Hardware Reference
In-Depth Information
also handles process scheduling, which is based on priorities. Signals, which are a
form of (asynchronous) software interrupt, are also managed here. Finally, memo-
ry management is done here as well. Most
UNIX
systems support demand-paged
virtual memory, sometimes with a few extra features, such as the ability of multiple
processes to share common regions of address space.
From its inception,
UNIX
has tried to be a small system, in order to enhance
reliability and performance. The first versions of
UNIX
were entirely text based,
using terminals that could display 24 or 25 lines of 80 ASCII characters. The user
interface was handled by a user-level program called the
shell
, which offered a
command-line interface. Since the shell was not part of the kernel, adding new
shells to
UNIX
was easy, and over time a number of increasingly sophisticated ones
were invented.
Later on, when graphics terminals came into existence, a windowing system
for
UNIX
, called
X Windows
, was developed at M.I.T. Still later, a full-fledged
GUI
(
Graphical User Interface
), called
Motif
, was put on top of X Windows.
These GUIs eventually developed into full-blown desktop environments with beau-
tifully rendered window management, productivity tools, and utilities. Examples of
these desktop environments include GNOME and KDE. In keeping with the
UNIX
philosophy of having a small kernel, nearly all the code of X Windows and its
accompanying GUIs run in user mode, outside the kernel.
Windows 7
When the original IBM PC was launched in 1981, it came equipped with a
16-bit real-mode, single-user, command-line-oriented operating system called
MS-
DOS
1.0 This operating system consisted of 8 KB of memory-resident code. Two
years later, a much more powerful 24-KB system,
MS-DOS
2.0, appeared. It con-
tained a command-line processor (shell), with a number of features borrowed from
UNIX
. When IBM released the 286-based PC/AT in 1984, it came equipped with
MS-DOS
3.0, by now 36 KB. Over the years,
MS-DOS
continued to acquire new
features, but it was still a command-line-oriented system.
Inspired by the success of the Apple Macintosh, Microsoft decided to give
MS-
DOS
a graphical user interface that it called
Windows
. The first three versions of
Windows, culminating in Windows 3.x, were not true operating systems but graph-
ical user interfaces on top of
MS-DOS
, which was still in control of the machine.
All programs ran in the same address space and a bug in any one of them could
bring the whole system to a grinding halt.
The release of Windows 95 in 1995 still did not eliminate
MS-DOS
, although it
introduced a new version, 7.0. Together, Windows 95 and
MS-DOS 7.0
contained
most of the features of a full-blown operating system, including virtual memory,
process management, and multiprogramming. However, Windows 95 was not a
full 32-bit program. It contained large chunks of old 16-bit code (as well as some
32-bit code) and still used the
MS-DOS
file system, with nearly all its limitations.
