Classic Computing

Sections.

The I/O station Control Program outputs the PROGRAM? prompt and, upon receipt of a valid reply, overlays the station’s core partition with the requested program and passes control to it. The program must return to the Control Program on completion (or on operator’s ESCAPE request) (by JUMP Z 1400or JUMP Z 1402), whereupon the PROGRAM? prompt will be re-output.

The program must return to the Control Program upon completion (by JUMP Z 1406). If there is no work in the queue, the Control Program waits for postings to the queue and the printer is IDLE. The operators may, by command, change the print queue assignment at any time; the Control Program will switch to the new queue as soon as it receives such instructions.

I/O Station ® Printer

For any application requiring printout, the I/O station task does not directly output to a printer; rather all output is written to the disc-held Spool File for later processing by a printer task. The format and content of these intermediate Spool records is defined by the application, except that each record is 124 words long (the last 4 words of the sector are reserved for system use: double-word program name and two chain-linking words).

As a general rule, the I/O station task should create compact spool records with the minimum of processing beyond input validation; wherever possible processing and updating should be carried out (at a lower priority level) by the printer task. This improves I/O station response time, reduces the spool storage requirement and eases the problems of recovery after a cancellation request or machine fault.

An application may require several spool records to define a printout (e.g. an invoice may be of any length), but usually a single record will suffice. The first record is referred to as the header record. When spooling is complete, the spooling task posts the header record number to a pre-selected print queue, normally determined by the type of paper required, and the system adds the posting to the end of the queue.

The system inserts the station’s task number into the first word of the Spool Buffer when an I/O station starts a new program, and after each SPOOL and each POST subroutine call (thus the receiving printer program may identify the station from which it has received work). Note: this provision does not reserve the first word of spool records.

Printer ® I/O Station

The FLASH SINGLE STATION subroutine allows any task to send a message to a specific I/O Station. A printer may identify the Station from which it has received work by the task number in the first word of the spool buffer. If this task number is stored at 3742- (within the Printer’s Task Control Area) the system will flash the station pppp PRINTED BY x when the program returns to the control program (by JUMP Z 1406).

System ® Operator

The general FLASH subroutine allows any task to send a message to all I/O Stations.

Operator ® System

All I/O Stations are provided with a key which interrupts the current program and allows the operator to enter system commands. Available commands are listed in section 3 of this guide.

Operator ® Operator

Program S is available to send a message to the screen of any or all I/O Stations.

Each task has its own core partition. All task partitions are of the same size and format (as defined in the Configuration Table). Because the partition maps are pre-defined and known throughout the system, there is a considerable saving in the number of parameters required by subroutines. Experience has shown a 2K partition size to be the optimum choice for Commercial applications, and all offsets quoted in this guide will refer only to a 2K partition, with a base offset of zero (i.e. first word of the partition is offset 0000-, last word is offset 3777-).

The task OVERLAY AREA is 13 sectors long and occupies 0000- to 3177-. This will contain program code, data and work areas. Programs and Overlay Modules are loaded into the area by FETCH subroutine.

The task MASTER BUFFER is one sector (128 words) long and occupies 3200- to 3377-. Used by FETCH, REWRITE and OVERWRITE subroutines as a one sector disc transfer buffer.

The task SPOOL BUFFER is one sector (128 words) long and occupies 3400- to 3577-. Used by the SPOOL, UNSPOOL, POST, SPOOL & POST subroutines. A spooling task creates spool records here; an unspooling task receives spool records here (3577- contains the number of the next spool record in the chain). This buffer may be used as workspace (or as an extension to the Master Buffer) when spooling is not required.

I/O STATIONS only: The task INPUT BUFFER is 128 characters long and starts at 3600-. Used by the GET and SPLIT subroutines and contains the most recent input from the station (ASCII format terminated by NUL byte).

PRINTERS only: The task INNER PRINT BUFFER is 132 characters long and starts at 3600-. Used by the PRINT LINE subroutine, which always space-fills the buffer. The buffer holds one line of ASCII printout; a map of print positions against offsets is provided.

The task CONTROL AREA occupies 3704- to 3777-. This is reserved for system use. The current program name (four ASCII characters) is held at 3716-.

Note Task Control Programs and Outer Print Buffers are within the system core space, not part of the task partition.

All Applications programs are held on disc as Overlay Modules. An overlay Module may be from 1 to 13 (decimal) sectors in length, and is identified by an Octal Number, range 001 to 400 (Module numbers 001 to 040 are reserved for Systems programs).

Currently, all overlay modules are contained on disc in the Overlay Module Library, assigned file identifier 02. An index to this library is also held on disc, stating the length of each module and the offset within the-task partition at which it is to be loaded.

A task may fetch overlay modules into its own core partition at any time (see FETCH Overlay Module subroutine), but in practice only the most complex of applications will overrun the 13 sectors available to the initial overlay. On the contrary, it may be found convenient and practical to group several related programs into the same overlay module.

Programs within LOS are identified by four character name (space filled if necessary) and type (I/O Station or Printer). A directory is held on disc giving, against each I/O station program name, the corresponding Module Number and logical Entry Point within the module. A similar directory exists for printer programs.

Overlay Modules are loaded and amended by the utility program OP. A systems utility program (PLM) is available to allocate disc space to a new overlay module, and to maintain the library index and program directories.

When an overlay module is fetched into core by program name, an option of the system may automatically resolve a block of offset addresses placed at the beginning of the module. (The library index contains flags indicating the modules to which the option applies.)

Direct Access

All tasks may transfer data to and from disc held files. Programs refer to files by a two-digit octal number, the File Identifier. Identifiers 00 to 10 are reserved for Systems Files, 11 to 77 for Applications Files.

The association between File Identifier and Data-Set on disc is, in principle, flexible. But as yet LOS does not provide file opening and closing facilities; therefore all files should be regarded as pre-opened by the system. This should be adequate where the machine is dedicated to the application.

Records within a Direct Access data set are identified by their relative record number; the first record number one. Records may be buffered several per sector (128 words), but records exceeding one sector cannot be contained within the task’s Master Buffer, which is the implied transfer buffer for the FETCH, REWRITE and OVERWRITE subroutines. The system provides automatic de-buffering and re-buffering facilities.

Disc Updates

The possibility of a simultaneous update of the same record (or, more precisely, the same sector) by two different tasks is eliminated within LOS because no mechanism is provided to write a record to disc without having first read that record and locked the entire disc transfer system. The lock must still be in force when the record is rewritten or overwritten.

Where it is necessary to prevent updating an entire file for a period of time this must be recognised and dealt with by the programs concerned.

To initiate LOS proceed as follows.

1. Place any exchangeable disc (MASTER or SECURITY) holding a copy of the Operating System and holding the configuration Table appropriate to the installation onto Device 70 Drive 0.

2. Insert key, turn to MANUAL.

3. Press RESET. Wait for +15 volt light to come on.

4. Load the five-word bootstrap at 000100:

004400

010670

010570

011470

020104

Note: This bootstrap is permanently core resident whilst LOS is running.

5. Load MA and PC with 000100

6. CLEAR THE SWITCHES.

7. When the disc is READY, press CONTINUE. Turn key to NORMAL and remove.

Initiation takes approximately one second. Provided the disc holding the System Control Record and the disc holding the program library are on-line, all I/O stations will bleep and display PROGRAM?; otherwise all I/O stations will flash a message indicating which disc is required.

 

Start Sector (octal)	Number of Sectors (decimal)	Description
000000	1	BOOTSTRAP
000001	31	not used (to avoid writing data on the vulnerable outermost track of early cartridges)
000040	1	DISC LABEL
000041	1	MASTER BOOTSTRAP (backup copy of Sector 000000)
000042	3	CONFIGURATION TABLE
000045	3
000050	95	OPERATING SYSTEM (12K)
000207		Start of User Datasets
014537		D818 cartridge max (6,496 sectors)
031277		D1600 cartridge max (12,992 sectors)

CONFIGURATION TABLE