What is First-stage boot loader

The first-stage bootloader (FSBL) is the first boot stage for the HPS. In FPGA Configuration First mode, the SDM extracts and loads the FSBL into the on-chip RAM of the HPS. The SDM releases the HPS from reset after the FPGA has entered user mode. After the HPS exits reset, it uses the FSBL hardware handoff file to setup the clocks, HPS dedicated I/Os, and peripherals. Typically, the FSBL then loads the SSBL into HPS SDRAM and passes the control to the SSBL.