Specifications

Computer Architecture and Maintenance (G-Scheme-2014)
programs; it otherwise can use only the 16-bit instructions and access memory within
the same 1 MB memory map of the original 8088. Therefore, if you have a system with
a current 32-bit or 64-bit processor running Windows 3.x or DOS, you are effectively
using only the first megabyte of memory, leaving all the other RAM largely unused!
New OSs and applications that ran in the 32-bit protected mode of the modern
processors were needed.
Note : Windows XP was the first true 32-bit OS that became a true mainstream product,
and that is primarily because Microsoft coerced us in that direction with Windows
9x/Me (which are mixed 16-bit/32-bit systems). Windows 3.x was the last 16-bit OS,
which some did not really consider a complete OS because it ran on top of DOS.
IA-32 Virtual Real Mode
The key to the backward compatibility of the Windows 32-bit environment is the
third mode in the processor : virtual real mode. Virtual real is essentially a virtual real
mode 16-bit environment that runs inside 32-bit protected mode. When you run a DOS
prompt window inside Windows, you have created a virtual real mode session.
Because protected mode enables true multitasking, you can actually have several real
mode sessions running, each with its own software running on a virtual PC. These can
all run simultaneously, even while other 32-bit applications are running.
Note : any program running in a virtual real mode window can access up to only 1MB
of memory, which that program will believe is the first and only megabyte of memory
in the system. In other words, if you run a DOS application in a virtual real window, it
will have a 640 KB limitation on memory usage. That is because there is only 1 MB of
total RAM in a 16-bit environment, and the upper 384KB is reserved for system use.
The virtual real window fully emulates an 8088 environment, so that aside from speed,
the software runs as if it were on an original real mode–only PC. Each virtual machine
gets its own 1 MB address space, an image of the real hardware basic input/output
system (BIOS) routines, and emulation of all other registers and features found in real
mode.
Virtual real mode is used when you use a DOS window to run a DOS or
Windows 3.x 16-bit program. When you start a DOS application, Windows creates a
virtual DOS machine under which it can run.
Note : All Intel and Intel-compatible (such as AMD and VIA/Cyrix) processors power
up in real mode. If you load a 32-bit OS, it automatically switches the processor into 32-
bit mode and takes control from there.
Prepared By – Prof. Manoj.kavedia (9860174297 – 9324258878 ) (www.kavediasir.yolasite.com)
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