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- .ds OF \\fBtest~off:~\\fR
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- Wiskundig Seminarium
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- De Boelelaan 1081
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- . IMPLEMENTATION 1
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- .br
- .fi
- .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.
- .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.
- .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)
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