Day 11 Processes

Operating Systems Control Tables

Process image Consists of The program(one or more files) Data System stack Process Control Block/Process descriptor The PCB has 3 types of information Process Identification - process id, parent id Process State info - PC, PSW(carry, zero, parity, ieb, overflow, user/supervisor mode), user-visible registers, SP, etc Process Control Information - Process state and Scheduling info, Process Privileges, Resource allocation, Priority for scheduling, memory management (pointers to segment or page tables) PCB is the most important data structure in the OS. Used by scheduler, resource allocator, interrupt processing and performance monitoring, etc.

Execution of the Operating System

Non-process kernel The operating system is not a process – It is just the code that is called frequently to manage resources and processes. It has its own address space. Execute kernel outside of any process The concept of process applies only to user programs Operating system code is executed as a separate entity that operates in kernel mode, with own region of memory and stack.

Execution within user processes Operating system software runs within the context of a user process. OS is a collection of routines that the user calls to perform various functions. Process executes in kernel mode when executing operating system code Process has user stack and kernel stack. Fewer process switches as system calls are now treated as a part of the process itself. Process does not get blocked when a supervisor call is made. Calling an OS routine is like making a function call and all one has to do is change the mode. Process switching software exists separate from the process itself.

Process-based operating system Major kernel functions are separate processes Process switching code is outside of the process. Modular structure implies easier to modify the operating system e.g. a system process that measures the rate of progress of user programs. Never called by the user process. Given a certain priority level by the OS and is scheduled to run by the O.S. Useful in multi-processor where certain processors can be dedicated to certain OS operations.