Incremental simulation using previous simulation results and knowledge of changes to simulation model to achieve fast simulation time

A method for designing and simulating a circuit comprising the steps of:

• a. providing a first circuit model describing the circuit as a network of blocks, including blocks for receiving and processing input signals to produce output signals, wherein the output signals produced by some of the blocks are supplied as the input signals to others of the blocks;
• b. performing a first simulation of behavior of the circuit described by the first circuit model during successive first simulation cycles, the first simulation producing dump file data indicating states of block input and output signals between successive first simulation cycles;
• c. modifying the first circuit model to change its description of at least one of the circuit blocks while descriptions of others of the blocks remain unchanged, thereby to produce a second circuit model;
• d. processing the first and second circuit models to classify each block of the second circuit model having a changed description as a variant block, to classify each block having an unchanged description that is within a fan-out cone of a variant block as a conditional block, and to classify each other unchanged block as an invariant block;
• and
• e. performing a second simulation of the behavior of the circuit described by the fist circuit model in response to the input signal during each of a plurality of successive second simulation cycles, wherein a separate decision made relative to each conditional block immediately preceding each second simulation cycle determines whether a state of each output signal of the conditional block at the end of the second simulation cycle is thereafter determined from dump file data produced by the first simulation or determined by simulating the behavior of the conditional block during the second simulation cycle.