Some operating systems include the command interpreter in the kernel. Others,
such as Windows and UNIX, treat the command interpreter as a special program
that is running when a job is initiated or when a user first logs on (on interactive
systems)
[shell也是一个程序,这个程序为用户提供系统功能的接入]
In one approach, the command interpreter itself contains the code to
execute the command. For example, a command to delete a file may cause
the command interpreter to jump to a section of its code that sets up the
parameters and makes the appropriate system call. In this case, the number of
commands that can be given determines the size of the command interpreter,
since each command requires its own implementing code.
An alternative approach—used by UNIX, among other operating systems
—implements most commands through system programs. In this case, the
command interpreter does not understand the command in any way; it merely
uses the command to identify a file to be loaded into memory and executed.
Thus, the UNIX command to delete a file
rm file.txt
would search for a file called rm, load the file into memory, and execute it with
the parameter file.txt. The function associated with the rm command would
be defined completely by the code in the file rm. In this way, programmers can
add new commands to the system easily by creating new files with the proper
names. The command-interpreter program, which can be small, does not have
to be changed for new commands to be added.
We begin our consideration of process synchronization by discussing the so-
called critical-section problem. Consider a system consisting of n processes
{P0 , P1 , ..., Pn−1 }. Each process has a segment of code, called a critical section,
in which the process may be changing common variables, updating a table,
writing a file, and so on. The important feature of the system is that, when
one process is executing in its critical section, no other process is allowed to
execute in its critical section. That is, no two processes are executing in their
critical sections at the same time.
每一个进程都有自己的临界段
The test and set() instruction can be defined as shown in Figure 5.3.
The important characteristic of this instruction is that it is executed atomically.
Thus, if two test and set() instructions are executed simultaneously (each
on a different CPU), they will be executed sequentially in some arbitrary order. If
the machine supports the test and set() instruction, then we can implement
mutual exclusion by declaring a boolean variable lock, initialized to false.
The structure of process Pi is shown in Figure 5.4.
每一个进程都有自己的test_and_set()instruction,即是在多处理器中，它的处理都是原子性的
Each semaphore has an integer value and a list of processes list. When
a process must wait on a semaphore, it is added to the list of processes. A
signal() operation removes one process from the list of waiting processes
and awakens that process.
每一个信号量都有自己的等待队列