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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 98
SEC. 1.7
OPERATING SYSTEM STRUCTURE
67
User
mode
Microkernel handles interrupts, processes,
scheduling, interprocess communication
Sys
Clock
FS
Proc.
Reinc.
Other
...
Servers
Disk
TTY
Netw
Print
Other
...
Drivers
Shell
Make
...
Process
User programs
Other
Figure 1-26.
Simplified structure of the
MINIX
system.
the kernel to do the write. This approach means that the kernel can check to see
that the driver is writing (or reading) from I/O it is authorized to use. Consequently
(and unlike a monolithic design), a buggy audio driver cannot accidentally write on
the disk.
Above the drivers is another user-mode layer containing the servers, which do
most of the work of the operating system. One or more file servers manage the file
system(s), the process manager creates, destroys, and manages processes, and so
on. User programs obtain operating system services by sending short messages to
the servers asking for the POSIX system calls. For example, a process needing to
do a
read
sends a message to one of the file servers telling it what to read.
One interesting server is the
reincarnation server
, whose job is to check if the
other servers and drivers are functioning correctly.
In the event that a faulty one is
detected, it is automatically replaced without any user intervention. In this way,
the system is self healing and can achieve high reliability.
The system has many restrictions limiting the power of each process.
As men-
tioned, drivers can touch only authorized I/O ports, but access to kernel calls is also
controlled on a per-process basis, as is the ability to send messages to other proc-
esses. Processes can also grant limited permission for other processes to have the
kernel access their address spaces. As an example, a file system can grant permis-
sion for the disk driver to let the kernel put a newly read-in disk block at a specific
address within the file system’s address space. The sum total of all these restric-
tions is that each driver and server has exactly the power to do its work and nothing
more, thus greatly limiting the damage a buggy component can do.
An idea somewhat related to having a minimal kernel is to put the
mechanism
for doing something in the kernel but not the
policy
.
To make this point better,
consider the scheduling of processes.
A relatively simple scheduling algorithm is
to assign a numerical priority to every process and then have the kernel run the
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