Real mode

Real mode is a processor memory addressing status where a pointer points to a exact place in memory with out redirection, either through Virutal memory, Task virtualization, or relocateable task execution.

Real Mode was coined for the ability of processors to change where a program assumed a memory address was, so that it could be either relocated able, or virtualized and moved.

on Intel Processors, the first 16/8 bit microprocessors could only address 1Mb of memory, and later Intel reffered to these processors as real mode only. LIM memory was developed to swap pages over the bus into the 1Mb address space.

Beginning with the 80286 and later members of the Intel x86 line, refers to a mode of the CPU in which the extended architectures of the later chips, including hardware support for virtual memory, is not enabled. So the archictecutures had developing memory modes:

The Intel 8088, and 8086, had a real mode memory of 1Mb, ( 16-bit addresses ). Since 384K was reserved for I/O pages, it created a real mode barrier of 640Kb of memory.

The Intel 80286, had a real mode of 1Mb and a protected mode of 16Mb, which memory address could not only be relocated, but they could be swapped out to disk. ( Virtual memory ). Switching to protected mode was relativity simple, but switching back to real mode, required a processor reset.

The Intel 80386, had a real mode of 16Mb, and both a protected mode of 16Mb, which memory address could not only be relocated, but also could be swapped out to disk.

The Intel 80386, also had a virtual 8086 mode, where memory address could not only be moved ( relocated ), and swapped to disk ( virtualized ), but with the ability to swap in register files ( a complete set of 8086 registers ), could support a virtual machine environment.

Starting with the Westmere Processor core, not only were memory addresses virtualized, (VT-x) and register files virtualised, but I/O address could be virtualized (VT-d) which is direct I/O. A virtual machine can address Input/Output directly.

These chips power on in real mode, in which they appear to be 8086's; this is for purposes of backward compatibility at the object code level. A small extension to that architecture allows them to switch into protected mode, where all the extended features of the later architectures are available.

Real mode in the later x86 machines is almost entirely the same as the 8086, but there are minor differences (e.g. different Instruction Clock Counts).