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TABLE OF CONTENTS
1. Introduction
2. Language Description
2.1 General Language Features
2.1.1 SNOBOL4 features not implemented
2.1.2 SNOBOL4 features implemented differently
2.1.3 Additions to SNOBOL4
2.2 Declarations
2.2.1 PURGE and UNPURGE
2.2.2 GLOBAL, ENTRY, and EXTERNAL
2.2.3 Type declarations
2.2.4 Other declarations
2.3 Control
2.4 Predefined primitives
2.4.1 Pattern primitives
2.4.2 Expression primitives
2.4.3 FORTRAN primitives
2.5 Keywords
3. FASBOL II Programming
3.1 Using the compiler and runtime library
3.2 Programming techniques
3.2.1 Dedicated expressions
3.2.2 Use of unary ? and . operators
3.2.3 Pattern matching
3.2.4 Timing and storage management
3.2.5 FORTRAN and other non-FASBOL interfaces
Appendix 1 - Predefined Symbols
Appendix 2 - Runtime Errors
Appendix 3 - Differences between FASBOL and SPITBOL
References
FASBOL
A SNOBOL4 COMPILER FOR THE PDP-10
Paul Joseph Santos, Jr.
Department of Electrical Engineering and Computer Sciences
and the Electronics Research Laboratory
University of California, Berkeley, California 94720
revised by Mike Clancy
ABSTRACT
The FASBOL compiler system represents a new approach to the processing and
execution of programs written in the SNOBOL4 language. In contrast to the
existing interpretive and semi-interpretive systems, the FASBOL compiler
produces independent, assembly-language programs. These programs, when
assembled, and using a small run-time library, execute much faster than under
other SNOBOL4 systems.
While being almost totally compatible with SNOBOL4, Version 3, FASBOL offers
the same advantages as other compiler systems, such as:
1. Up to two orders of magnitude decrease in execution times over interpretive
processing for most problems.
2. Much smaller storage requirements at execution time than in-core sysems,
permitting either small partitions or larger programs.
3. Capability of independent compilation of deifferent program segments,
simplifying program structure and debugging.
4. Capability of interfacing with FORTRAN, FAIL, and MACRO programs, providing
any division of labor required by the nature of a problem.
5. Measurement and runtime parameter facilities to aid in optimizing execution
time and storage utilization.
------------------
Research supported in part by the National Science Foundation, Grant GJ-821.
CHAPTER 1
1. Introduction
It is presumed that the reader is familiar with SNOBOL4, version 3 as described
by the second edition (1971) of the Prentice-Hall publication [2]. Using [2]
as a base description of SNOBOL4, the following chapters explain any differences
and additions present in FASBOL, as well as describe how to use it to compile
and run programs.
The FASBOL compiler is itself written in FASBOL and is like FORTRAN, FAIL, and
MACRO in that it accepts specifications for source, listing, and object files
(the object is a FAIL program which must be assembled). The reason for writing
the compiler in FASBOL was for speed of implementation, automatic checkout of
the run-time library, and ease of modification. If after some use the compiler
should prove to be unsatisfactory in terms of core utilization or execution
speed, the FAIL stage can be hand-tailored, using the measurement techniques
available in FASBOL, into a more efficient program. A further enhancement would
be the direct production of relocatable code by a one-pass compiler written in
either FASBOL or assembler language.
The FASBOL run-time library is written in MACRO, since its efficiency is
paramount, and is searched in library mode, after loading all FASBOL programs,
in order to satisfy program references to predefined primitives and system
routines. Sections of the FORTRAN library may also be loaded, provided they
do not compete with FASBOL for UUOs and traps.
Use of the male gender in third-person references in this manual in no way implies
that FASBOL is not useful for female persons; the author is simply not aware of
any easy way to write in neuter.
CHAPTER 2
2. Language Description
In addition to the detailed changes mentioned below, this syntax differs from
that given in [2] only in the sense that it is more restrictive of compile-time
syntax. For example, since FASBOL does not permit redefinition of operators,
the expression
( A . B ) + ( C . D )
is flagged as a compilation syntax error, whereas the interpreter (i.e. the
system described in [2]) would accept it and then produce an "illegal type"
error message during execution. Most SNOBOL4 programs should run "as is"
under FASBOL; sections 2.1.1 and 2.1.2 describe exactly all features that may
cause incompatibility, and the remaining sections deal with enhancements.
2.1 General language features
The following three sections discuss, respectively, features of SNOBOL4 not
implemented in FASBOL, features of SNOBOL4 implemented differently in FASBOL,
and additional features available in FASBOL and described more completely in
sections 2.2, 2.3, 2.4 and 2.5.
2.1.1 SNOBOL4 features not implemented
The predefined primitives EVAL and CODE, the datatypes CODE and EXPRESSION,
and direct gotos (as used with CODE) are not implemented. They imply a
run-time compilation capability which is not available in the FASBOL
library at this time.
The redefinition of operators via OPSYN, or the redefinition of predefined
primitive pattern variables (e.g. ARB) or functions (e.g. SPAN) is not permitted
in FASBOL, which considers all these items as a structural part of the language
essential to generating efficient code. For this reason, the keywords &ARB,
&BAL, &FAIL, &FENCE, &REM, and &SUCCEED are not needed and therefore not
implemented. Also, &CODE has no meaning for the PDP-10, and is not available
either.
The SNOBOL4 tracing capability is not implemented in FASBOL at this time.
However, the &SYNTRACE keyword (see section 2.5) provides some tracing capability.
Although QUICKSCAN mode for pattern matching is implemented in FASBOL, two
features of this mode, available in SNOBOL4, are not implemented. These are
a) continual comparison of the number of characters remaining in the subject
string against the number of characters required by non-string-valued patterns,
and b) assumption that unevaluated expressions must match at least one character.
This implies that some matches may last a little longer and perhaps have a few
more side-effects (e.g. via $), and that left-recursive pattern definitions will
loop indefinitely (see Section 3.2.3).
2.1.2 SNOBOL4 features implemented differently
The FASBOL I/O structure is time-sharing oriented and does not use FORTRAN I/O,
so that it differs somewhat from the SNOBOL4 I/O. Both input and output can be
either in line or character mode. Line mode is similar to SNOBOL4 I/O, with input
records being terminated just prior to a carriage return - linefeed sequence,
and with this sequence being added to output records. Trailing blanks seldom
occur on input, and so the &TRIM keyword is not implemented (but the TRIM function
is). Character mode gets one character (including a carriage return or linefeed)
on input, and outputs a string without appending a carriage return - linefeed
sequence to it. INPUT, INPUTC, OUTPUT and OUTPUTC have predefined associations
(see 2.4.2, I/O primitives) corresponding to line and character mode teletype
input and output, respectively. PUNCH does not have a predefined association.
There are additional predefined primitives for device and file selection, etc.,
discussed in Section 2.4.2.
Changes in program syntax are as follows:
a) Compiler generated statement numbers are always on the left.
b) Source lines are always truncated after 72 characters.
c) The character codes and extended syntax are like the S/360 version,
except the character ! (exclamation point) may be used for |
(vertical stroke) and \ (back slash) may be used for ¬ (not sign).
d) Binary $ and . (immediate and conditional pattern assignment) have
lower precedence than the binary arithmetic operators, but higher
precedence than concatenation. Thus, the expression
X A + B $ C
is taken to mean
X ((A + B) $ C)
In SNOBOL4, $ and . have the highest precedence of all binary operators, and
would give the meaning
X (A + (B $ C))
to the above expression.
Changes in program semantics and operation are as follows:
a) The binary . (name) operator always returns a value of type
NAME (SNOBOL4 sometimes returns a STRING). Indirection (unary $)
applied to a value of type NAME returns the name as value. Names
of TABLE entries are permitted.
b) Some predefined primitive functions operate differently than in
SNOBOL4 (see Section 2.4.2).
c) &MAXLNGTH is initially set to 262143 (in SNOBOL4, the value is
5000). This value is also the absolute upper limit on string size.
d) If &ABEND is nonzero at program termination, an abnormal (EXIT 1,)
exit to the system is taken.
e) Primitive functions may be called with either too few or too many
arguments, even via OPSYN or APPLY.
2.1.3 Additions to SNOBOL4
Declarations are provided in FASBOL for the enhancement of programs. No
declarations are ever required, but if they are used they must all precede
the first executable statement in a program. Declarations are described
in Section 2.2.
Additional control cards and compilation features are available, discussed in
Section 2.3, and additional predefined primitive functions and keywords are
described respectively in Sections 2.4 and 2.5.
Additions to program syntax are as follows:
a) Quoted strings may be continued onto a new line, with the
continuation character removed from the literal.
b) Single and double quotes may be included in a literal that
is bracketed by the same, by use of the construction '' to
stand for ' and "" to stand for " inside of literals bracketed
by ' and ", respectively.
c) Comment and control lines (i.e. starting with * or -) may start
inside a line image (i.e. after a ;), and consume the remainder
of the line image.
d) The run-time syntax for DEFINE and DATA prototypes has been
loosened to conform with the rest of FASBOL syntax by permitting
blanks and tabs after ( (open parenthesis), around , (comma),
and before ) (close parenthesis).
2.2 Declarations
FASBOL declarations have two primary purposes. One purpose is to optimize a
program in space or time. The second purpose is to allow inter-program
linkage and communication. The general form of a declaration is a call on the
pseudofunction DECLARE, with two or three arguments, the first of which is
always a string literal identifying the type of declaration, and the remaining
arguments specifying the parameters or program symbols upon which the
declaration has effect. Single quotes must be used in specifying literal
arguments to DECLARE. As has been noted, all declarations must precede the
first executable statement; this also implies no declaration line may
contain a label. A FASBOL program with declarations can be made otherwise
compatible with a SNOBOL4 interpreter by inserting the statement
DEFINE('DECLARE()', 'RETURN')
at the beginning of the program.
2.2.1 PURGE and UNPURGE
Normally, a FASBOL application will involve a main program and several
independently compiled subroutines. During execution, the run-time system
maintains a run-time symbol table for each separately compiled program, as
well as a global symbol table. In the absence of declarations to the
contrary, all explicitly mentioned variables, labels, and functions are
put into the local symbol table for that program. Thus, program X and
program Y may both have labels LAB to which they perform indirect gotos. The
global symbol table contains such global symbols as OUTPUT and RETURN, and any
new symbols that arise during execution of any of the programs. A symbol
lookup in program X first searches the local symbol table for program X, then
the global symbol table, and then, if still not found, creates a new entry in
the global symbol table. Thus a local symbol table never grows beyond the size
determined for it at compilation time.
The purpose of the PURGE.VARIABLE, PURGE.LABEL, and PURGE.FUNCTION declarations
is to eliminate symbols from the local symbol table and thus conserve space.
This can be safely done for labels provided that the label is never referenced
indirectly ($ goto), or explicitly or implicitly in a DEFINE call. A similar
criterion applies to safely eliminating variables, only the number of cases to
watch for is greater; any situation that requires an association between the
string representing the variable name, and the actual location assigned to that
variable, is such a case. For example, the statement
INPUT( 'VARB',0,60)
implies that the variable VARB, if it is mentioned explicitly in the program
and thus assigned a location, must be in the run-time symbol table. An explicit
reference to VARB would be, for example,
TTYLIN = VARB
On the other hand, a variable that is never referenced explicitly need not be
in the local symbol table, but the first symbol lookup for it will create an
entry for it in the global symbol table. In the case of functions, the only
symbols that can be safely purged are the ones corresponding to predefined
primitives, since all others are needed to be able to define the user functions,
via DEFINE or otherwise.
When there appear to be more symbols of a given type to be purged than left in
the symbol table, the second argument to the declaration can be the pseudo-
variable ALL; then, the UNPURGE.VARIABLE, UNPURGE.LABEL or UNPURGE.FUNCTION
declarations can be used to place specific symbols into the symbol table.
2.2.2 GLOBAL, ENTRY, and EXTERNAL
These declarations permit interprogram communication on an indirect (i.e. symbol
lookup) or direct (i.e. loader linking) basis. The GLOBAL.LABEL, GLOBAL.VARIABLE,
and GLOBAL.FUNCTION declarations override PURGE/UNPURGE and cause the specified
symbols to be placed in the global symbol table instead of the local one. Only
one subprogram may globalize a particular symbol, since the implication is that
the variable, label, or function belongs to that program. Any other program
that does not have a similar symbol in its local symbol table will then be able
to reference the global symbol.
While GLOBAL provides for interprogram communication via the symbol table, the
ENTRY/EXTERNAL declarations provide for more direct inter-program communication
by using the linking loader to connect external references. The ENTRY.VARIABLE,
ENTRY.LABEL, and ENTRY.FUNCTION declarations make the specified local entities
accessible to external programs. The second and third arguments to the
ENTRY.FUNCTION declaration are like the arguments to DEFINE, and the function
is automatically DEFINED the first time it is called, so no extra DEFINE is
necessary. The ENTRY.FORTRAN.FUNCTION declaration is similar to ENTRY.FUNCTION
except that the compiler assumes the entry will be called by a FORTRAN program.
Any combination of FASBOL, FORTRAN, and assembler language programs is
permitted, provided the main program is FASBOL, and certain restrictions on
FORTRAN code (see Section 3.2.5) are observed.
The EXTERNAL.VARIABLE, EXTERNAL.FUNCTION, EXTERNAL.FORTRAN.FUNCTION, and
EXTERNAL.LABEL declarations are the converse of the ENTRY declarations, and
imply that the specified entities be outside the program. The
EXTERNAL.FORTRAN.FUNCTION declaration has a special form for its parameter
list (second argument) where the number of arguments expected by each function
is given in parentheses. The function value type is either declared implicitly
(first character = I,J,K,L,M,N means integer, otherwise real), or explicitly by
appending =INTEGER or =REAL to the function value, before the argument
specification.
2.2.3 Type declarations
Normally, FASBOL variables may contain data of any type, referred to here as
descriptor mode variables. Sometimes it is known in advance that certain
variables will always take on values that are integers or reals; in this
case it becomes advantageous to declare them INTEGER or REAL. In addition
to the execution speed advantages (see Section 3.2.1), they can be passed
to FORTRAN directly in function calls. Also, the only way to pass a string
(other than a literal) to FORTRAN is to use a variable declared to be STRING,
which is the only real use for that declaration; a fixed amount of storage
is allocated for the variable based on the max character count given in
parenthesis after each name in the parameter list (second argument). The
only restrictions on these variables, referred to here as dedicated mode
variables, is that they may not have I/O associations. All keywords
(except for &RTNTYPE and &ALPHABET) are treated as dedicated integers.
2.2.4 Other declarations
The OPTION declaration serves to specify various compilation options. The
HASHSIZE=n declaration, ignored in all but the main program, is used to
cause a larger or smaller than normal hash bucket table to be allocated for
use by the run-time symbol table. The number n should be a prime and
represents the number of buckets in a linked hash table; the standard value
is 127. This bucket table is at the center of all symbol lookups in the
runtime system, including TABLE references, so that there is a distinct
tradeoff between the sparsity of the table and the time required for a lookup.
The NO.STNO option causes the compiler to eliminate the normal bookkeeping
on &STNO, &STCOONT, etc. that occurs each time a statement is entered, and is
helpful to speed up slightly the execution of debugged programs. The TIMER
option, which is incompatible with NO.STNO in the same program, adds to the
normal bookkeeping a valuable statement timing feature (see Section 3.2.4).
The timing statistics on each program being timed are printed out at the end of
execution, and intermediate timing statistics can be printed out during
execution by using the primitive EXTIME (see Section 2.4.2).
The SNOBOL.MAIN and SNOBOL.SUBPROGRAM declarations indicate whether the
program is a main program or a subprogram, and gives it a name (i.e. TITLE
in FAIL). In the absence of either declaration,
DECLARE( 'SNOBOL.MAIN', '.MAIN.')
is assumed.
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once it has been created, it exists independently from its use in the
program. Thus, to reclaim the storage, it must be explicitly deleted by
TABLE(table). Once a table has been deleted, further references to it
are illegal.
APPLY(fun, args) will accept either more or fewer arguments than required by the
function; it will reject extra ones or fill in missing arguments with
null values.
OPEN(device, chan) opens an I/O device on a software channel, assigns buffers,
and returns the channel number.
If chan < 0 or > 15, illegal I/O unit error.
If chan = 0, an unused channel is assigned; if channel table is full
(> 15 channels), error.
If chan is already in use, illegal I/O unit error.
If device is not a string of the form:
devnam [([outbuf] [, [inbuf] ])],
it is a bad prototype or illegal arg error.
If devnam is not recognized, or is not a file structure and is already
assigned to a channel, illegal I/O unit error.
If ([outbuf] [, [inbuf] ]) is missing, (2,2) is assumed. If either
outbuf or inbuf is missing, 0 is assumed for the missing value.
If the device allows only input (or output), the other buffer parameter
is ignored.
Examples:
OPEN('DTA3(,4)', 5)
OUTPUT('DUMP', OPEN('MTA0'), 1000)
RELEASE(chan) releases the software channel and all associations to it, returns
all buffers to free storage, and returns a null value.
If chan < 0 or > 15, illegal I/O unit error.
If chan = 0, release all channels in use.
If channel not in use, ignore and return.
LOOKUP(file, chan) opens file for input (reading) on software channel, returns
channel.
ENTER(file, chan) opens file for output (writing) on software channel, returns
channel.
If chan < 0 or > 15, illegal I/O unit error.
If chan = 0, a preliminary OPEN('DSK') is performed, the new channel
returned.
If file is not of the form
filenam [. ext]
then bad prototype error.
If file is not found, or if channel is not open for operations,
illegal I/O unit error.
If input (LOOKUP) or output (ENTER) side of channel already selects
a file, the old file is closed.
CLOSE(chan, inhib, outhib) closes the input or output side of the software
channel, returns null.
If outhib is non-null, the output side is not closed.
If inhib is non-null, the input side is not closed.
If chan < 1 or > 15, illegal I/O unit error.
If channel is not in use, ignore and return.
INPUT(var, chan, len)
OUTPUT(var, chan, len) create an input(output) association between the variable
var and software channel chan, with line/character mode and association
length specified by len, and return null.
If len > 0, line mode.
If len = 0, line mode with default length (72).
If len < 0 or not INTEGER, character mode.
If chan > 15, illegal I/O unit.
If chan > 0 use channel table to determine I/O device.
If chan = 0 use TTY I/O
If chan < 0 or not INTEGER, disconnect association but do not DETACH it.
If the input (output) side of the channel has not been opened,
illegal I/O unit error.
If var is not a string, illegal arg.
If an association for var already exists, it is changed.
If variable is dedicated, illegal arg.
Examples:
INPUT('SOURCE',LOOKUP('SRCELT.SNO'),80)
OUTPUT('TYPEOUT.CHARS',0,-1)
The initial I/O configuration is equivalent to:
INPUT('INPUT')
INPUT('INPUTC',0,-1)
OUTPUT('OUTPUT')
OUTPUT('OUTPUTC',0,-1)
During execution, all system messages are output via the variables
OUTPUT and OUTPUTC (which should always both be associated to the same
channel). In order to switch system output to the printer, for example,
LPT = OPEN('LPT')
OUTPUT('OUTPUT',LPT,132)
OUTPUT('OUTPUTC',LPT,-1)
Channel 0 is never assigned, but when used in INPUT and OUTPUT associations
implies the user TTY and TTCALL operation. On input, line mode reads up
to (but not including) the next carriage return - line feed (CRLF) sequence,
and then these are discarded. Character mode reads only one character
(including CR or LF). Line mode discards any characters beyond the
association length. An EOF causes failure in either mode, but cannot occur
on some devices (such as the user TTY).
On output, line mode writes out the string value with a CRLF appended,
whereas character mode does not append the CRLF. In line mode, if the
string length is greater than the association length, extra CRLF
characters are inserted every association length substring.
DETACH(var) disconnects input and output associations for the variable and
detaches it from I/O processing, returns null.
If the variable had no association, ignore and return.
SUBSTR(string, len, pos) returns the substring of string starting at pos of
length len, and fails if len < 0, pos < 0, or pos + len > SIZE(string).
The position convention is the same as that for patterns, and the
operation is similar (but faster and less space-consuming) to:
string TAB(pos) LEN(len) . SUBSTR
INSERT(substring, string, len, pos) returns the string formed by substituting
for the one specified by the last 3 arguments into string, failing
under that same conditions as:
string TAB(pos) . PART1 LEN(len) REM . PART2
INSERT = PART1 substring PART2
LPAD(string, len, padchr) returns the string formed by padding string on the
left with padchr characters to a length of len. If string is already
too long, it is returned unchanged; if padchr has more than one
character, only the first is used. If the third argument is null,
blanks are used.
RPAD(string, len, padchr) is like LPAD, but pads to the right.
REVERS(string) returns the string formed by reversing the order of the
characters of its argument.
EXTIME(progname) causes the runtime system to output current timing
statistics for the program progname and returns null, or fails
if the program is not being timed.
REAL(x) is like INTEGER for reals
EJECT() causes a page eject (form feed) to be assigned to OUTPUTC
DAYTIM() returns an 11-character string representing the time of day
(since midnight), as
HH:MM:SS.HH
meaning hours, minutes, and seconds to the nearest hundredth.
2.4.3 FORTRAN primitives
These are predefined EXTERNAL.FORTRAN functions that, except for FREEZE, merely
perform some simple arithmetic task and have integer values.
FREEZE() can be called to freeze the state of the FASBOL execution for resumption
at some future date. When FREEZE is called, it exits to the monitor; the
job may be SAVED, and when run again, it will start off by returning from
the call to FREEZE. This is particularly useful for some applications
that perform a considerable amount of initialization and wish to be able
to start after that point on a repeated basis. No I/O devices (other than
the console TTY) may be open at the time of the FREEZE and the call should
be made from function level 0 if any timing is active.
ILT(int, int) or ILT(real, real)
[also ILE, IEQ, INE, IGE, IGT] Like LT, etc. except more efficient for dedicated
variables or expressions. Note that both arguments must be supplied for
these functions, since they have been declared as EXTERNAL.FORTRAN.
AND(int, int)
[also OR, XOR, RSHIFT, LSHIFT, REMDR] perform the specified arithmetic or logical
operation on their integer arguments and return the value (logical AND;
inclusive OR; exclusive OR; logical right and left shift of first by
second argument; remainder of integer division of first by second
argument).
NOT(int) returns one's complement of its argument.
2.5 Keywords
Three new keywords, all unprotected, have been added in FASBOL.
&STNTRACE is initially 0, but if assigned a nonzero value it causes a trace output
for each statement, giving statement number, program name, and time. This
slows down execution considerably, so it is best to turn it on as close to
the suspected bug as possible. Programs compiled under the NO.STNO option
will ignore the value of &STNTRACE, however, so another approach is to run
with all but the suspect program under NO.STNO, with &STNTRACE on all the
time.
&DENSITY is initially 75, and represents the desired density of free storage
immediately following a garbage collection. For example, &DENSITY = 75
means that the free storage system will try to maintain at least a 1:4
ratio between available and total storage immediately following a
garbage collection, and will expand total storage as far as necessary or
possible in order to try to maintain this ratio. See Section 3.2.4.
&SLOWFRAG is initially 0, but if assigned a nonzero value it serves to switch in
a heuristic in the free storage mechanism that slows down the rate of
fragmentation of blocks at the expense of some wasted storage. See
Section 3.2.4.
CHAPTER 3
3. FASBOL Programming
Using FASBOL involves two separate stages, as in FORTRAN: compilation and
execution. The first requires the compiler, an absolute program named
FASBOL.DMP. Execution of compiled (and then assembled) programs requires a
library search, during loading, of the FASBOL library; this is a collection of
relocatable programs named FASLIB.REL. The compiler requires a minimum of 35K
to run, and requires more core in proportion to the program being compiled.
The core requirements for execution of user programs depends on the size of
the compiled programs plus at most 5K (if every single facility in the library
is used) for the library.
3.1 Using the compiler and runtime library
To compile a FASBOL program type
.R FASBOL n
where the core argument (n) is optional. It is best to give the compiler an
amount of core commensurate with the size of program being compiled; this
will increase compilation speed by minimizing garbage collections, since the
compiler will expand core on its own only when it absolutely has to.
The compiler will respond with
*
to which the user is expected to respond with a set of file specifications of
the form
faifil,lstfil←srcfil or
faifil←srcfil or
,lstfil←srcfil or
srcfil or
,srcfil
Each file specification is of the standard form, as would be given to FAIL, for
instance. The FAIL output file, faifil, is given a default version of FAI if
not specified. Lstfil is the listing file; its default extension is LST. One
of faifil and lstfil (but not both) may be omitted. The source file, srcfil,
is given a default extension of SNO if not specified. Only one source file is
permitted. In the last two forms, the faifil or lstfil (respectively) is
assumed to have the same name as the srcfil, but with the default extension.
Thus the following are equivalent: "X.FOO", "X←X.FOO", and "X.FAI←X.FOO".
Examples:
DTA3:SAMPLE,LPT:←SAMPLE
,TAPE.LST←MTA0:
NEW.NEW←TEST.NEW
Once the compiler produces the FAIL output file, it will automatically be
assembled by FAIL. The FAIL file can be deleted after assembly, as it will
be of little interest to most users; it is mainly a shortcut for the compiler
to avoid having to generate relocatable code. On the other hand, those
individuals who understand the workings of the run-time system may wish to
hand-tailor these intermediate programs to suit their own needs; caveat emptor.
While FASBOL may call or may be called by FORTRAN or user assembler language
programs, the main program must be a FASBOL program. Furthermore, the FASBOL
runtime system enables traps and uses user UUOs 1 through 10, so it is
incompatible with the FORTRAN runtime system. What this means is that FORTRAN
programs used within a FASBOL execution must not do any I/O or otherwise cause
FORSE. to be loaded. FASBOL does provide an infinite stack (all FASBOL stacks
are infinite, up to user core limits) in register 17, however, so a broad class
of FORTRAN user programs and library routines is permissible.
Unless changed by the user's program, all system output during execution is sent
to the user's console; upon either error or normal termination of execution,
the appropriate messages and statistics will be printed out, and control
returned to the monitor. The error messages are listed in appendix 2.
3.2 Programming techniques
Because of the basic difference between interpretive and compiler systems,
and the additional features available in FASBOL, some programming techniques
besides those discussed in [2], Ch. 11, are described here. An interested
user may wish to get a listing of the compiler itself to see examples of some
of these techniques.
3.2.1 Dedicated expressions
Dedicated expressions in FASBOL are those that are known, because of some
component, to have a numerical value of a predetermined type. At one extreme
is the totally dedicated statement that involves nothing but declared dedicated
variables, constants, and perhaps FORTRAN calls. For example, if I were declared
INTEGER, the statement
I = 2 * I + 10
would be totally dedicated, and compile into
MOVE 1, I
IMULI 1, 2
ADDI 1, 10
MOVEM 1, I
Even if an expression is mixed, with both dedicated and descriptor-mode
subexpressions, in-line arithmetic code is compiled for as much of the expression
as is possible to commit to a specific type of value (i.e. INTEGER or REAL) at
compile time. It is therefore to the user's advantage to declare as many
variables as he perceives will be dedicated in use to be of that dedicated type.
Not only will the program run faster, it may even use less core. In a situation
where all entities are descriptor mode, even arithmetic operators have to check
the type of, and possibly convert each argument.
In this connection it should also be noted that the predefined FORTRAN
primitives ILT, ILE, IEQ, INE, IGE, IGT have been provided in order to do a
much more efficient job than LT, ..., GT when the arguments are dedicated.
For example, if R and S are REAL, the test
ILT(R,S)
take up several fewer words and runs about 100 times faster than the test
LT(R,S)
FASBOL permits mixed mode (INTEGER and REAL) arithmetic, the general rule
being that the result of an operation is INTEGER only if both sides are
integer; furthermore, an arithmetic operation involving dedicated and descriptor
mode values always has a dedicated result. A value being combined with a stronger
mode is first converted, and then the operation is performed in that mode; for
example, if I is INTEGER but D has not been declared dedicated,
I + D
implies the value of D will first be converted to an integer, and then added to I.
The only exception to this rule is the ** (exponentiation) operator, which permits
a REAL raised to an INTEGER power.
Finally, it should be noted that whereas the range of values of dedicated variables
is the same as in FORTRAN, descriptor mode integers have a range two powers of 2
less in magnitude, and descriptor mode reals have two fewer bits of precision in
the mantissa. The reason for this is that the two bits are needed for the
descriptor type.
3.2.2 Use of the unary ? and . operators
Unary ? (interrogation) is useful to indicate to the compiler that an expression
is evaluated for its effect, rather than value. For example, a frequent
occurrence in SNOBOL programs is the concatenation of null-valued functions for
their sequential effects or succeed/fail potential. If the compiler knows
that an element has a null value, it does not generate code to include it in
the concatenation. Therefore it is efficient to precede predicates and other
null-valued elements in a concatenation with the ? operator. This technique is
especially valuable when combining predicates and dedicated arithmetic, as in
I = ?IDENT(A, B) ?IGT(I, 25) I + 1
since concatenation is avoided entirely and the dedicated arithmetic is performed
after the execution of the predicates without any need for conversion between
dedicated and descriptor values.
Another frequent occurrence in SNOBOL programs is the repetitive access of the
indirect variable, array element, or field of a programmer-defined datatype.
Each of these accesses, whether to retrieve or store a value, involve some
overhead which is repeated for each access. For example, in the statements
$X = $X + 1
the variable represented by the string value of X is looked up in the symbol
table twice, the first time to retrieve its value, the second time to store into
it. The unary . (name) operator can be used to save the result of one lookup
by creating a NAME datatype, and then the NAME can be used in an indirect
reference wherever the original expression was used. Instead of the above
statement, a more efficient sequence would be
Z = .$X
$Z = $Z + 1
where Z contains the NAME of the variable pointed to by X. The same
considerations apply to array references and field references as apply to
indirection; it is efficient to save the NAME of the variable referenced if
it will be used more than once in close succession. For example, the
statements
A<25> = F(A<25>)
NEXT(LIST) = NODE(VAL, NEXT(LIST))
would be more efficient if coded as
Z = .A<25>
$Z = F($Z)
Z = .NEXT(LIST)
$Z = NODE(VAL, $Z)
It should be noted that TABLE references are array references, and the symbol
lookup involved makes it even more efficient to save the NAME. The NAME of
an ARRAY or TABLE element, or of a field of a programmer-defined datatype, is
only valid as long as that array, table or datatype exist; attempts to
retrieve or store using the NAME afterwards will have unpredictable results.
Also, the NAME of a variable, evaluated before that variable acquires an
I/O association, does not reflect that association.
3.2.3 Pattern matching
Frequently a programmer wishes to write a degenerate-type statement
consisting of a concatenation of elements executed for their effect, as in
F(A) F(B) F(C) F(D)
This syntax, however, is parsed as a pattern match, and, though having the
same effect as intended (provided the match is successful), is less efficient
in both space and time. The original intent can best be achieved by enclosing
the concatenation in parentheses, and, in this case, using the ? operator
(?F(A) ?F(B) ?F(C) ?F(D))
which will suppress string concatenation.
One particularly unique feature of FASBOL is that explicit pattern
expressions, i.e. those involving the pattern operators or primitives, are
compiled as re-entrant subroutines, rather than constructed at run-time into
intermediate-language structures. The significance of this to the programmer,
aside from the improvement in execution speed, is that there is less of a need
to pre-assign subpatterns that will appear in pattern matches later on; in
fact, an unnecessary pre-assignment will be slightly less efficient because
the pattern match will have to recurse one level deeper than otherwise during
execution. The way to determine the need for pre-assignment is to note how
much evaluation is actually required in a subpattern; if little or none is
required, it can just as efficiently be included in the body of the match.
Of course, if a subpattern is large or used in several matches, the programmer
may wish to pre-assign it anyway for convenience sake. Pattern evaluation
involves only the elements of the pattern, not the structure itself. Literals
and other constant values do not require evaluation, so the pattern
TAB(7) (SPAN('XYZ') ! BREAK(';')) $ SYM ';??'
requires no evaluation at all. A generally applicable rule for all FASBOL
programming is that it is more efficient to use, wherever possible, literals
instead of variables with constant value. Simple variables appearing in a
pattern require little evaluation (only a determination if they have a string
or pattern value), and even character class primitives (i.e. SPAN, BREAK, etc.)
require little evaluation, if their argument is non-literal, provided the
argument is a variable with a constant value. Examples of pattern elements
requiring more extensive evaluation are (non-pattern primitive) function
calls, non-pattern expressions requiring considerable evaluation in their own
right, and character-class primitives whose arguments are other than literals
or simple variables. An example of the latter case would be
ANY('XYZ' OTHER)
Even if the value of OTHER remains constant, the concatenation produces a
new string each time, which prevents ANY from immediately using the break table
it has generated on the last execution of the call. It has been assumed in all
this discussion of pattern evaluation that the value of the pattern element would
not change value between the time of assignment of the subpattern and its use
in a match. Should this not be the case, of course, the alternative of
including the subpattern in the match does not exist.
Even when integer constants cannot be used, it is still helpful to use dedicated
integer variables or expressions in patterns, if possible. Dedicated integer
expressions are ideally suitable as arguments to the positional pattern
primitives (i.e. POS, LEN, etc.), and integer variables are ideally suitable as
objects of the cursor assignment operator (@). For example, suppose one wishes
to take a string composed of sentences separated by semicolons (and terminated by
a semicolon) and output the sentences on separate lines. A single pattern match
to do this would be (P is INTEGER):
STRING @P SUCCEED TAB(*P) BREAK(';' $ OUTPUT LEN(1)
. @P RPOS(0)
Note that the pattern requires no evaluation.
Since FASBOL does not employ the QUICKSCAN heuristic of assuming at least one
character for unevaluated expressions, left-recursive patterns will loop
indefinitely at execution time, as they would in SNOBOL4 under FULLSCAN. Usually
a set of patterns involving left recursion can be re-written to eliminate it. To
take a simple example, the pattern
P = *P 'Z' ! 'Y'
which matches strings of the form 'Y', 'YZ', 'YZZ', 'YZZZ', etc, could be
re-written as the pair of patterns
P1 = 'Z' *P1 | ''
P = 'Y' P1
3.2.4 Timing and storage management
The TIMER option permits the programmer to monitor the operation of
any (or all) separately compiled programs, and provides feedback on where
the time is being spent. Initial programming of some problem can be done
rapidly with not much attention being paid to optimization. It is usually
the case that some small sections of a program account for a large percentage
of the execution time; these are identified using the TIMER option. The
programmer's time is then spent most efficiently optimizing the critical areas
and ignoring the rest. Of course, after a series of optimizations, a new
bottleneck will develop; the process can then be iterated until the law of
diminishing returns takes hold. Finally, the TIMER declarations can be
removed and the programs run in production mode.
The programmer has a large degree of control over storage management
in FASBOL, which in turn means control over the space/time tradeoff that
exists due to the dynamic storage allocation system (free storage). To begin
with, requests from the free storage system prior to the first garbage
collection (regeneration of dynamic storage) have very little overhead
compared to ones subsequent to the first garbage collection. Unless there
are good reasons for the contrary, the user should capitalize on this by
starting his execution with approximately the amount of core he expects will
be required -- past experience with the program is the best guide. Thus the
number of garbage collections will be reduced to a minimum, and initial
execution speeded up. In the absence of a core specification, the program
will begin with the minimum required for loading, and will expand core as it
becomes necessary, but undergoing more garbage collections.
The &DENSITY keyword is also useful in controlling the space/time
tradeoff. &DENSITY may be set dynamically to any value between 1 and 100;
immediately following a garbage collection, the dynamic storage allocation
mechanism attempts to satisfy this value, interpreted as the percentage of
total storage allocated that is in use at that time. Nothing is done unless
the actual ratio is greater than the desired one, in which case core is
expanded to satisfy the desired ratio, or until user core limits are
reached. For example, a user who sets &DENSITY to 99 is saying he wishes to
keep his core size to a minimum, and is willing to pay a (rather large)
premium in repeated garbage collections. On the other hand, a user who sets
&DENSITY to 1 is asking for all the core he can get, in order that his
program execute as rapidly as possible. It is also perfectly feasible to
use a strategy where &DENSITY is set to different values at different times
during execution. The initial value of &DENSITY is 75, which represents a
general-purpose compromise.
If a user's application will occasionally require large contiguous
blocks of storage, he may give himself 100% insurance by reserving dummy
arrays of the appropriate size at the very beginning of his program. An
alternative is to turn on the keyword &SLOWFRAG, which activates a heuristic
which tends to slow down the fragmentation of large blocks at the expense of
some wasted storage. While not 100% guaranteed, it will give the desired
effect in most cases, minimizing the situation where a large block is called
for, and though enough total storage is available, no contiguous area is
large enough to satisfy the request.
Finally, the COLLECT primitive may be invoked at appropriate times,
both to force a regeneration and also measure the amount of storage that is
available.
3.2.5 FORTRAN interface
External FORTRAN subroutines, whether user-written or from the
FORTRAN library, must be declared, including the number of arguments
expected. A function call to the external subroutine may have any expression
(except patterns) as arguments, but all but a few recognized expressions are
assumed to evaluate to integers and will cause an error exit if not.
Dedicated integer and real variables are passed directly to the subroutine,
as they would in a call from a FORTRAN program. Also, dedicated integer
and real expressions are evaluated, the value saved in a temporary location,
and this location passed to the FORTRAN routine. Finally, dedicated string
variables and literals are passed to FORTRAN as a vector, the first word of
which is pointed to by the calling sequence, and the FORTRAN routine may
interpret it as a one-dimensional array of ASCII characters packed five to
a word. In addition to returning an integer or real value, the function
may modify the value of any dedicated integer, real, or string variable
that is passed to it. In the last case, not only may the characters be
modified, but the character count may be changed by storing it in the right
half of the word immediately preceding the first word of the string (array
element 0). For example, suppose a FASBOL program contains the declarations
DECLARE('INTEGER' , 'I')
DECLARE('REAL' , 'R')
DECLARE('STRING' , 'S(15)')
DECLARE('EXTERNAL.FORTRAN.FUNCTION' , 'GETDAT(3)')
and the FORTRAN function GETDAT is defined as
FUNCTION GETDAT(INDEX, ISTR, IDAT)
COMMON IDATA(1000, 4)
EQUIVALENCE (RDATA, IDATA)
DIMENSION ISTR(3), RDATA(1000, 4)
ISTR(1) = IDATA(INDEX, 1)
ISTR(2) = IDATA(INDEX, 2)
ISTR(0) = (ISTR(0) / 2**18) * 2**18 + 10
IDAT = IDATA(INDEX, 3)
GETDAT = RDATA(INDEX, 4)
RETURN
END
then a typical use of GETDAT within the FASBOL program might be
R = GETDAT(2, S, I)
which would have the effect of setting I to some integer value, R to some
real value, and S to a 10-character string.
Entries that are expected from FORTRAN must be declared with the
ENTRY.FORTRAN.FUNCTION declaration. This works like ENTRY.FUNCTION in that
an automatic DEFINE is performed on the first call. Valid actual arguments
in the FORTRAN call to FASBOL can be integers, reals, and Hollerith arrays
(as described above, denoted by the codes 0, 2, and 5 in the calling sequence).
Upon entering FASBOL, the actual arguments are copied, and converted if
necessary, into the formal arguments, which are dedicated or descriptor mode
FASBOL variables. The right half of the word immediately preceding the first
word of a Hollerith array is considered to be the character count, and may be
modified. Upon return to FORTRAN (via RETURN), the function value is
determined by dedicated or descriptor value in the variable corresponding to
the function name, but must be integer or real. The formal argument values
are copied back (and reconverted, if necessary) into the actual arguments in
the FORTRAN calling sequence, thus providing a means of passing back
additional values, besides the function value, to FORTRAN.
It should be noted that FORTRAN is not recursive, and therefore any
recursive combination of FASBOL and FORTRAN calls will not work. Even when
a series of calls is not recursive, care must be taken not to re-enter a
FASBOL routine which has a FORTRAN call pending, because the FASBOL routine
uses the same temporary storage locations for all FORTRAM calls, including
the predefined primitives IGT, etc.
In writing FORTRAN programs to be used with FASBOL programs, care
should be taken not to perform any I/O, or use any other FORTRAN facility
that requires the FORTRAN runtime system (FORSE.) to be loaded.
APPENDIX 1
Predefined symbols:
1. GLOBAL and EXTERNAL variables
INPUT INPUTC OUTPUT OUTPUTC
2. GLOBAL and EXTERNAL labels
END FRETURN NRETURN RETURN
3. EXTERNAL.FORTRAN functions (all integer-valued)
AND(2) FREEZE(0) IEQ(2) IGE(2) IGT(2) ILE(2) ILT(2) INE(2)
LSHIFT(2) NOT(1) OR(2) REMDR(2) RSHIFT(2) XOR(2)
4. Primitive pattern variables
ABORT ARB BAL FAIL FENCE REM SUCCEED
5. Primitive pattern functions
ANY ARBNO BREAK BREAKQ BREAKX LEN NOTANY NSPAN POS RPOS
RTAB SPAN TAB
6. Predefined primitive functions
APPLY ARRAY CLOSE COLLECT CONVERT COPY DATA DATATYPE DATE
DAYTIM DEFINE DETACH DIFFER DUPL EJECT ENTER EQ EXTIME GE
GT IDENT INPUT INSERT INTEGER ITEM LE LGT LOOKUP LPAD LT
NE OPEN OPSYN OUTPUT PROTOTYPE REAL RELEASE REPLACE REVERS
RPAD SIZE SUBSTR TABLE TIME TRIM
APPENDIX 2
Run-time errors:
Conditionally fatal
1. Illegal data type.
2. Error in arithmetic operation.
3. Erroneous array or table reference.
4. Null string in illegal context.
5. Undefined function or operation.
6. Erroneous prototype.
7. Dedicated string overflow.
8. Variable not present where required.
9. Real to string conversion overflow.
10. Illegal argument to primitive function.
11. Reading error.
12. Illegal I-O unit.
13. Limit on defined datatypes or tables exceeded.
14. Negative number in illegal context.
15. String overflow.
16. Bug in FASBOL system.
Unconditionally fatal
17. Error in FASBOL system.
18. Return from zero level.
19. Failure during goto evaluation.
20. Insufficient storage to continue.
21. Illegal memory reference (try more core if this happens).
22. Limit on statement execution exceeded.
23. Object exceeds size limit.
24. Undefined or erroneous goto.
25. Error in FASBOL system.
26. Error in FASBOL system.
27. Writing error.
28. Execution of statement with compilation error.
29. Failure under NOFAIL.
30. Divide check.
31. Arithmetic overflow.
APPENDIX 3
Differences between Fasbol and Spitbol/360:
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REFERENCES
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