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The first timescale indicates that the time units for "module1" are in multiples of 1 ns and are precise to 10 ps. Thus, the smallest timestep for the simulator is 10 ps.
The second timescale is 100 ns/1 ns. Since 1 ns is greater than 10 ps, the smallest timestep remains 10 ps.
The third timescale is 1 ps/100 fs. Since 100 fs is smaller than 10 ps, the smallest simulator timestep now becomes 100 fs.
The `timescale compiler directive is optional. Designs that do not contain this require no modification, and the default of 1 ns/1 ns is used.
However, if the design includes a `timescale compiler directive for any one module definition, then all other module definitions require this as well. If this does not happen, ModelSim Vlog issues this warning.