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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 806
SEC. 10.5
INPUT/OUTPUT IN LINUX
775
high-end workstations, with their small and unchanging sets of I/O devices, this
scheme worked well. Basically, a computer center built a kernel containing drivers
for the I/O devices and that was it.
If next year the center bought a new disk, it
relinked the kernel. No big deal.
With the arrival of Linux on the PC platform, suddenly all that changed. The
number of I/O devices available on the PC is orders of magnitude larger than on
any minicomputer.
In addition, although all Linux users have (or can easily get)
the full source code, probably the vast majority would have considerable difficulty
adding a driver, updating all the device-driver related data structures, relinking the
kernel, and then installing it as the bootable system (not to mention dealing with
the aftermath of building a kernel that does not boot).
Linux solved this problem with the concept of
loadable modules
.
These are
chunks of code that can be loaded into the kernel while the system is running. Most
commonly these are character or block device drivers, but they can also be entire
file systems, network protocols, performance monitoring tools, or anything else de-
sired.
When a module is loaded, several things have to happen. First, the module has
to be relocated on the fly, during loading.
Second, the system has to check to see if
the resources the driver needs are available (e.g., interrupt request levels) and if so,
mark them as in use. Third, any interrupt vectors that are needed must be set up.
Fourth, the appropriate driver switch table has to be updated to handle the new
major device type. Finally, the driver is allowed to run to perform any device-spe-
cific initialization it may need. Once all these steps are completed, the driver is
fully installed, the same as any statically installed driver. Other modern UNIX sys-
tems now also support loadable modules.
10.6 THE LINUX FILE SYSTEM
The most visible part of any operating system, including Linux, is the file sys-
tem. In the following sections we will examine the basic ideas behind the Linux
file system, the system calls, and how the file system is implemented. Some of
these ideas derive from MULTICS, and many of them have been copied by MS-
DOS, Windows, and other systems, but others are unique to UNIX-based systems.
The Linux design is especially interesting because it clearly illustrates the principle
of
Small is Beautiful
.
With minimal mechanism and a very limited number of sys-
tem calls, Linux nevertheless provides a powerful and elegant file system.
10.6.1 Fundamental Concepts
The initial Linux file system was the MINIX 1 file system. However, because
it limited file names to 14 characters (in order to be compatible with UNIX Version
7) and its maximum file size was 64 MB (which was overkill on the 10-MB hard
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