Specifications

Computer Architecture and Maintenance (G-Scheme-2014)

It’s also important to note that some 16-bit (DOS and Windows 3.x) applications

misbehave in a 32-bit environment, which means they do things that even virtual real

mode does not support. Diagnostics software is a perfect example of this. Such

software does not run properly in a real mode (virtual real) window under Windows.

In that case, you can still run your modern system in the original no-frills real mode by

booting to a DOS or Windows 9x/Me startup floppy or by using a self-booting CD or

DVD that contains the diagnostic software.

Although 16-bit DOS and “standard” DOS applications use real mode, special

programs are available that “extend” DOS and allow access to extended memory (over

1 MB). These are sometimes called DOS extenders and usually are included as part of

any DOS or Windows 3.x software that uses them. The protocol that describes how to

make DOS work in protected mode is called DOS protected mode interface (DPMI).

Windows 3.x used DPMI to access extended memory for use with Windows 3.x

applications. It allowed these programs to use more memory even though they were

still 16-bit programs. DOS extenders are especially popular in DOS games because they

enable them to access much more of the system memory than the standard 1 MB that

most real mode programs can address. These DOS extenders work by switching the

processor in and out of real mode. In the case of those that run under Windows, they

use the DPMI interface built into Windows, enabling them to share a portion of the

system’s extended memory.

Another exception in real mode is that the first 64 KB of extended memory is

actually accessible to the PC in real mode, despite the fact that it’s not supposed to be

possible. This is the result of a bug in the original IBM AT with respect to the

memory address line, known as A20 (A0 is the first address line). By manipulating

the A20 line, real mode software can gain access to the first 64 KB of extended memory

—the first 64 KB of memory past the first megabyte. This area of memory is called

the high memory area (HMA).

IA-32e 64-Bit Extension Mode (x64, AMD64, x86

64-bit extension mode is an enhancement to the IA-32 architecture originally designed

by AMD and later adopted by Intel.

In 2003, AMD introduced the first 64-bit processor for x86-compatible desktop

computers—the Athlon 64—followed by its first 64-bit server processor, the Opteron.

In 2004, Intel introduced a series of 64-bit-enabled versions of its Pentium 4 desktop

processor. The years that followed saw both companies introducing more and more

processors with 64-bit capabilities.

Prepared By – Prof. Manoj.kavedia (9860174297 – 9324258878 ) (www.kavediasir.yolasite.com)