Page 335 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Below we will first examine the various components of the system starting at the bottom and
working our way up.

Figure 11.1: The structure of Windows 2000 (slightly simplified). The shaded
area is the executive. The boxes indicated by D are device drivers.
The service processes are system daemons.

POSIX program Win32 program OS/2 program
Service
process mode
POSIX subsystem Win32 subsystem OS/2 subsystem
User
System interface (NT DLLDLL)

System services

I/O mgr
Object Process Memory Security Cache PnP Power Config LPC Win32 mode
mgr mgr mgr mgr mgr mgr mgr mgr mgr GDI
File sys
Video Kernel
Kernel
D driver
Hardware Abstraction layer (HAL)
Hardware

The software component of a computer system that is responsible for the
management and coordination of activities and the sharing of the resources
of the computer. The operating system (OS) acts as a host for application
programs that are run on the machine.

11.1.1 Hardware Abstraction Layer

One of the goals of Windows 2000 (and Windows NT before it) was to make the operating system
portable across platforms. Ideally, when a new machine comes along, it should be possible to just
recompile the operating system with the new machine’s compiler and have it run for the first
time. Unfortunately, the upper layers of the operating system can be made completely portable
(because they mostly deal with internal data structures), while the lower layers deal with device
registers, interrupts, DMA, and other hardware features that differ appreciably from machine to
machine. Even though most of the low-level code is written in C, it cannot just be scooped up
from a Pentium, plopped down on, say, an Alpha, recompiled, and rebooted due to the many
small hardware differences between the Pentium and the Alpha that have nothing to do with
the different instruction sets and which cannot be hidden by the compiler.
Fully aware of this problem, Microsoft made a serious attempt to hide many of the machine
dependencies in a thin layer at the bottom called the HAL (Hardware Abstraction Layer). (The
name HAL was inspired by the computer HAL in the late Stanley Kubrick’s movie 2001: A
Space Odyssey. Rumor has it that Kubrick chose the name “HAL” by taking the name of the
then-dominant computer company—IBM—and subtracting 1 from each letter.)

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