During configuration, the dedicated Global Set/Reset (GSR) signal is asserted. The GSR signal is deasserted upon completion of device configuration. All the flip-flops and latches receive this reset, and are set or reset depending on how the registers are defined.
Although you can access the GSR net after configuration, avoid use of the GSR circuitry in place of a manual reset. This is because the FPGA devices offer high-speed backbone routing for high fanout signals such as a system reset. This backbone route is faster than the dedicated GSR circuitry, and is easier to analyze than the dedicated global routing that transports the GSR signal.
In post-synthesis and post-implementation simulations, the GSR signal is automatically asserted for the first 100 ns to simulate the reset that occurs after configuration.
A GSR pulse can optionally be supplied in pre-synthesis functional simulations, but is not necessary if the design has a local reset that resets all registers.