When booting from disk the PROM finds the system boot block at sectors 1-15 of the boot partition. The bootblock program, bootblk, reads in the boot program, /boot (SunOS 4.X). For SunOS 5.X bootblk, the generic part of the boot program, reads in the file-specific part of the boot program: /ufsboot for diskfull boots, or /inetboot for diskless boots; these use the device driver on the PROM or on the SBus F-code PROM, so the boot block no longer contains the actual location of the disk block where the boot program resides. Under SunOS 5.X bootblk can read the ufs file system to locate the boot program, /ufsboot.
The boot program then locates the kernel and passes control to it.