Too Long; Didn't Read
In x86 CPU starts running in real mode which you probably don't want to use and will either want to switch to 32bit mode with paging enabled or 64 bit mode (aka long mode) We need to change mode of execution somehow. To do that we need to configure CPU and execute certain instructions. In protected mode we use the same form A:B (where A ∈ [CS, SS, DS, ES, FS, GS] is used. This logical address is then translated into physical address by using A * 0x10 + B.