Embedded Systems/Protected Mode and Real Mode

x86 Processor Modes edit

Real mode and protected mode are two operating modes of the Intel x86 processor. However, there are certain other modes as well.

V86 (Virtual 86 mode)
This is the mode in which DOS applications run on Windows machine. This was done mainly to maintain compatibility with older DOS applications.
SMM (System Management Mode)
This mode was introduced, as the name suggests, for managing the system transparently without applications or OS getting the hint of it. Its primarily meant to be used by the BIOS code.
Big Real Mode
Now this mode is more like Real mode, but in this we can access full 4Gb address space of the 32-bit processor.
However in this page, we will be focussing mainly on Real mode and Protected mode only.

Real Mode edit

This is the only mode which was supported by the 8086 (the very first processor of the x86 series). The 8086 had 20 address lines, so it was capable of addressing "2 raised to the power 20" i.e. 1 MB of memory.

Protected Mode edit

This is the mode used most commonly by modern 32-bit operating systems.

Entering Protected Mode edit

For instructions to enter protected mode, see: X86 Assembly/Protected Mode.