Page 337 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes Devices are referred by their logical addresses. The HAL also implements software scatter/
gather (writing or reading from noncontiguous blocks of physical memory).
The HAL also manages clocks and timers in a portable way. Time is kept track of in units of
100 nsec starting at 1 January 1901, which is far more precise than MS-DOS’s keeping track of
time in units of 2 sec since 1 January 1980 and provides support for the many computer-related
activities in the 17th, 18th, and 19th centuries. The time services decouple the drivers from the
actual frequencies at which the clocks run.

Kernel components sometimes need to synchronize at a very low level, especially to prevent
race conditions in multiprocessor systems. The HAL provides some primitives to manage this
synchronization, such as spin locks, in which one CPU simply waits for a resource held by
another CPU to be released, particularly in situations where the resource is typically only held
for a few machine instructions.

Finally, after the system has been booted, the HAL talks to the BIOS and inspects the CMOS
configuration memory, if any, to find out which buses and I/O devices the system contains and
how they have been configured. This information is then put into the registry so other system
components can look it up without having to understand how the BIOS or configuration memory
work. A summary of some of the things the HAL does is given in Figure 11.2.
Since the HAL is highly-machine dependent, it must match the system it is installed on perfectly,
so a variety of HALs are provided on the Windows 2000 CD-ROM. At system installation time,
the appropriate one is selected and copied to the system directory \winNT\system32 on the
hard disk as hal.dll. All subsequent boots use this version of the HAL. Removing this file will
make the system unbootable.
Although the HAL is reasonably efficient, for multimedia applications, it may not be fast enough.
For this reason, Microsoft also produced a software package called DirectX, which augments
the HAL with additional procedures and allows user processes much more direct access to the
hardware. DirectX is somewhat specialized, so we will not discuss it further in this unit.

Figure 11.2: Some of the Hardware Functions the HAL Manages

Hardware refers to a physical piece of a computer. This could be a hard drive,
monitor, memory chip, or CPU. The key idea is that the item is something
you can touch.

330 LOVELY PROFESSIONAL UNIVERSITY