.\" Copyright (c) 1980 Regents of the University of California.
.\" All rights reserved.  The Berkeley software License Agreement
.\" specifies the terms and conditions for redistribution.
.\"
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.NH 
Appendix to Wirth's Pascal Report
.PP
This section is an appendix to
the definition of the Pascal language in Niklaus Wirth's
.I "Pascal Report"
and, with that Report, precisely defines the
Berkeley
implementation.
This appendix includes a summary of extensions to the language,
gives the ways in which the undefined specifications were resolved,
gives limitations and restrictions of the current implementation,
and lists the added functions and procedures available.
It concludes with a list of differences with the commonly available
Pascal 6000\-3.4 implementation,
and some comments on standard and portable Pascal.
.NH 2
Extensions to the language Pascal
.PP
This section defines non-standard language constructs available in
.UP .
The
.B s
standard Pascal option of the translators
.PI
and
.PC
can be used to detect these extensions in programs which are to be transported.
.SH
String padding
.PP
.UP
will pad constant strings with blanks in expressions and as
value parameters to make them as long as is required.
The following is a legal
.UP
program:
.LS
\*bprogram\fP x(output);
\*bvar\fP z : \*bpacked\fP \*barray\fP [ 1 .. 13 ] \*bof\fP char;
\*bbegin\fP
    z := 'red';
    writeln(z)
\*bend\fP;
.LE
The padded blanks are added on the right.
Thus the assignment above is equivalent to:
.LS
z := 'red          '
.LE
which is standard Pascal.
.SH
Octal constants, octal and hexadecimal write
.PP
Octal constants may be given as a sequence of octal digits followed
by the character `b' or `B'.
The forms
.LS
write(a:n \*boct\fP)
.LE
and
.LS
write(a:n \*bhex\fP)
.LE
cause the internal representation of
expression
.I a,
which must be Boolean, character, integer, pointer, or a user-defined enumerated
type,
to be written in octal or hexadecimal respectively.
.SH
Assert statement
.PP
An
.B assert
statement causes a
.I Boolean
expression to be evaluated
each time the statement is executed.
A runtime error results if any of the expressions evaluates to be 
.I false .
The
.B assert
statement is treated as a comment if run-time tests are disabled.
The syntax for
.B assert
is:
.LS
\*bassert\fP <expr>
.LE
.SH
Enumerated type input-output
.PP
Enumerated types may be read and written.
On output the string name associated with the enumerated
value is output.
If the value is out of range,
a runtime error occurs.
On input an identifier is read and looked up
in a table of names associated with the
type of the variable, and
the appropriate internal value is assigned to the variable being
read.
If the name is not found in the table
a runtime error occurs.
.SH
Structure returning functions
.PP
An extension has been added which allows functions 
to return arbitrary sized structures rather than just
scalars as in the standard.
.SH
Separate compilation
.PP
The compiler
.PC
has been extended to allow separate compilation of programs.
Procedures and functions declared at the global level
may be compiled separately.
Type checking of calls to separately compiled routines is performed
at load time to insure that the program as a whole
is consistent.
See section 5.10 for details.
.NH 2
Resolution of the undefined specifications
.SH
File name \- file variable associations
.PP
Each Pascal file variable is associated with a named
.UX
file.
Except for
.I input
and
.I output,
which are
exceptions to some of the rules, a name can become associated
with a file in any of three ways:
.IP "\ \ \ \ \ 1)" 10
If a global Pascal file variable appears in the
.B program
statement
then it is associated with
.UX
file of the same name.
.IP "\ \ \ \ \ 2)"
If a file was reset or rewritten using the
extended two-argument form of
.I reset
or
.I rewrite
then the given name
is associated.
.IP "\ \ \ \ \ 3)"
If a file which has never had
.UX
name associated
is reset or rewritten without specifying a name
via the second argument, then a temporary name
of the form `tmp.x'
is associated with the file.
Temporary names start with
`tmp.1' and continue by incrementing the last character in the
.SM
USASCII
.NL
ordering.
Temporary files are removed automatically
when their scope is exited.
.SH
The program statement
.PP
The syntax of the
.B program
statement is:
.LS
\*bprogram\fP <id> ( <file id> { , <file id > } ) ;
.LE
The file identifiers (other than
.I input
and
.I output )
must be declared as variables of
.B file
type in the global declaration part.
.SH
The files input and output
.PP
The formal parameters
.I input
and
.I output
are associated with the
.UX
standard input and output and have a
somewhat special status.
The following rules must be noted:
.IP "\ \ \ \ \ 1)" 10
The program heading
.B must
contains the formal parameter
.I output.
If
.I input
is used, explicitly or implicitly, then it must
also be declared here.
.IP "\ \ \ \ \ 2)"
Unlike all other files, the
Pascal files
.I input
and
.I output
must not be defined in a declaration,
as their declaration is automatically:
.LS
\*bvar\fP input, output: text
.LE
.IP "\ \ \ \ \ 3)"
The procedure
.I reset
may be used on
.I input.
If no
.UX
file name has ever been associated with
.I input,
and no file name is given, then an attempt will be made
to `rewind'
.I input.
If this fails, a run time
error will occur.
.I Rewrite
calls to output act as for any other file, except that
.I output
initially has no associated file.
This means that a simple
.LS
rewrite(output)
.LE
associates a temporary name with
.I output.
.SH
Details for files
.PP
If a file other than
.I input
is to be read,
then reading must be initiated by a call to the
procedure
.I reset
which causes the Pascal system to attempt to open the
associated
.UX
file for reading.
If this fails, then a runtime error occurs.
Writing of a file other than
.I output
must be initiated by a
.I rewrite
call,
which causes the Pascal system to create the associated
.UX
file and
to then open the file for writing only.
.SH
Buffering
.PP
The buffering for
.I output
is determined by the value of the
.B b
option
at the end of the
.B program
statement.
If it has its default value 1,
then
.I output
is
buffered in blocks of up to 512 characters,
flushed whenever a writeln occurs
and at each reference to the file
.I input.
If it has the value 0,
.I output
is unbuffered.
Any value of
2 or more gives block buffering without line or
.I input
reference flushing.
All other output files are always buffered in blocks of 512 characters.
All output buffers are flushed when the files are closed at scope exit,
whenever the procedure
.I message
is called, and can be flushed using the
built-in procedure
.I flush.
.PP
An important point for an interactive implementation is the definition
of `input\(ua'.
If
.I input
is a teletype, and the Pascal system reads a character at the beginning
of execution to define `input\(ua', then no prompt could be printed
by the program before the user is required to type some input.
For this reason, `input\(ua' is not defined by the system until its definition
is needed, reading from a file occurring only when necessary.
.SH
The character set
.PP
Seven bit
.SM USASCII
is the character set used on
.UX .
The standard Pascal
symbols `and', 'or', 'not', '<=', '>=', '<>',
and the uparrow `\(ua' (for pointer qualification)
are recognized.\*(dg
.FS
\*(dgOn many terminals and printers, the up arrow is represented
as a circumflex `^'.
These are not distinct characters, but rather different graphic
representations of the same internal codes.
.FE
Less portable are the 
synonyms tilde `~'
for
.B not ,
`&' for
.B and ,
and `|' for
.B or .
.PP
Upper and lower case are considered to be distinct.\*(st
.FS
\*(stThe proposed standard for Pascal considers them to be the same.
.FE
Keywords and built-in
.B procedure
and
.B function
names are
composed of all lower case letters.
Thus the identifiers GOTO and GOto are distinct both from each other and
from the keyword
\*bgoto\fP.
The standard type `boolean' is also available as `Boolean'.
.PP
Character strings and constants may be delimited by the character
`\''
or by the character `#';
the latter is sometimes convenient when programs are to be transported.
Note that the `#' character has special meaning 
.up
when it is the first character on a line \- see
.I "Multi-file programs"
below.
.SH
The standard types
.PP
The standard type
.I integer
is conceptually defined as
.LS
\*btype\fP integer = minint .. maxint;
.LE
.I Integer
is implemented with 32 bit twos complement arithmetic.
Predefined constants of type
.I integer
are:
.LS
\*bconst\fP maxint = 2147483647; minint = -2147483648;
.LE
.PP
The standard type
.I char
is conceptually defined as
.LS
\*btype\fP char = minchar .. maxchar;
.LE
Built-in character constants are `minchar' and `maxchar', `bell' and `tab';
ord(minchar) = 0, ord(maxchar) = 127.
.PP
The type
.I real
is implemented using 64 bit floating point arithmetic.
The floating point arithmetic is done in `rounded' mode, and
provides approximately 17 digits of precision
with numbers as small as 10 to the negative 38th power and as large as
10 to the 38th power.
.SH
Comments
.PP
Comments can be delimited by either `{' and `}' or by `(*' and `*)'.
If the character `{' appears in a comment delimited by `{' and `}',
a warning diagnostic is printed.
A similar warning will be printed if the sequence `(*' appears in
a comment delimited by `(*' and `*)'.
The restriction implied by this warning is not part of standard Pascal,
but detects many otherwise subtle errors.
.SH
Option control
.PP
Options of the translators may be controlled
in two distinct ways.
A number of options may appear on the command line invoking the translator.
These options are given as one or more strings of letters preceded by the
character `\-' and cause the default setting of
each given option to be changed.
This method of communication of options is expected to predominate
for
.UX .
Thus the command
.LS
% \*bpi \-l \-s foo.p\fR
.LE
translates the file foo.p with the listing option enabled (as it normally
is off), and with only standard Pascal features available.
.PP
If more control over the portions of the program where options are enabled is
required, then option control in comments can and should be used.
The
format for option control in comments is identical to that used in Pascal
6000\-3.4.
One places the character `$' as the first character of the comment
and follows it by a comma separated list of directives.
Thus an equivalent to the command line example given above would be:
.LS
{$l+,s+ listing on, standard Pascal}
.LE
as the first line of the program.
The `l'
option is more appropriately specified on the command line,
since it is extremely unlikely in an interactive environment
that one wants a listing of the program each time it is translated.
.PP
Directives consist of a letter designating the option,
followed either by a `+' to turn the option on, or by a `\-' to turn the
option off.
The
.B b
option takes a single digit instead of
a `+' or `\-'.
.SH
Notes on the listings
.PP
The first page of a listing
includes a banner line indicating the version and date of generation of
.PI
or
.PC .
It also
includes the
.UX
path name supplied for the source file and the date of
last modification of that file.
.PP
Within the body of the listing, lines are numbered consecutively and
correspond to the line numbers for the editor.
Currently, two special
kinds of lines may be used to format the listing:
a line consisting of a form-feed
character, control-l, which causes a page
eject in the listing, and a line with
no characters which causes the line number to be suppressed in the listing,
creating a truly blank line.
These lines thus correspond to `eject' and `space' macros found in many
assemblers.
Non-printing characters are printed as the character `?' in the listing.\*(dg
.FS
\*(dgThe character generated by a control-i indents
to the next `tab stop'.
Tab stops are set every 8 columns in
.UX .
Tabs thus provide a quick way of indenting in the program.
.FE
.SH
The standard procedure write
.PP
If no minimum field length parameter is specified
for a
.I write,
the following default
values are assumed:
.KS
.TS
center;
l n.
integer	10
real	22
Boolean	length of `true' or `false'
char	1
string	length of the string
oct	11
hex	8
.TE
.KE
The end of each line in a text file should be explicitly
indicated by `writeln(f)', where `writeln(output)' may be written
simply as `writeln'.
For
.UX ,
the built-in function `page(f)' puts a single
.SM ASCII
form-feed character on the output file.
For programs which are to be transported the filter
.I pcc
can be used to interpret carriage control, as
.UX
does not normally do so.
.NH 2
Restrictions and limitations
.SH
Files
.PP
Files cannot be members of files or members of dynamically
allocated structures.
.SH
Arrays, sets and strings
.PP
The calculations involving array subscripts and set elements
are done with 16 bit arithmetic.
This
restricts the types over which arrays and sets may be defined.
The lower bound of such a range must be greater than or equal to
\-32768, and the upper bound less than 32768.
In particular, strings may have any length from 1 to 65535 characters,
and sets may contain no more than 65535 elements.
.SH
Line and symbol length
.PP
There is no intrinsic limit on the length of identifiers.
Identifiers
are considered to be distinct if they differ
in any single position over their entire length.
There is a limit, however, on the maximum input
line length.
This limit is quite generous however, currently exceeding 160
characters.
.SH
Procedure and function nesting and program size
.PP
At most 20 levels of 
.B procedure 
and 
.B function 
nesting are allowed.
There is no fundamental, translator defined limit on the size of the
program which can be translated.
The ultimate limit is supplied by the
hardware and thus, on the \s-2PDP\s0-11,
by the 16 bit address space.
If
one runs up against the `ran out of memory' diagnostic the program may yet
translate if smaller procedures are used, as a lot of space is freed
by the translator at the completion of each 
.B procedure
or
.B function
in the current
implementation.
.PP
On the \s-2VAX\s0-11, there is an implementation defined limit
of 65536 bytes per variable.
There is no limit on the number of variables.
.SH
Overflow
.PP
There is currently no checking for overflow on arithmetic operations at
run-time on the \s-2PDP\s0-11.
Overflow checking is performed on the \s-2VAX\s0-11 by the hardware.
.br
.ne 15
.NH 2
Added types, operators, procedures and functions
.SH
Additional predefined types
.PP
The type
.I alfa
is predefined as:
.LS
\*btype\fP alfa = \*bpacked\fP \*barray\fP [ 1..10 ] \*bof\fP \*bchar\fP
.LE
.PP
The type
.I intset
is predefined as:
.LS
\*btype\fP intset = \*bset of\fP 0..127
.LE
In most cases the context of an expression involving a constant
set allows the translator to determine the type of the set, even though the
constant set itself may not uniquely determine this type.
In the
cases where it is not possible to determine the type of the set from
local context, the expression type defaults to a set over the entire base
type unless the base type is integer\*(dg.
.FS
\*(dgThe current translator makes a special case of the construct
`if ... in [ ... ]' and enforces only the more lax restriction
on 16 bit arithmetic given above in this case.
.FE
In the latter case the type defaults to the current
binding of
.I intset,
which must be ``type set of (a subrange of) integer'' at that point.
.PP
Note that if 
.I intset 
is redefined via:
.LS
\*btype\fP intset = \*bset of\fP 0..58;
.LE
then the default integer set is the implicit
.I intset 
of
Pascal 6000\-3.4
.SH
Additional predefined operators
.PP
The relationals `<' and `>' of proper set
inclusion are available.
With
.I a
and
.I b
sets, note that
.LS
(\*bnot\fR (\fIa\fR < \fIb\fR)) <> (\fIa\fR >= \fIb\fR)
.LE
As an example consider the sets
.I a
= [0,2]
and
.I b
= [1].
The only relation true between these sets is `<>'.
.SH
Non-standard procedures
.IP argv(i,a) 25
where
.I i 
is an integer and
.I a
is a string variable
assigns the (possibly truncated or blank padded)
.I i \|'th
argument
of the invocation of the current
.UX
process to the variable
.I a .
The range of valid
.I i
is
.I 0
to
.I argc\-1 .
.IP date(a) 
assigns the current date to the alfa variable 
.I a
in the format `dd mmm yy ', where `mmm' is the first
three characters of the month, i.e. `Apr'.
.IP flush(f) 
writes the output buffered for Pascal file
.I f
into the associated
.UX
file.
.IP halt 
terminates the execution of the program with
a control flow backtrace.
.IP linelimit(f,x)\*(dd
.FS
\*(ddCurrently ignored by pdp-11
.X .
.FE
with
.I f 
a textfile and
.I x 
an integer expression
causes
the program to be abnormally terminated if more than
.I x 
lines are
written on file
.I f .
If
.I x 
is less than 0 then no limit is imposed.
.IP message(x,...) 
causes the parameters, which have the format of those
to the
built-in
.B procedure
.I write,
to be written unbuffered on the diagnostic unit 2,
almost always the user's terminal.
.IP null
a procedure of no arguments which does absolutely nothing.
It is useful as a place holder,
and is generated by
.XP
in place of the invisible empty statement.
.IP remove(a) 
where
.I a 
is a string causes the
.UX
file whose
name is
.I a, 
with trailing blanks eliminated, to be removed.
.IP reset(f,a) 
where
.I a 
is a string causes the file whose name
is
.I a 
(with blanks trimmed) to be associated with
.I f 
in addition
to the normal function of
.I reset.
.IP rewrite(f,a) 
is analogous to `reset' above.
.IP stlimit(i)
where
.I i 
is an integer sets the statement limit to be
.I i 
statements.
Specifying the
.B p
option to
.I pc
disables statement limit counting.
.IP time(a) 
causes the current time in the form `\ hh:mm:ss\ ' to be
assigned to the alfa variable
.I a.
.SH
Non-standard functions
.IP argc 25
returns the count of arguments when the Pascal program
was invoked.
.I Argc
is always at least 1.
.IP card(x) 
returns the cardinality of the set
.I x, 
i.e. the
number of elements contained in the set.
.IP clock 
returns an integer which is the number of central processor
milliseconds of user time used by this process.
.IP expo(x) 
yields the integer valued exponent of the floating-point
representation of
.I x ;
expo(\fIx\fP) = entier(log2(abs(\fIx\fP))).
.IP random(x) 
where
.I x 
is a real parameter, evaluated but otherwise
ignored, invokes a linear congruential random number generator.
Successive seeds are generated as (seed*a + c) mod m and
the new random number is a normalization of the seed to the range 0.0 to 1.0;
a is 62605, c is 113218009, and m is
536870912.
The initial seed
is 7774755.
.IP seed(i) 
where
.I i 
is an integer sets the random number generator seed
to
.I i 
and returns the previous seed.
Thus seed(seed(i))
has no effect except to yield value
.I i.
.IP sysclock
an integer function of no arguments returns the number of central processor
milliseconds of system time used by this process.
.IP undefined(x) 
a Boolean function.
Its argument is a real number and
it always returns false.
.IP wallclock 
an integer function of no arguments returns the time
in seconds since 00:00:00 GMT January 1, 1970.
.NH 2
Remarks on standard and portable Pascal
.PP
It is occasionally desirable to prepare Pascal programs which will be
acceptable at other Pascal installations.
While certain system dependencies are bound to creep in,
judicious design and programming practice can usually eliminate
most of the non-portable usages.
Wirth's
.I "Pascal Report"
concludes with a standard for implementation and program exchange.
.PP
In particular, the following differences may cause trouble when attempting
to transport programs between this implementation and Pascal 6000\-3.4.
Using the
.B s
translator option may serve to indicate many problem areas.\*(dg
.FS
\*(dgThe
.B s
option does not, however, check that identifiers differ
in the first 8 characters.
.I Pi
and
.PC
also do not check the semantics of
.B packed .
.FE
.SH
Features not available in Berkeley Pascal
.IP
Segmented files and associated functions and procedures.
.IP
The function
.I trunc 
with two arguments.
.IP
Arrays whose indices exceed the capacity of 16 bit arithmetic.
.SH
Features available in Berkeley Pascal but not in Pascal 6000-3.4
.IP
The procedures
.I  reset 
and
.I  rewrite 
with file names.
.IP
The functions
.I argc,
.I seed,
.I sysclock,
and
.I wallclock.
.IP
The procedures
.I argv,
.I flush,
and
.I remove.
.IP
.I Message
with arguments other than character strings.
.IP
.I Write
with keyword
.B hex .
.IP
The
.B assert
statement.
.IP
Reading and writing of enumerated types.
.IP
Allowing functions to return structures.
.IP
Separate compilation of programs.
.IP
Comparison of records.
.SH
Other problem areas
.PP
Sets and strings are more general in \*
.UP ;
see the restrictions given in
the 
Jensen-Wirth
.I "User Manual"
for details on the 6000\-3.4 restrictions.
.PP
The character set differences may cause problems,
especially the use of the function
.I chr,
characters as arguments to
.I ord,
and comparisons of characters,
since the character set ordering
differs between the two machines.
.PP
The Pascal 6000\-3.4 compiler uses a less strict notion of type equivalence.
In
.UP ,
types are considered identical only if they are represented
by the same type identifier.
Thus, in particular, unnamed types are unique
to the variables/fields declared with them.
.PP
Pascal 6000\-3.4 doesn't recognize our option
flags, so it is wise to
put the control of
.UP
options to the end of option lists or, better
yet, restrict the option list length to one.
.PP
For Pascal 6000\-3.4 the ordering of files in the program statement has
significance.
It is desirable to place
.I input 
and
.I output 
as the first two files in the
.B program
statement.
.sp 2
.SH
Acknowledgments
.PP
The financial support of William Joy and Susan Graham by
the National Science Foundation under grants
MCS74-07644-A04, MCS78-07291, and MCS80-05144,
and the William Joy by an
.SM IBM
Graduate Fellowship are gratefully acknowledged.
