4.2.5 Precision of the scheduler clock used in ns

Precision of the scheduler clock can be defined as the smallest time-scale of the simulator that can be correctly represented. The clock variable for ns is represented by a double. As per the IEEE std for floating numbers, a double, consisting of 64 bits must allocate the following bits between its sign, exponent and mantissa fields.

sign     exponent      mantissa
 1 bit   11 bits        52 bits

Any floating number can be represented in the form ($X * 2^n$) where X is the mantissa and n is the exponent. Thus the precision of timeclock in ns can be defined as ($1/2^(52)$). As simulation runs for longer times the number of remaining bits to represent the time educes thus reducing the accuracy. Given 52 bits we can safely say time upto around ($2^(40)$) can be represented with considerable accuracy. Anything greater than that might not be very accurate as you have remaining 12 bits to represent the time change. However ($2^(40)$) is a very large number and we donot anticipate any problem regarding precision of time in ns.