Question

elaborate real and protected modes of cpu

Answer 1

x86 CPUs boot up in a "real mode" - basically, 16-bit mode where the CPU can only see the first 1MB of RAM. One of the first tasks of a BIOS bootloader (or UEFI may do this directly) is to switch the CPU into "protected" mode. Protected memory is available in this mode, and the CPU has privilege levels in this mode - generally "kernel" and "user."