1. A computer implemented simulation method comprising: during a simulation, simulating, using a computer, a first portion of a sequence of software instructions by a first cycle accurate simulation model; and during said simulation, simulating, using the computer, a second portion of said sequence by a second instruction accurate simulation model that provides a faster simulation in comparison to said first cycle accurate simulation model, wherein said simulating said second portion of said sequence is responsive to a run-time simulation model switch between said first cycle accurate simulation model and said second instruction accurate simulation model, and wherein said second simulation model does not simulate said first portion of a sequence of software instructions.