Page 336 - DCAP103_Principle of operating system

Unit 11: Operating System Structure

The job of the HAL is to present the rest of the operating system with abstract hardware devices, Notes
in particular, devoid of the warts and idiosyncracies with which real hardware is so richly
endowed. These devices are presented in the form of machine-independent services (procedure
calls and macros) that the rest of the operating system and the drivers can use. By using the
HAL services (which are identical on all Windows 2000 systems, no matter what the hardware
is) and not addressing the hardware directly, drivers and the kernel require fewer changes
when being ported to new hardware. Porting the HAL itself is straightforward because all
the machine-dependent codes are concentrated in one place and the goals of the port are well
defined, namely, implement all of the HAL services.

The services chosen for inclusion in the HAL are those that relate to the chip set on the parentboard
and which vary from machine to machine within reasonably predictable limits. In other words,
it is designed to hide the differences between one vendor’s parentboard and another one’s, but
not the differences between a Pentium and an Alpha. The HAL services include access to the
device registers, bus-independent device addressing, interrupt handling and resetting, DMA
transfers, control of the timers and real-time clock, low-level spin locks and multiprocessor
synchronization, interfacing with the BIOS and its CMOS configuration memory. The HAL does
not provide abstractions or services for specific I/O devices such as keyboards, mice, or disks
or for the memory management unit.

As an example of what the hardware abstraction layer does, consider the issue of memory-
mapped I/O versus 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 machine that did it the other way, the hardware
abstraction layer offers three procedures 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 implemented.
Drivers often 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 (ISA, PCI, SCSI, USB, 1394, etc.), it can happen that two or more devices have the same bus
address, so some way is needed to distinguish them. The HAL provides a service for identifying
devices by mapping bus-relative device addresses onto system-wide logical addresses. In this
way, drivers are not required to keep track of which device is on which bus. These logical
addresses are analogous to the handles the operating system gives user programs to refer to
files and other system resources. This mechanism also shields higher layers from properties 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 the interrupts in a system-wide way and also provides services to allow drivers
to attach interrupt service routines to interrupts in a portable way, without having to know
anything about which interrupt vector is for which bus. Interrupt request level management is
also handled in the HAL.

Another HAL service is setting up and managing DMA transfers in a device-independent way.
Both the system—wide DMA engine and DMA engines on specific I/O cards can be handled.

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