It shall be an error if j is zero in 'i div j'.
.IS
It depends on the EM implementation whether this error is detected:
.I1
\*(ONdetected (divide by 0, trap 6, non-fatal).
.br
\*(OFnot detected.
.I2
not detected.
.IE
.IT 6.7.2.2
It shall be an error if
j is zero or negative in i MOD j.
.IS
This error is detected (only positive j in 'i mod j', trap 71, non-fatal).
.IT 6.7.2.2
It shall be an error if the result of any operation on integer
operands is not performed according to the mathematical
rules for integer arithmetic.
.IS
The reaction depends on the EM implementation:
.I1
\*(ONerror detected if
.EQ
     (result >= 32768) or (result < -32768).
.EN
(integer overflow, trap 3, non-fatal).
Note that if the result is -32768 the use of this value in further operations
may cause an error.
.br
\*(OFnot detected.
.I2
not detected.
.IT 6.8.3.5
It shall be an error if none of the case-constants is equal to the value of the
case-index upon entry to the case-statement.
.IS
This error is detected (case error, trap 20, fatal).
.IT 6.8.3.9
It shall be an error if the final-value of a for-statement is not
assignment-compatible with the control-variable when the
initial-value is assigned to the control-variable.
.IS
It is detected if the control variable leaves
its allowed range of values while stepping
from initial to final value.
This is equivalent with the requirements if the
for-statement is not terminated before
the final value is reached.
.IT 6.9.2
It shall be an error if the sequence of characters read looking for an integer does not
form a signed-integer as specified in 6.1.5.
.IS
This error is detected (digit expected, trap 105, non-fatal).
.IT 6.9.2
It shall be an error if the sequence of characters read looking for a real does not
form a signed-number as specified in 6.1.5.
.IS
This error is detected (digit expected, trap 105, non-fatal).
.IT 6.9.2
It shall be an error if read is applied to f while f is undefined or
not opened for reading.
.IS
This error is detected (see get(f)).
.IT 6.9.4
It shall be an error if write is applied to f while f is undefined or
not opened for writing.
.IS
This error is detected (see put(f)).
.IT 6.9.4
It shall be an error if TotalWidth or FracDigits as specified in
write or writeln procedure calls are less than one.
.IS
This error is not detected. Moreover, it is considered an extension to
allow zero or negative values.
.IT 6.9.6
It shall be an error if page is applied to f while f is undefined or
not opened for writing.
.IS
This error is detected (see put(f)).
.CH "Extensions to the standard"
.IS
.ti -3
1.~\
Separate compilation.
.sp
The compiler is able to (separately) compile a collection of declarations,
procedures and functions to form a library.
The library may be linked with the main program, compiled later.
The syntax of these modules is
.EQ
     module = [constant-definition-part]
              [type-definition-part]
              [var-declaration-part]
              [procedure-and-function-declaration-part]
.EN
The compiler accepts a program or a module:
.EQ
     unit = program | module
.EN
All variables declared outside a module must be imported
by parameters, even the files input and output.
Access to a variable declared in a module is only possible
using the procedures and functions declared in that same module.
By giving the correct procedure/function heading followed by the
directive 'extern' you may use procedures and functions declared in
other units.
.sp
.ti -3
2.~\
Assertions.
.sp
The Ack-Pascal compiler recognizes an additional statement, the assertion.
Assertions can be used as an aid in debugging and documentation.
The syntax is:
.EQ
     assertion = 'assert' Boolean-expression
.EN
An assertion is a simple-statement, so
.EQ
     simple-statement = [assignment-statement |
                         procedure-statement |
                         goto-statement |
                         assertion
                        ]
.EN
An assertion causes an error if the Boolean-expression is false.
That is its only purpose.
It does not change any of the variables, at least it should not.
Therefore, do not use functions with side-effects in the Boolean-expression.
If the a-option is turned off, then assertions are skipped by the
compiler. 'assert' is not a word-symbol (keyword) and may be used as identifier.
However, assignment to a variable and calling of a procedure with that name will be impossible.
.sp
.ti -3
3.~\
Additional procedures.
.sp
Three additional standard procedures are available:
.IS
.IS
.ti -8
halt:~~~a call of this procedure is equivalent to jumping to the
end of your program. It is always the last statement executed.
The exit status of the program may be supplied
as optional argument.
.ti -8
release:
.ti -8
mark:~~~for most applications it is sufficient to use the heap as second stack.
Mark and release are suited for this type of use, more suited than dispose.
mark(p), with p of type pointer, stores the current value of the
heap pointer in p. release(p), with p initialized by a call
of mark(p), restores the heap pointer to its old value.
All the heap objects, created by calls of new between the call of
mark and the call of release, are removed and the space they used
can be reallocated.
Never use mark and release together with dispose!
.sp
.in -10
.ti -3
4.~\
UNIX interfacing.
.sp
If the c-option is turned on, then some special features are available
to simplify an interface with the UNIX environment.
First of all, the compiler allows you to use a different type
of string constants.
These string constants are delimited by double quotes ('"').
To put a double quote into these strings, you must repeat the double quote,
like the single quote in normal string constants.
These special string constants are terminated by a zero byte (chr(0)).
The type of these constants is a pointer to a packed array of characters,
with lower bound 1 and unknown upper bound.
.br
Secondly, the compiler predefines a new type identifier 'string' denoting
this just described string type.
.PP
The only thing you can do with these features is declaration of
constants and variables of type 'string'.
String objects may not be allocated on the heap and string pointers
may not be de-referenced.
Still these strings are very useful in combination with external routines.
The procedure write is extended to print these zero-terminated strings correctly.
.sp
.ti -3
5.~\
Double length (32 bit) integers.
.sp
If the d-option is turned on, then the additional type 'long' is known to the compiler.
Long variables have integer values in the range -2147483647..+2147483647.
Long constants may be declared.
It is not allowed to form subranges of type long.
All operations allowed on integers are also
allowed on longs and are indicated by the same
operators: '+', '-', '*', '/', 'div', 'mod'.
The procedures read and write have been extended to handle long arguments correctly.
The default width for longs is 11.
The standard procedures 'abs' and 'sqr' have been extended to work on long arguments.
Conversion from integer to long, long to real,
real to long and long to integer are automatic, like the conversion from integer to real.
These conversions may cause a
.IS
conversion error, trap 10, non-fatal
.IE
This last error is only detected in implementation 1, with 'test on'.
Note that all current implementations use target
machine floating point instructions
to perform some of the long operations.
.sp
.ti -3
6.~\
Underscore as letter.
.sp
The character '_' may be used in forming identifiers, if the u-option is turned on.
.sp
.ti -3
7.~\
Zero field width in write.
.sp
Zero or negative TotalWidth arguments to write
are allowed.
No characters are written for character, string or Boolean type arguments then.
A zero or negative FracDigits argument for fixed-point representation of reals causes the
fraction and the character '.' to be suppressed.
.sp
.ti -3
8.~\
Alternate symbol representation.
.sp
The comment delimiters '(*' and '*)' are recognized and treated like '{' and '}'.
The other alternate representations of symbols are not recognized.
.sp
.ti -3
9.~\
Pre-processing.
.sp
If the very first character of a file containing a Pascal
program is the sharp ('#', ASCII 23(hex)) the file is preprocessed
in the same way as C programs.
Lines beginning with a '#' are taken as preprocessor command lines
and not fed to the Pascal compiler proper.
C style comments, /*......*/, are removed by the C preprocessor,
thus C comments inside Pascal programs are also removed when they
are fed through the preprocessor.
.CH "Deviations from the standard"
Ack-Pascal deviates from the (March 1980) standard proposal in the following ways:
.IS
.ti -3
1.~\
Only the first 8 characters of identifiers are significant,
as requested by all standard proposals prior to March 1980.
In that proposal, however, the sentence
.DS
"A conforming program should not have its meaning altered
by the truncation of its identifiers to eight characters
or the truncation of its labels to four digits."
.DE
is missing.
.sp
.ti -3
2.~\
The character sequences 'procedur', 'procedur8', 'functionXyZ' etc. are
all erroneously classified as the word-symbols 'procedure' and 'function'.
.sp
.ti -3
3.~\
Standard procedures and functions are not allowed as parameters in Ack-Pascal,
conforming to all previous standard proposals.
You can obtain the same result with negligible loss of performance
by declaring some user routines like:
.EQ
     function sine(x:real):real;
     begin
         sine:=sin(x)
     end;
.EN
.sp
.ti -3
4.~\
The scope of identifiers and labels should start at the beginning of the block
in which these identifiers or labels are declared.
The Ack-Pascal compiler, as most other one pass compilers, deviates in this respect,
because the scope of variables and labels start
at their defining-point.
.CH "Compiler options"
Some options of the compiler may be controlled by using "{$....}".
Each option consists of a lower case letter followed by +, - or an unsigned
number.
Options are separated by commas.
The following options exist:
.in 8
.sp
.ti -8
a~+/-~~~\
this option switches assertions on and off.
If this option is on, then code is included to test these assertions
at run time. Default +.
.sp
.ti -8
c~+/-~~~\
this option, if on, allows you to use C-type string constants
surrounded by double quotes.
Moreover, a new type identifier 'string' is predefined.
Default -.
.sp
.ti -8
d~+/-~~~\
this option, if on, allows you to use variables of type 'long'.
Default -.
.sp
.ti -8
f~<num>~\
the size of reals can be changed by this option. <num> should be specified in 8-bit bytes.
The default in most implementations is 8, but other values can
occur.
.sp
.ti -8
i~<num>~\
with this flag the setsize for a set of integers can be
manipulated.
The number must be the number of bits per set.
The default value is 16, just fitting in one word on the PDP and many other minis.
.sp
.ti -8
l~+/-~~~\
if + then code is inserted to keep track of the source line number.
When this flag is switched on and off, an incorrect line number may appear
if the error occurs in a part of your program for which this flag is off.
These same line numbers are used for the profile, flow and count options
of the EM interpreter em [6].
Default +.
.sp
.ti -8
p~<num>~the size of pointers can be changed by this option. <num> should be specified in bytes.
Default 2 in most implementations.
.sp
.ti -8
r~+/-~~~\
if + then code is inserted to check subrange variables against
lower and upper subrange limits.
Default +.
.sp
.ti -8
s~+/-~~~\
if + then the compiler will hunt for places in your program
where non-standard features are used, and for each place found
it will generate a warning. Default -.
.sp
.ti -8
t~+/-~~~\
if + then each time a procedure is entered, the routine 'procentry'
is called.
The compiler checks this flag just before the first symbol that follows the
first 'begin' of the body of the procedure.
Also, when the procedure exits, then the procedure 'procexit' is called
if the t flag is on just before the last 'end' of the procedure body.
Both 'procentry' and 'procexit' have a packed array of 8 characters as a parameter.
Default procedures are present in the run time library.
Default -.
.sp
.ti -8
u~+/-~~~\
if + then the character '_' is treated like a lower case letter,
so that it may be used in identifiers.
Procedure and function identifiers starting with an underscore may cause problems,
because they may collide with library routine names.
Default -.
.in 0
.sp
Seven of these flags (c, d, f, i, p, s and u) are only effective when they appear
before the 'program' symbol. The others may be switched on and off.
.PP
A second method of passing options to the compiler ia available.
This method uses the file on which the compact EM code will be written.
The compiler starts reading from this file scanning for options
in the same format as used normally, except for the comment delimiters and
the dollar sign.
All options found on the file override the options set in your program.
Note that the compact code file must always exist before the compiler is called.
.PP
The user interface program \fIack\fP[4]
takes care of creating this file normally
and also writes one of its options onto this file.
The user can specify, for instance, without changing any character in its
Pascal program, that the compiler must include code for
procedure/function tracing.
.PP
Another very powerful debugging tool is the knowledge that inaccessible
statements and useless tests are removed by the EM optimizer.
For instance, a statement like:
.sp
.nf
        if debug then
          writeln('initialization done');
.fi
.sp
is completely removed by the optimizer if debug is a constant with
value false.
The first line is removed if debug is a constant with value true.
Of course, if debug is a variable nothing can be removed.
.PP
A disadvantage of Pascal, the lack of preinitialized data, can be
diminished by making use of the possibilities of the EM optimizer.
For instance, initializing an array of reserved words is sometimes
optimized into 3 EM instructions. To maximize this effect you must initialize
variables as much as possible in order of declaration and array entries
in order of decreasing index.
.CH "References"
.in +5
.ti -5
[1]~~\
ISO standard proposal ISO/TC97/SC5-N462, dated February 1979.
The same proposal, in slightly modified form, can be found in:
A.M.Addyman e.a., "A draft description of Pascal",
Software, practice and experience, May 1979.
An improved version, received March 1980,
is followed as much as possible for the
current Ack-Pascal.
.sp
.ti -5
[2]~~\
A.S.Tanenbaum, J.W.Stevenson, Hans van Staveren, E.G.Keizer,
"Description of a machine architecture for use with block structured languages",
Informatica rapport IR-81.
.sp
.ti -5
[3]~~\
W.S.Brown, S.I.Feldman, "Environment parameters and basic functions
for floating-point computation",
Bell Laboratories CSTR #72.
.sp
.ti -5
[4]~~\
UNIX manual ack(I).
.sp
.ti -5
[5]~~\
UNIX manual ld(I).
.sp
.ti -5
[6]~~\
UNIX manual em(I).
.sp
.ti -5
[7]~~\
UNIX manual libpc(VII)
.sp
.ti -5
[8]~~\
UNIX manual pc_prlib(VII)