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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 912 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 912
SEC. 11.3
SYSTEM STRUCTURE
881
By using the HAL services and not addressing the hardware directly, drivers
and the kernel require fewer changes when being ported to new processors—and in
most cases can run unmodified on systems with the same processor architecture,
despite differences in versions and support chips.
The HAL does not provide abstractions or services for specific I/O devices
such as keyboards, mice, and disks or for the memory management unit.
These
facilities are spread throughout the kernel-mode components, and without the HAL
the amount of code that would have to be modified when porting would be sub-
stantial, even when the actual hardware differences were small.
Porting the HAL
itself is straightforward because all the machine-dependent code is concentrated in
one place and the goals of the port are well defined: implement all of the HAL ser-
vices. For many releases Microsoft supported a
HAL Development Kit
allowing
system manufacturers to build their own HAL, which would allow other kernel
components to work on new systems without modification, provided that the hard-
ware changes were not too great.
As an example of what the hardware abstraction layer does, consider the issue
of memory-mapped I/O vs. I/O ports.
Some machines have one and some have the
other.
How should a driver be programmed: to use memory-mapped I/O or not?
Rather than forcing a choice, which would make the driver not portable to a ma-
chine that did it the other way, the hardware abstraction layer offers three proce-
dures for driver writers to use for reading the device registers and another three for
writing them:
uc = READ
PORT
UCHAR(port);
WRITE
PORT
UCHAR(port, uc);
us = READ
PORT
USHORT(port);
WRITE
PORT
USHORT(port, us);
ul = READ
PORT
ULONG(port);
WRITE
PORT
LONG(port, ul);
These procedures read and write unsigned 8-, 16-, and 32-bit integers, respectively,
to the specified port.
It is up to the hardware abstraction layer to decide whether
memory-mapped I/O is needed here.
In this way, a driver can be moved without
modification between machines that differ in the way the device registers are im-
plemented.
Drivers frequently need to access specific I/O devices for various purposes.
At
the hardware level, a device has one or more addresses on a certain bus. Since
modern computers often have multiple buses (PCI, PCIe, USB, IEEE 1394, etc.), it
can happen that more than one device may have the same address on different
buses, so some way is needed to distinguish them.
The HAL provides a service for
identifying devices by mapping bus-relative device addresses onto systemwide log-
ical addresses.
In this way, drivers do not have to keep track of which device is
connected to which bus. This mechanism also shields higher layers from proper-
ties of alternative bus structures and addressing conventions.
Interrupts have a similar problem—they are also bus dependent.
Here, too, the
HAL provides services to name interrupts in a systemwide way and also provides
ways to allow drivers to attach interrupt service routines to interrupts in a portable
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