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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
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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 911
880
CASE STUDY 2: WINDOWS 8
CHAP. 11
The Hardware Abstraction Layer
One goal of Windows is to make the system portable across hardware plat-
forms. Ideally, to bring up an operating system on a new type of computer system
it should be possible to just recompile the operating system on the new platform.
Unfortunately, it is not this simple.
While many of the components in some layers
of the operating system can be largely portable (because they mostly deal with in-
ternal data structures and abstractions that support the programming model), other
layers must deal with device registers, interrupts, DMA, and other hardware fea-
tures that differ significantly from machine to machine.
Most of the source code for the NTOS kernel is written in C rather than assem-
bly language (only 2% is assembly on x86, and less than 1% on x64).
However, all
this C code cannot just be scooped up from an x86 system, plopped down on, say,
an ARM system, recompiled, and rebooted owing to the many hardware differ-
ences between processor architectures that have nothing to do with the different in-
struction sets and which cannot be hidden by the compiler.
Languages like C make
it difficult to abstract away some hardware data structures and parameters, such as
the format of page-table entries and the physical memory page sizes and word
length, without severe performance penalties.
All of these, as well as a slew of
hardware-specific optimizations, would have to be manually ported even though
they are not written in assembly code.
Hardware details about how memory is organized on large servers, or what
hardware synchronization primitives are available, can also have a big impact on
higher levels of the system.
For example, NT’s virtual memory manager and the
kernel layer are aware of hardware details related to cache and memory locality.
Throughout the system NT uses
compare&swap
synchronization primitives, and it
would be difficult to port to a system that does not have them. Finally, there are
many dependencies in the system on the ordering of bytes within words. On all the
systems NT has ever been ported to, the hardware was set to little-endian mode.
Besides these larger issues of portability, there are also minor ones even be-
tween different parentboards from different manufacturers. Differences in CPU
versions affect how synchronization primitives like spin-locks are implemented.
There are several families of support chips that create differences in how hardware
interrupts are prioritized, how I/O device registers are accessed, management of
DMA transfers, control of the timers and real-time clock, multiprocessor synchron-
ization, working with firmware facilities such as
ACPI
(
Advanced Configuration
and Power Interface
), and so on.
Microsoft made a serious attempt to hide these
types of machine dependencies in a thin layer at the bottom called the HAL, as
mentioned earlier.
The job of the HAL is to present the rest of the operating sys-
tem with abstract hardware that hides the specific details of processor version, sup-
port chipset, and other configuration variations. These HAL abstractions are pres-
ented in the form of machine-independent services (procedure calls and macros)
that NTOS and the drivers can use.
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