如何通过API检索Facebook中的所有电子邮件地址(How to retrieve all email addresses in Facebook through API)

 正如你所说的，“程序计数器”（也称为“指令指针”）是硬件的一部分; 更具体地说，它是一个处理器寄存器。 该寄存器的全部用途是指向处理器正在执行的内存中的当前指令。 一旦该指令被执行，PC就被更改为指向要执行的下一条指令。 
 
 当今大多数现代操作系统都是多任务处理。 这基本上意味着它们可以同时运行多个进程。 但是，如果你只有一个处理器，那么你不可能同时执行多个进程，对吧？ 为了创造出这样的错觉：多个进程在同一时间只在一个处理器上执行，多任务操作系统可以非常快速地在可运行进程之间切换：它们推进一个进程，暂停进程，然后推进一些其他进程，等等，所有这一切在几分之一秒内。 
 
 为了实现这个机制，操作系统必须有适当的结构来保持所有正在运行的进程的当前状态。 操作系统应该保留在这些结构中的一个最重要的值是进程的当前PC值，该值指示其程序代码中当前正在执行的位置。 

As you said, the "program counter" (also called "instruction pointer") is part of the hardware; more specifically, it's a processor register. The whole purpose of this register is to point to the current instruction in memory that is being executed by the processor. Once that instruction is executed, the PC is altered to point to the next instruction to be executed.
 
Most modern operating systems today are multitasking. This essentially means that they can run multiple processes at the same time. However, if you have just one processor, there's no way you can execute more than a process simultaneously, right? To create this illusion that more than one process is executing at the same time on only one processor, multitasking operating systems switch between runnable processes very fast: They advance one process, they pause it, and then they advance some other process, and so on, all this in fractions of a second.
 
To implement this mechanism, the operating system must have appropriate structures to keep the current status of all running processes. One of the most important values the operating system should keep in those structures is the current PC value for the process, which indicates where, in its program's code, it is currently executing.