S3 (programming language): Difference between revisions

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{{Infobox programming language

| name = S3

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'''S3''' is a [[structured programming|structured]], [[Imperative programming|imperative]] [[high-level language|high-level]] [[computer programming|computer]] [[programming language]].<ref>{{cite book|title=Software engineering environments|publisher=North-Holland Pub. Co.|year=1981|edition=illustrated|last=Hünke|first=Horst|pages=410|isbn=978-0-4448444-613386133-7|page=83}}</ref> It was developed by the UK company [[International Computers Limited]] (ICL) for its [[ICL 2900 Series|2900 Series]] mainframes. It is a [[system programming language]] basedwith onsyntax influenced by [[ALGOL 68]] but with data types and operators aligned to those offered by the 2900 Series. It was the implementation language of the [[operating system]] [[ICL VME|VME]].

==Annotated Example==

A rare example of an S3 program available in the public ___domain is the implementation of [[Kermit (protocol)|Kermit]] developed at the South-West Universities Regional Computer Centre, and archived in the [[Columbia University]] archive of Kermit implementations.<ref>{{cite web

|url=http://www.columbia.edu/kermit/archive

|publisher=Columbia University's Kermit Project

|title=Kermit Software Source Code Archive

|date=22 Aug 2011

|access-date=1 March 2013

}}</ref> The examples below are selected highlights of the main module (kmt_main_module).

The program starts with a module identification, and comments which we quote by way of acknowledgment to the authors:

MODULE KMT_MAIN_MODULE; @ Version 1.01 @

@------------------------------------------------------------------------------@

@ @

@ @

@ ----- S W U R C C V M E K E R M I T ----- @

@ @

@ @

@ ---------------------------------------------------------------- @

@ @

@ @

@ Version 1.00 (February 1986) @

@ @

@ Written by : Richard Andrews and David Lord, @

@ South West Universities Regional Computer Centre, @

@ Claverton Down, Bath BA2 7AY, U.K. @

@ @

@ @

@ ---------------------------------------------------------------- @

@ @

@ @

@ Version 1.01 (October 1986) @

@ @

@ Fixes by : Dave Allum and David Lord, SWURCC. @

@ ---------------------------------------------------------------- @

Next follow a number of "mode declarations". Mode is the Algol 68 term for a type.

<pre>

MODE KMT_BUFFER IS (96)BYTE;

MODE KMT_STRING IS REF()BYTE;

MODE KMT_WORD IS REF()BYTE;

MODE KMT_MTM_VALUES IS ANY

(LONG WORD LW_VALUE,

LONG INT LI_VALUE,

REF WORD RW_VALUE,

REF INT RI_VALUE,

REF LONG WORD RLW_VALUE,

REF LONG INT RLI_VALUE,

REF()BYTE RVB_VALUE,

REF()REF()BYTE RVRVB_VALUE);

MODE KMT_PP_PACKET_STATISTICS_S IS STRUCT

(INT INPUT_TOTAL,

OUTPUT_TOTAL);

</pre>

The first type is an array of 96 bytes; the next two are references (pointers) to arrays of bytes. KMT_MTM_VALUES is a union type allowing a variety of different types to appear. Note that WORD is a 32-bit unsigned integer, INT is a 32-bit signed integer; LONG makes it 64 bits. The last option in the union is marked REF()REF()BYTE, which means it is a pointer to an array whose members are pointers to arrays of bytes.

The final type declared here is a STRUCT, specifically a tuple containing two integers.

The program continues by declaring external procedures on which the module depends. RESPONSE indicates a return value containing error information:

<pre>

EXT PROC (RESPONSE) KMT_UI;

EXT PROC (REF INT,INT,RESPONSE) KMT_PH;

EXT PROC (REF INT,REF INT,RESPONSE) KMT_PP_GET_PACKET,

PROC (INT,INT,BOOL,RESPONSE) KMT_PP_SEND_PACKET,

PROC (REF()BYTE,RESPONSE) KMT_PP_BUILD_STRING_PACKET_DATA;

</pre>

and also some external variables:

<pre>

EXT REF () BYTE KMT_VERSION;

EXT REF BOOL ASG_ROUTE;

EXT REF()KMT_MTM_VALUES KMT_MTM_AREA;

EXT REF()BYTE MTM_TEXT;

EXT REF INT MTM_TEXT_LEN;

EXT REF ()REF ()BYTE MTM_RECALL_DATA;

</pre>

The rest of the program consists of a number of procedure definitions. One of these, which actually defines the entry point to the program, is reproduced here:

<pre>

GLOBAL STATIC (<STATUS 5;PSPACE 10001; TEMPLATE>) PROC KERMIT_THE_FROG IS

((<LIT "COMMAND">) REF()BYTE OPTION,

(<LIT "" >) REF()BYTE VME_FILE,

(<LIT "" >) REF()BYTE REM_FILE,

(<KEY RESPONSE;DEF N'RESULT>) RESPONSE RESULT):

BEGIN

()BYTE JSV_NAME := "ASG"; @ obtain value for ASG_ROUTE bool @

CTM_JS_READ(JSV_NAME,NIL,NIL,ASG_ROUTE,RC_IGNORED);

IF RC_IGNORED NE 0 THEN ASG_ROUTE := FALSE FI;

@ verify parameter references (parameter values validated later): @

@ OPTION must be of mode REF () BYTE, may not be ZLR or NIL @

@ VME_FILE must be of mode REF () BYTE, may be ZLR, must not be NIL @

@ REM_FILE must be of mode REF () BYTE, may be ZLR, must not be NIL @

UNLESS (VERIFY OPTION AND VALIDR OPTION)

AND (VERIFY VME_FILE AND (VALIDR VME_FILE OR NOT(VME_FILE IS NIL)))

AND (VERIFY REM_FILE AND (VALIDR REM_FILE OR NOT(REM_FILE IS NIL)))

THEN @ invalid parameter reference @

RESULT := 10002 @ ARCH_INACCESSIBLE_PARAMETER @

ELSF @ create resource block @

CTM_JS_BEGIN(RESULT);

RESULT <= 0

THEN @ resource block created @

LONG LONG WORD KERMIT_RESULT;

ANY((3)LONG WORD AS_LW,(6) WORD AS_W) PARAMS;

PARAMS.AS_LW := (BDESC OPTION,BDESC VME_FILE,BDESC REM_FILE);

@ set up program error handler @

IF KMT_EH_INFORM_PE_CONTINGENCY(RESULT);

RESULT > 0

THEN @ failed to set error handler @

SKIP

ELSF CTM_JS_CALL(NIL,PDESC KERMIT_SUPPORT,PARAMS.AS_W,KERMIT_RESULT,

RESULT); @ create firewall @

RESULT <= 0

THEN @ either exited normally or via CTM_STOP @

RESULT := IF (S'S'KERMIT_RESULT) <= 0

THEN 0 @ ignore warnings @

ELSE 52000 @ error return common resultcode @

FI

FI;

CTM_JS_END(RC_IGNORED) @ end resource block @

FI

END

</pre>

Features to note here include:

* The declaration of the procedure is decorated with annotations that define a command line syntax allowing the program to be called from SCL, or used from an interactive shell with prompting for default parameter values.

* Procedure calls prefixed CTM are calls to the "Compiler Target Machine", an [[API]] offered by the VME operating system.

* "JSV" means "job space variable", VME's term for an [[environment variable]], and the call on CTM_JS_READ reads the value of the variable.

* UNLESS means "if not"; ELSF means "else if".

* LONG LONG WORD declares a 128-bit integer, which is a native type supported by the 2900 architecture

* The bulk of the processing is delegated to another procedure, KERMIT_SUPPORT, which can be found in the same module. This is called indirectly via the operating system CTM_JS_CALL, similar to an exec() call on Unix systems; this ensures clean failure handling and tidying up of any resources in the event of a fatal error. The PDESC keyword constructs a "procedure descriptor": essentially it treats KERMIT_SUPPORT as a [[first-class function]] which can be passed as an argument to another function, making CTM_JS_CALL a [[higher-order function]] that calls its supplied argument with appropriate error handling.

==References==

{{reflist}}

[[Category: ALGOL 68 dialect]]

[[Category:ICL programming languages]]

{{compu-lang-stub}}