This document discusses real addressing mode and virtual addressing mode in computer systems. Real addressing mode provides a 20-bit segmented memory address space of 1 MB and direct access to memory, I/O, and hardware. It was the original mode for x86 CPUs. Virtual addressing mode allows real mode applications to run on protected mode operating systems through hardware virtualization. It uses a segmentation scheme like real mode but subjects addresses to protected mode paging. A virtual 8086 mode monitor manages access between the real mode program and system resources.

1. SUBMITED BY:-
ARIJIT CHAKRABORTY
BETI1EC16002
ELECTRONIC & COMMUNICATION
III year, V sem
SUBMITED TO:-
DR. SHYAM AKASHE
PROFESSOR
ELECTRONIC &
COMMUNICATION

2. TABLE OF CONTAINS:-
 What is Real Addressing Mode
 What is Virtual Addressing Mode

3. REAL ADDRESSING MODE:-
o Real mode, also called real address mode, is an operating mode of
all x86-compatible CPUs. Real mode is characterized by a 20-
bit segmented memory address space (giving exactly 1 MB of
addressable memory) and unlimited direct software access to all
addressable memory, I/O addresses and peripheral hardware. Real
mode provides no support for memory protection, multitasking, or
code privilege levels.

4. o Before the release of the 80286, which introduced protected mode, real
mode was the only available mode for x86 CPUs; and for backward
compatibility, all x86 CPUs start in real mode when reset, though it is
possible to emulate real mode on other systems when starting on other
modes.
CONT.….

5. CONT.….
o The 80286 chip was introduced on 1 February 1982 and was made to
start in 'real mode' – that is, in a mode which turned off the new memory
protection features, so that it could run operating systems written for
the 8086 and the 8088. As of 2018, current x86 CPUs (including x86-
64 CPUs) are able to boot real mode operating systems and can run
software written for almost any previous x86 chip without emulation or
virtualization.
o The PC BIOS which IBM introduced operates in real mode, as do
the DOS operating systems (MS-DOS, DR-DOS, etc.). Early versions
of Microsoft Windows ran in real mode.

6. VIRTUAL ADDRESSING MODE:-
o In the 80386 microprocessor and later, virtual addressing mode (also
called virtual real mode, virtual 8086 mode, V86-mode or VM86)
allows the execution of real mode applications that are incapable of
running directly in protected mode while the processor is running a
protected mode operating system. It is a hardware
virtualization technique that allowed multiple 8086 processors to be
emulated by the 386 chip; it emerged from the painful experiences with
the 80286 protected mode, which by itself was not suitable to run
concurrent real mode applications well.

7. CONT.….
o VM86 mode uses a segmentation scheme identical to that of real mode
(for compatibility reasons) which creates 20-bit linear addresses in the
same manner as 20-bit physical addresses are created in real mode, but
are subject to protected mode's memory paging mechanism.
o To use virtual 8086 mode, an operating system sets up a virtual 8086
mode monitor, which is a program that manages the real-mode program
and emulates or filters access to system hardware and software
resources. The monitor must run at privilege level 0 and in protected
mode. Only the 8086 program runs in VM86 mode and at privilege level
3.

8. o When the real-mode program attempts to do things like access certain
I/O ports to use hardware devices or access certain regions in its
memory space, the CPU traps these events and calls the V86 monitor,
which examines what the real mode program is trying to do and either
acts as a proxy to interface with the hardware, emulates the intended
function the real-mode program was trying to access, or terminates the
real-mode program if it is trying to do something that cannot either be
allowed or be adequately supported (such as reboot the machine, set a
video display into a mode that is not supported by the hardware and is
not emulated, or write over operating system code).
CONT.….