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- .\" $Id$
- .\" tbl pcref.doc | troff
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- Vakgroep Informatica
- Vrije Universiteit
- De Boelelaan 1081
- Amsterdam
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- .TL "Amsterdam Compiler Kit-Pascal reference manual"
- .AU "Johan W. Stevenson"
- .DA "January 4, 1983"
- .OA "Hans van Eck"
- .DA "May 1, 1989"
- .VU
- .CH "Introduction"
- This document refers to the (1982) BSI standard for Pascal [1].
- Ack-Pascal complies with the requirements of level 1 of BS 6192: 1982, with
- the exceptions as listed in this document.
- .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 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.
- .PP
- There does not (yet) exist a hardware EM machine.
- Therefore, EM programs must be interpreted, or translated into
- instructions for a target machine.
- The Ack-Pascal compiler is currently available for use with the VAX,
- Motorola MC68020, Motorola MC68000,
- PDP-11, and Intel 8086 code-generators.
- For the 8086, MC68000, and MC68020,
- floating point emulation is used. This is made available with the \fI-fp\fP
- option, which must be passed to \fIack\fP[3].
- .IE
- .CH "Implementation-defined features"
- 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:
- .DS
- Possibly differing between processors, but defined for any particular
- processor.
- .DE
- .IT 6.1.7
- Each string-character shall denote an implementation-defined value of the
- required char-type.
- .IS
- All 7-bits ASCII characters except linefeed LF (10) are allowed.
- .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 bu signed real.
- .IS
- The format of reals is not defined in EM.
- Even the size of reals depends on the EM-implementation.
- The compiler can be instructed, by the V-option, to use a different
- size for real values.
- The size of reals is preset by the calling program \fIack\fP
- [3] to the proper size.
- .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 6.6.5.2
- The post-assertions imply corresponding activities on the external entities,
- if any, to which the file-variables are bound. These activities, and the
- point at which they are actually performed, shall be
- implementation-defined.
- .IS
- The reading and writing writing of objects on files is buffered.
- This means that when a program terminates abnormally, IO may be
- unfinished. Terminal IO is unbuffered.
- Files are closed whenever they are rewritten or reset, or on
- program termination.
- .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 of 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 range of available integers depends on the EM implementation:
- For 2-byte machines, the integers range from -32767 to +32767. For 4-byte
- machines, the integers range from -2147483647 to 2147483647.
- The number -maxint-1 may be used to indicate 'undefined'.
- .IE
- .IT 6.7.2.2
- The result of the real arithmetic operators and functions shall be
- approximations to the corresponding mathematical results. The accuracy of
- this approximation shall be implementation-defined
- .IS
- Since EM doesn't specify floating point format, it is not possible to
- specify the accuracy. When the floating point emulation is used, and the
- default size of reals is 8 bytes, the accuracy is 11 bits for the exponent,
- and 53 bits for the mantissa. This gives an accuracy of about 16 digits,
- and exponents ranging from -309 to +307.
- .IE
- .IT 6.9.3.1
- The default TotalWidth values for integer, Boolean and real types
- shall be implementation-defined.
- .IS
- The defaults are:
- integer 6 for 2-byte machines, 11 for 4-byte machines
- Boolean 5
- real 14
- .IT 6.9.3.4.1
- ExpDigits, the number of digits written in an exponent part of a real,
- shall be implementation-defined.
- .IS
- ExpDigits is defined as 3. This is sufficient for all implementations
- currently available. When the representation would need more than 3
- digits, then the string '***' replaces the exponent.
- .IT 6.9.3.4.1
- 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 6.9.3.5
- 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 6.9.5
- 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 [6].
- .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 BSI standard,
- 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
- Possibly differing between processors and not necessarily defined for any
- particular processor.
- .DE
- .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.
- .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
- .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
- .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
- The decision as to the order of accessing the variable and evaluating
- the expression in an assignment-statement, 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 6.9.5
- 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
- that a processor is permitted to leave undetected.
- .DE
- .SH "Compiler errors"
- Error are written on the standard error output. Each line has the form:
- .br
- <file>, line <number>: <description>
- .br
- 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
- Sometimes the compiler produces several errors for the same line. They are
- only shown up to a maximum of 5 errors per line. Warning are also shown up
- to a maximum of 5 per line.
- .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 on, 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.
- .PP
- 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 [7].
- .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 lower bound on the index, the number of elements minus one and the
- element-size.
- It depends on the EM implementation whether these bounds are checked. Since
- most implementations don't, an extra compiler flag is added to force these
- checks.
- .br
- 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 on, 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.
- .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 6.5.3.3
- It shall be an error if a component of a variant-part of a variant,
- where the selector of the variant-part is not a field,
- is accessed unless the variant is active for the entirety of each
- reference and access to each component of the variant.
- .IS
- This error is not detected.
- .IT 6.5.4
- It shall be an error if
- the pointer-variable of an identified-variable either denotes a
- nil-value or is undefined.
- .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 [3].
- The compiler can be instructed, by the V-option, to use
- another 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.
- .IE
- .IT 6.5.4
- It shall be an error to remove the identifying-value of an identified
- variable from its pointer-type when a reference to the variable exists.
- .IS
- When the identified variable is an element of the record-variable-list of
- a with_statement, a warning is given at compile-time. Otherwise, this error
- is not detected.
- .IT 6.5.5
- It shall be an error to alter the value of a file-variable f when a
- reference to the buffer-variable f^ exists.
- .IS
- When f is altered when it is an element of the record-variable-list of a
- with-statement, a warning is given. When a buffer-variable is used as a
- variable-parameter, an error is given. This is done at compile-time.
- .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 the 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 if a variant of a variant-part within the new
- variable becomes active and a different variant of the variant-part is
- one of the specified variants.
- .IS
- This error is not detected.
- .IT 6.6.5.3
- It shall be an error to use dispose(q) if the identifying variable has been
- allocated using the form new(p,c1,...,cn).
- .IS
- This error is not detected. However, 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 to use dispose(q,k1,...,km) if the identifying
- variable has been allocated using the form new(p,c1,...,cn) and m is not
- equal to n.
- .IS
- This error is not detected. However, 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 created using the second form of new is
- accessed by the identified variable of the variable-access of a factor,
- of an assignment-statement, or of an actual-parameter.
- .IS
- This error is not detected.
- .IT 6.6.6.2
- It shall be an error if the value of sqr(x) does not exist.
- .IS
- This error is detected for real-type arguments (real overflow,
- trap 4, non-fatal).
- .IT 6.6.6.2
- It shall be an error if x in ln(x) is smaller than or equal to 0.
- .IS
- This error is detected (error in ln, trap 66, non-fatal)
- .IT 6.6.6.2
- It shall be an error if x in sqrt(x) is smaller than 0.
- .IS
- This error is detected (error in sqrt, trap 67, non-fatal)
- .sp
- In addition to these errors, overflow in the expression exp(x) is
- detected (error in exp, trap 65, non-fatal; real overflow, trap 4, non-fatal)
- .sp
- .IT 6.6.6.3
- It shall be an error if
- the integer value of trunc(x) does not exist.
- .IS
- It depends on the implementations whether this error is detected.
- The floating-point emulation detects this error (conversion error,
- trap 10, non-fatal).
- .IT 6.6.6.3
- It shall be an error if
- the integer value of round(x) does not exist.
- .IS
- It depends on the implementations whether this error is detected.
- The floating-point emulation detects this error (conversion error,
- trap 10, non-fatal).
- .IT 6.6.6.4
- 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 6.6.6.4
- It shall be an error if
- the character value of chr(x) does not exist.
- .IS
- Except when the R-option is off, the compiler generates an EM
- range check instruction. The effect of this instruction depends on the
- EM implementation.
- .IT 6.6.6.4
- It shall be an error if the value of succ(x) does not exist.
- .IS
- Same comments as for chr(x).
- .IT 6.6.6.4
- 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 a variable-access used as an operand
- in an expression is undefined at the time of its use.
- .IS
- The compiler performs some limited checks to see if identifiers are
- used before they are set. Since it can not always be sure (one could, for
- instance, jump out of a loop), only a warning is generated. When an
- expression contains a function-call, an error occur if the
- function is not assigned at run-time.
- .IT 6.7.2.2
- A term of the form x/y shall be an error if y is zero.
- .IS
- It depends on the EM implementation whether this error is detected. On some
- machines, a trap may occur.
- .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. On some
- machines, a trap may occur.
- .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. Most implementations,
- however, will not notice integer overflow.
- .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 6.9.1
- 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 6.9.1
- 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 6.9.1
- When read is applied to f, it shall be an error if the buffer-variable f^
- is undefined or the pre-assertions for get do not hold.
- .IS
- This error is detected (see get(f)).
- .IT 6.9.3
- When write is applied to a textfile f, it shall be an error if f is
- undefined or f is opened for reading.
- .IS
- This error is detected (see put(f)). Furthermore, this error is also
- detected when f is not a textfile.
- .IT 6.9.3.1
- The values of TotalWidth or FracDigits shall be greater than or equal to
- one; it shall be an error if either value is less then one.
- .IS
- When either value is less than zero, an error (illegal field width, trap
- 75, non-fatal) occurs. Zero values are allowed, in order to maintain some
- compatibility with the old Ack-Pascal compiler.
- .IT 6.9.5
- It shall be an error if the pre-assertion required for writeln(f) doe not
- hold prior to the invocation of page(f);
- .IS
- This error is detected (see put(f)).
- .CH "Extensions to the standard"
- .IS
- .ti -3
- 1.~\
- External routines
- .sp
- Except for the required directive 'forward' the Ack-Pascal compiler recognizes
- the 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 for accessing 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.
- .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
- .TS
- l l.
- 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 ;
- }
- .TE
- The Pascal runtime system uses the following algorithm when calling
- function/procedures passed as parameters.
- .TS
- l l.
- if ( static_link ) (*a)(static_link,pars) ;
- else (*a)(pars) ;
- .TE
- .ti -3
- 2.~\
- 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' procedures and functions declared in
- other units may be used.
- .sp
- .ti -3
- 3.~\
- Assertions.
- .sp
- When the s-option is off, 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 on, 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.
- If the s-option is turned on, the compiler will not know a thing about
- assertions, so using assertions will then give a parse error.
- .sp
- .ti -3
- 4.~\
- 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 the program. It is always the last statement executed.
- The exit status of the program may be supplied
- as optional argument. If not, it will be zero.
- .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
- 5.~\
- 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 for a different type
- of string constants.
- These string constants are delimited by double quotes ('"').
- To put a double quote into these strings, the double quote must be repeated,
- 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
- These features are only useful for 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
- 6.~\
- Double length (32 bit) integers.
- .sp
- If the d-option is turned on, then the additional type 'long' is known
- to the compiler.
- By default, long variables have integer values in the
- range -2147483647..+2147483647, but this can be changed with the -V option
- (if the backend can support this).
- Long constants can not be declared.
- Longs can not be used as control-variables.
- 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. It is possible to read longs from a file of integers
- and vice-versa, but only if longs and integers have the same size.
- 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
- .sp
- .ti -3
- 7.~\
- Underscore as letter.
- .sp
- The character '_' may be used in forming identifiers, if the u- or U-option
- is turned on. It is forbidden to start identifiers with underscores, since
- this may cause name-clashes with run-time routines.
- .sp
- .ti -3
- 8.~\
- Zero field width in write.
- .sp
- Zero TotalWidth arguments are allowed. No characters are written for
- character, string or Boolean type arguments then. A zero FracDigits
- argument for fixed-point representation of reals causes the fraction and
- the character '.' to be suppressed.
- .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 standard proposal in the following ways:
- .IS
- .ti -3
- 1.~\
- Standard procedures and functions are not allowed as parameters in Ack-Pascal.
- The same result can be obtained 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
- 2.~\
- The standard procedures read, readln, write and writeln are implemented as
- word-symbols, and can therefore not be redeclared.
- .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 the use of 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 the use of variables of type 'long'.
- Default -.
- .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 wordsize-1.
- .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 the 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 [5].
- Default +.
- .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 the 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, and each time a procedure exits, the procedure 'procexit' is
- called. Both 'procentry' and 'procexit' have a 'string' as parameter. This
- means that when a user specifies his or her own procedures, the c-option
- must be used. Default procedures are present in the run time library.
- Default -.
- .sp
- .ti -8
- u~+/-~~~\
- if + then the character '_' is treated like a letter,
- so that it may be used in identifiers.
- Procedure and function identifiers are not allowed to start with an
- underscore because they may collide with library routine names.
- Default -.
- .in 0
- .sp
- Some of these flags (c, d, i, s, u, C and U) are only effective when
- they appear before the 'program' symbol. The others may be switched
- on and off.
- .PP
- A 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
- variables must be initialized as much as possible in order of declaration and array entries
- in order of decreasing index.
- .CH "References"
- .in +5
- .ti -5
- [1]~~\
- BSI standard BS 6192: 1982 (ISO 7185).
- .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]~~\
- UNIX manual ack(I).
- .sp
- .ti -5
- [4]~~\
- UNIX manual ld(I).
- .sp
- .ti -5
- [5]~~\
- UNIX manual em(I).
- .sp
- .ti -5
- [6]~~\
- UNIX manual libpc(VII)
- .sp
- .ti -5
- [7]~~\
- UNIX manual pc_prlib(VII)
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