Intel® processors since the Intel386™ processor can run one of three modes. They are the Real mode, Protected mode and SMM mode. You can also add a fourth mode called Virtual 8088 mode, which is considered a pseudo mode of the protected mode.
When the processor starts booting the computer, the processor starts in real mode where it operates like a 8086 processor that can see up to 1 MB of RAM.
The native mode for the processor is the Protected mode which it will switch into while it loads Windows* or some other advanced operating system. While in protected mode, the processor uses segmented (non-linear) addressing, as opposed to linear addressing.
Segmented addressing means that memory (physical memory and virtual memory) is divided into 64K blocks. This is the maximum value for the Instruction Pointer (IP) register. The IP register works with the Code Segment (CS) register to point to the memory location from where the microprocessor should fetch its next instruction. The IP uses 4 bytes for memory addressing, therefore making 0FFFFH the maximum memory location (0FFFFH = 64K).
This applies to: