1.6.2. Process Implementation

To let the kernel manage processes, each process is represented by a process descriptor that includes information about the current state of the process.

NOTE: *process descriptor*在3.2. Process Descriptor中进行专门介绍

本节中的process所指为lightweight process，而不是标准的Process (computing)

When the kernel stops the execution of a process, it saves the current contents of several processor registers in the process descriptor. These include:

• The program counter (PC) and stack pointer (SP) registers
• The general purpose registers
• The floating point registers
• The processor control registers (Processor Status Word) containing information about the CPU state
• The memory management registers used to keep track of the RAM accessed by the process

NOTE: See also

• Program counter
• Stack register
• Processor register

When the kernel decides to resume executing a process, it uses the proper process descriptor fields to load the CPU registers. Because the stored value of the program counter points to the instruction following the last instruction executed, the process resumes execution at the point where it was stopped.

When a process is not executing on the CPU, it is waiting for some event. Unix kernels distinguish many wait states, which are usually implemented by queues of process descriptors ; each (possibly empty) queue corresponds to the set of processes waiting for a specific event.

NOTE: 参见3.2.4. How Processes Are Organized