1. Operating Systems CMPSC 473
Processes
August 26, Lecture 2
Instructor: Bhuvan Urgaonkar

2. Last class Definition of an operating system
Software that virtualizes physical resources for applications
CPU, memory, IO devices virtualized to process, virtual memory, and files
OS requires a variety of support from hardware
For protecting processes from each other and to control their resource usage -> User/Kernel Modes

3. Announcements A brief tutorial on using gdb in the next class
Slides up on Angel

4. Services & Hardware Support
Protection: Kernel/User mode, Protected Instructions, Base & Limit Registers
Scheduling: Timer
Interrupts: Interrupt Vectors
System Calls: Trap Instructions
Efficient I/O: Interrupts, Memory-mapping
Synchronization: Atomic Instructions
Virtual Memory: Translation Lookaside Buffer (TLB)

5. Interrupts Polling = “are we there yet?” “no!” (repeat…)
Inefficient use of resources
Annoys the CPU
Interrupt = silence, then: “we’re there”
I/O device has own processor
When finished, device sends interrupt on bus
CPU “handles” interrupt

6. CPU Interrupt Handling
Handling interrupts: relatively expensive
CPU must:
Save hardware state
Registers, program counter
(Often) disable interrupts (why?)
Invoke via in-memory interrupt vector (like trap vector, soon)
Enable interrupts
Restore hardware state
Continue execution of interrupted process

7. Traps Special conditions detected by architecture
E.g.: page fault, write to read-only page, overflow, system call
On detecting trap, hardware must:
Save process state (PC, stack, etc.)
Transfer control to trap handler (in OS)
CPU indexes trap vector by trap number
Jumps to address
Restore process state and resume

8. Timer OS needs timers for Interrupt vector for timer Time of day
CPU scheduling
Interrupt vector for timer

9. Processes

10. Process: Definition For us: Instance of a program in execution
For OS: The collection of data structures that fully describes how far the execution of the program has progressed
From kernel’s point of view, an entity to which system resources (CPU time, memory, …) are allocated

11. Overview of Process-related Topics
How a process is born
Parent/child relationship
fork, clone, …
How it leads its life
Loaded: Later in the course
Executed
CPU scheduling
Where a process “lives”: Address space
OS maintains some info. for each process: PCB
Process = Address Space + PCB
How processes request services from the OS
System calls
How processes communicate
A variant of processes: threads
How processes die

12. Process Address Space Text section Stack Data Heap Temporary data
Function params, return addresses, local variables
Data
Global variables
Heap
Used for dynamically allocating memory

13. Process Life-cycle New: Being created
Running: Instructions are being executed
Only one process can be in this state at any given time per-CPU
Waiting: Waiting for some event to occur (e.g., I/O completion or reception of a signal)
Ready: Waiting to be assigned the CPU
Terminated: Finished execution