操作系统设计与实现-习题答案

I/O devices. In user mode, many sensitive instructions are prohibited.

Operating systems use these two modes to encapsulate user programs. Run-

ning user programs in user mode keeps them from doing I/O and prevents

them from interfering with each other and with the kernel.

processes in memory. It is commonly used to keep the CPU busy while one

or more processes are doing I/O.

onto the disk, so that when the currently executing processes are finished,

wait while data is copied. A CPU could do other work if the I/O were slow

for other reasons (arriving on a serial line, for instance).

the operating system from malicious user programs.

user. Mainframe systems nearly always emphasize batch or timesharing with

many users. Protection is much more of an issue on mainframe systems, as is

of output. The user then logs out and goes on a three-week vacation to Lon-

don. Sooner or later the disk will fill up, and the superuser will have to manu-

exist.

working directories and relative paths would only be a minor inconvenience.

directory is, say, /home/ast/projects/research/proj1 one could refer to the

written independently of all the other ones. It is possible to seek to any block

user 2 linked to it. So the reference count will be nonzero and the file itself

when all directory entries for a file have been removed will its i-node and

data actually vanish.

its output to a file and program 2 could read the file. While this is less

efficient than using a pipe between them, and uses unnecessary disk space, in

most circumstances it would work adequately.

shell. It is a graphical command interface to the device.

program in it. It is effectively a combination of

fast.

call takes 1000 instructions, up to 500,000 system calls/sec are possible while

consuming only half the CPU.

It goes to the buffer cache. The update program issues

seconds to force the dirty blocks in the cache onto the disk, in order to limit

impossible to fully virtualize and run an arbitrary unmodified operating sys-

tem in user mode.

ming model allows this activity to be described and modeled better.

cess has the CPU and is executing. The blocked state means that the process

cannot run because it is waiting for an external event to occur, such as a mes-

service procedure). This process would then be started up.

hardware that is required. For instance, an interrupt handler may be required

to enable and disable the interrupt servicing a particular device, or to

the model in part (a) of the figure. Suppose that a client process sends a

request to server process 1 to update a file. This process updates the cache

entry in its memory. Shortly thereafter, another client process sends a request

to server 2 to read that file. Unfortunately, if the file is also cached there,

server 2, in its innocence, will return obsolete data. If the first process writes

local variables, return addresses, and so on.

perform some action. Depending on the exact timing, one or other goes first.

If one of the processes goes first, everything works, but if another one goes

first, a fatal error occurs.