Intel® Processors since the Intel386™ Processor can run one of three modes:
- Real mode
- Protected mode
- 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, it starts in real mode and operates like a 8086 processor. The processor can see up to 1 MB of RAM.
The native mode for the processor is the Protected mode. The processor switches into Protected mode while it loads Windows* or other advanced operating system. In protected mode, the processor uses segmented (non-linear) addressing, as opposed to linear addressing.
Segmented addressing means that memory (physical 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, making 0FFFFH the maximum memory location (0FFFFH = 64K).