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System for bumpless changeover between active units and backup units by establishing rollback points and logging write and read operations

  • US 4,941,087 A
  • Filed: 09/18/1987
  • Issued: 07/10/1990
  • Est. Priority Date: 09/19/1986
  • Status: Expired due to Term
First Claim
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1. In computer equipment, a method of bumpless changeover of function from active units to backup units in a central unit, the central unit having at least one active processor, at least one active memory, at least one redundant unit as a backup unit for at least one of said active units, and members for effecting the changeover of the function from an active unit to a backup unit, the processor handling interrupts with the aid of interrupt routines of which at least some are hardware initiated, each of the interrupt routines having one entry point, where the execution of the routine is started, and one interrupt acknowledgement point, the central unit effecting write and read operations against peripheral units included in the computer equipment, wherein the method comprises the following steps:

  • (a) establishing successive rollback points by the active processor during program execution, in at least all hardware initiated interrupt routines between the entry point and the interrupt acknowledgement point, by storing status of the active processor and at least contents of those parts of the active memory which have been modified by the active processor;

    (b) logging said write and read operations against the peripheral units after each rollback point;

    (c) after changeover of the function from an active unit to a backup unit, returning the program execution to the last established rollback point by restoring the contents of the active memory and the status of the active processor to the respective contents and status established by a corresponding unit previously active at said rollback point, whereafter resuming the program execution without carrying out any write and read operations against the peripheral units;

    (d) during the following program execution, fetching the results of the read operations already performed against the peripheral units from the log until all logged operations have been repeated; and

    (e) continuing the program execution while carrying out operations against the peripheral units.

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