A PCB or process control block, is defined like this on Wikipedia
Process Control Block (PCB, also called Task Controlling Block, Task Struct, or Switchframe) is a data structure in the operating system kernel containing the information needed to manage a particular process. The PCB is "the manifestation of a process in an operating system
and its duty is:
Process identification data Processor state data Process control data
So where can the PCB of a process be found?