Method and system for upgrading fault-tolerant systems

A method for synchronizing an off-line CPU to an on-line CPU, the method including the steps of:

• A. providing component state information from an on-line CPU to an I/O subsystem;
• B. providing the component state information from the I/O subsystem to an off-line CPU and directing the off-line CPU to set corresponding components to the same states;
• C. directing the off-line CPU to a known state;
• D. holding the off-line CPU in the known state until the on-line CPU is in the same known state;
• E. allowing the off-line CPU to go on-line and operate in lock-step with the on-line CPU in a next operating cycle.

A fault tolerant computer system including:

• A. one or more CPUs;
• B. an I/O subsystem that communicates with the CPUs, the I/O subsystem including
• i. registers for holding CPU status information,
• ii. comparison logic for comparing output streams from a plurality of on-line CPUs,
• iii. one or more selectors for selecting a comparison method based on the number of CPUs on-line at a given time, and
• iv. a voter delay buffer for holding the output streams from two on-line CPUs until the system can determine which of the two CPUs is malfunctioning,
• the one or more selectors selecting the voter delay buffer to hold the output streams when two CPUs are on-line and the output streams from the two on-line CPUs do not agree.

A method for selecting a valid output stream from one or more CPUs, the method including the steps of

• A. determining how many CPUs are on-line at a given time;
• B. selecting a comparison method based on the number of CPUs on-line;
• C. if three or more CPUs are on-line selecting one of the data streams that agrees with a majority of the other data steams;
• D. if two CPUs are on-line and the corresponding data streams do not agree,
• i. sending the data streams to a voter delay buffer,
• ii. holding the data streams in the buffer until one of the CPUs goes off-line, and
• iii. selecting the output stream that corresponds to the remaining on-line CPU.