The original version of LOS could, optionally, duplicate every write to disc onto a second cartridge. The purpose was to ensure a system could continue running (as described below), should one drive fail, by "mirroring" its data in real time onto second drive.

However, this only protected against mechanical drive failure (which proved to be inadequate) so the additional capacity provided by the second drive was soon reassigned for extra data storage.

On-Line Security

The two disc drives (units 70 and 71) provide double the on-line disc capacity required during normal operating. To enable operations to continue should one of the drives fail, all data is duplicated on each drive.

The fixed cartridges are Master Discs, labeled 027 and 017. Under normal circumstances programs read from and write to the master discs. The operating system intercepts all writes and duplicates them onto the corresponding Slave disc.

Disc 010 is the Slave of Disc 017, Disc 020 is the Slave of 027.

Note: The Slave cartridge will normally be loaded onto the opposite drive to its Master, but this is not an essential requirement.


Failure is indicated either by the permanent absence of the READY light or by a STATUS message flashed to all VDU's at approximately 20-second intervals.

To withdraw discs from the system:

  1. At the CPU control panel, check that all numbered switches are DOWN.
  2. RAISE switches 12, 11, 8, 7, 4 and 3. Processor will respond by HALTING (green run light goes out) with lights 12, 11, 8, 7, 4 and 3 lit.
  3. Determine the disc number(s) of the cartridge(s) to be withdrawn (refer to the diagram above to relate UNIT field to disc number if a STATUS message has been issued).
    Note: Only one cartridge of a MASTER/SLAVE pair may be withdrawn and only if neither is already withdrawn.
  4. Put all numbered switches DOWN.
    • To withdraw DISC 17 (Master) RAISE Switch 16
    • To withdraw DISC 10 (Slave) RAISE Switch 15
    • To withdraw DISC 27 (Master) RAISE Switch 14
    • To withdraw DISC 20 (Slave) RAISE Switch 13
  5. Double check that the switches correctly identify the cartridge(s) to be withdrawn. (A cartridge can be reinstated only by program MEND.)
  6. RAISE Switch 17.
  7. Insert Key; turn to MANUAL; press CONTINUE; remove key.

System resumes with the indicated cartridges withdrawn. If a MASTER has been withdrawn, all read/writes are directed to the SLAVE. If a SLAVE has been withdrawn, it is ignored by the system except if a write was pending when withdrawn it will remain pending (under these circumstances the Slave should be taken off-line to prevent the system from continuing to attempt to complete the write) (the message that the issuing task is still awaiting the Slave may be ignored until repairs have been carried out).

Operator Action Pending Repairs

  1. All programs with the exception of DISC SECURITY operate as normal.
  3. The time and particulars of all actions taken should be recorded in the logbook.
  4. Engineers on the scene should be advised that they must not interfere with the running of the Central Processor. The disc drives are designed to facilitate repairs off-line; however, if the engineer is of the opinion that an action (such as switching in or out) may cause interference, the processor may be temporarily “frozen” as follows:
    • Key in MANUAL;
    • Lower CONT.INT Switch (processor halts with lights 5 and 1 lit having invoked the MAINS FAIL routine);
    • Raise CONT. INT Switch;
    • Carry out operation;
    • Press RESET;
    • Wait for +15V light;
    • Press CONTINUE.
  5. STATUS 000003 (WRITE CHECK). This status can be caused by attempting to write to a cartridge which is protected by the WRITE PROTECT button. It is cleared simply by pressing this button (light goes out).
  6. STATUS 000021 (CHECKWORD ERROR). This status only occurs on Read and usually indicates that the data was incorrectly written to the cartridge. This is unlikely to clear however often it is re-read. The cartridge should be withdrawn and regenerated by program MEND at the earliest opportunity. This procedure can be carried out without the need to call an engineer.

Checkword Errors are usually caused by processor or core faults.

DISC SECURITY with both SLAVES withdrawn.

Use programs DS10 and DS20 in parallel as usual.

DISC SECURITY with both MASTERS withdrawn.

Use programs DS10 and DS20 one at a time, loading the security cartridge onto the opposite drive to usual and keeping the appropriate slave online.

DISC SECURITY with UNIT 70 (discs 10 and 27) withdrawn.

Use program DS10 only.

At the prompt PHASE 1 ONLY? press REJECT.

Load discs as and when called for. Allow 50 minutes for the whole operation.

Should it become necessary to recover the system from this security, security disc 11 (or 12) must be placed on Unit 70 first. (This cannot be carried out until Unit 70 is repaired, unless the engineer reverses the interface connections.)

DISC SECURITY with UNIT 71 (discs 20 and 17) withdrawn.

Use program DS20 only.

At the prompt PHASE 1 ONLY? press REJECT.

Load discs as and when called for. Allow 50 minutes for the whole operation.

Should it become necessary to recover the system from this security, security disc 21 (or 22) must be placed on Unit 70 first.

Regeneration Of On-Line Security After Repairs

1 Load discs 10 and 20 onto their usual drives.
2 Check that all VDU’s and all printers are idle.
3 Enter program MEND at any VDU.
4 The VDU will display the current on-line security situation.
5PROCESS?Either ACCEPT to go to step 6,
or REJECT to return to step 4.
6PASSWORDEnter Password 1.
7 Regeneration of the withdrawn cartridges now proceeds automatically. No VDU or printer may carry out any read/write to any disc whilst regeneration is in progress. Any that attempts to do so will HALT AT 03/0451.
8 The message REGENERATION COMPLETED is flashed to all VDU’s upon completion of MEND.

Should it become necessary to re-bootstrap the system whilst Master Disc 27 is withdrawn, the correct procedure is to return to the latest security. If, nevertheless, an attempt is made to proceed, the bootstrap will read the System Control Record from Disc 27. Unfortunately, this record will be out of date. The appropriate File Control Block should be temporarily modified to read from Disc 20.