Identifiers
PROCESS CONTROL BLOCK INFORMATION Process Identification
Numeric identifiers that may be stored with the process control block include Identifier of this process
Identifier of the process that created this process (parent process)
User identifier
Processor State Information
User-Visible Registers
A user-visible register is one that may be referenced by means of the machine language that the processor executes. Typically, there are from 8 to 32 of these registers, although some RISC implementations have over 100.
Control and Status Registers
These are a variety of processor registers that are employed to control the operation of the processor. These include Program counter: contains the address of the next instruction to be fetched
Condition codes: result of the most recent arithmetic or logical operations (e.g., sign, zero, carry, equal, and overflow) Status information: includes interrupt enabled/disabled flags, execution mode.
Stack Pointers
Each process has one or more last-in, first-out (LIFO) system stacks associated with it. A stack is used to store parameters and calling addresses for procedure and system calls. The stack pointer points to the top of the stack.
Process Control Information
Scheduling and State Information
This is information that is needed by the operating system to perform its scheduling function.
Typical items of information include
Process state: defines the readiness of the process to be scheduled for execution (e.g., running, ready, waiting, halted)
Priority: One or more fields may be used to describe the scheduling priority of the process. In some systems, several values are required (e.g., default, current, highest allowable).
Scheduling-related information: this will depend on the scheduling algorithm used. Examples are the amount of time that the process has been waiting and the amount of time that the process executed the last time it was running.
Event: identity of event the process is awaiting before it can be resumed.
Data Structuring
A process may be linked to other process in a queue, ring, or some other structure. For example, all processes in a
waiting state for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator-created) relationship with another process. The process control block may contain pointers to other processes to support
these structures.
Inter-process Communication
Various flags, signals, and messages may be associated with communication between two independent processes. Some
or all of this information may be maintained in the process control block.
Process Privileges
Processes are granted privileges in terms of the memory that may be accessed and the types of instructions that may be executed. In addition, privileges may apply to the use of system utilities and services.
Memory Management
This section may include pointers to segment and/or page tables that describe the virtual memory assigned to this
process.
Resource Ownership and Utilization resources controlled by the process may be indicated such as opened fi1es. A history of utilization of the processor or other resources may also be included; this information may be need by the scheduler.