bootblk opens the boot device and, using the file system you specify, finds and loads ufsboot.
The boot PROM loads the kernel, /kernel/genunix, after ufsboot is loaded into memory. SunOS release 4 systems used /vmunix; however, in the SunOS release 5.7 software the /kernel directory contains all platform-independent kernel modules, including unix, needed to boot the system.
The kernel, in turn, loads other drivers, such as esp, from the /kernel/drv directory. These drivers had to be built as part of the SunOS release 4 kernel but can be dynamically loaded in SunOS release 5.7 systems when they are needed.