Transcript Operating Systems

Operating Systems
CMPSC 473
Processes
August 26, 2010 - Lecture 2
Instructor: Bhuvan Urgaonkar
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
Announcements
• A brief tutorial on using gdb in the next class
• Slides up on Angel
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)
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
CPU Interrupt Handling
• Handling interrupts: relatively expensive
• CPU must:
– Save hardware state
• Registers, program counter
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(Often) disable interrupts (why?)
Invoke via in-memory interrupt vector (like trap vector, soon)
Enable interrupts
Restore hardware state
Continue execution of interrupted process
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
Timer
• OS needs timers for
– Time of day
– CPU scheduling
• Interrupt vector for timer
Processes
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
•
Overview of ProcessHow a process is born
related Topics
– 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
Process Address Space
• Text section
• Stack
– Temporary data
• Function params, return addresses,
local variables
• Data
– Global variables
• Heap
– Used for dynamically allocating
memory
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