CSC 501 Lecture 2: Process

CSC 501
Lecture 2: Processes
Von Neumann Model
• Both program and data reside in memory
• Execution stages in CPU:
Fetch instruction
Decode instruction
Execute instruction
Write back result
• Process is
• a running program
• a program in execution
• an “instantiation” of a program
• We want to have multiple running programs
• However, we only have a few CPUs
• So, limit the number of programs you are running
Time to invent CPU virtualization
• OS virtualizes the CPU
• By time sharing it
• Mechanisms <- this lecture
• Policies <- next lecture
A Process
• A running program includes:
• Memory
• Registers
• I/O, opened files
• APIs: create, destroy, wait,
control, status
Static data
Process Creation
• Loading: code and static data
• Eagerly vs lazily
• Allocate memory for stack, heap
• Initialize file descriptors
• Jump to the main
Process states
• As a process executes, it changes state
new: The process is being created
running: Instructions are being executed
blocked: The process is waiting for some event to occur
ready: The process is waiting to be assigned to a
• terminated: The process has finished execution
Process Lifecycle
I/O: done
I/O: initiate
Data Structures
• Process list
• Common elements in process structure
Process state
Program counter
CPU registers
CPU scheduling information
Memory-management information
Accounting information
I/O status information
Example PCB in XINU
Process API
• On Unix:
• fork, exec, wait, …
• Why:
• they are essential in building a Unix shell
Process Creation
• UNIX examples
• fork system call creates new process
• exec system call used after a fork to replace new
process’ memory space with a new program
int main()
pid_t pid;
/* fork another process */
pid = fork();
if (pid < 0) { /* error occurred */
fprintf(stderr, "Fork Failed");
else if (pid == 0) { /* child process */
execlp("/bin/ls", "ls", NULL);
else { /* parent process */
/* parent will wait for the child to complete */
wait (NULL);
printf ("Child Complete");
Process Termination
• Possible scenarios for process termination
• Exit (by itself)
• Abort (by parent)
• Kill (by sysadmin)
Process Termination
• Exit (by itself)
• Process executes last statement and asks operating
system to delete
• Abort (by parent)
• Child has exceeded allocated resources
• Task assigned to child is no longer required
• If parent is exiting
• Some operating system do not allow child to continue if its
parent terminates
• All children terminated - cascading termination
• Kill (by sysadmin)
• Administration purpose
Process Suspension
• Temporarily ‘‘stop’’ a process
• Prohibit from using the CPU
• Why?
• What should be done?
• Change its state in PCB
• Save its machine states for later resumption
• Process table entry retained
• Complete state saved
Limited Direct Execution
• Time sharing the CPU:
• Run one process for a little while, then run another one,
and so forth
• Performance and control
• The “direct execution” part of the idea is simple:
• Just run the program directly on the CPU.
Protocol without limits
• Restricted Operations
• How to take over control
Restricted Operations
• The need to perform restricted operations
• Let processes do whatever they want
• Two modes: user mode and kernel mode
• How to perform restricted operations
• System calls, and they are procedure calls
How to execute system call
• Trap instruction
• The program executes trap, jump into kernel and raise
• Kernel does the work
• Kernel calls return-from-trap, return into the calling user
program and reducing the privilege level
• Save caller’s registers
• On x86, PC, flags, and a few others saved to a per-process
kernel stack
Which code to run
• Let the calling process specify the address
• Set up a trap table at boot time
• The hardware remembers the locations of trap handlers
Switching Between Processes
• Since OS is not running, how can it do anything?
• A cooperative approach
• Used in early version of the Macintosh OS
• Wait for systems calls which include yield
• Wait if illegal actions which generate trap
• What if a process gets stuck in an infinite loop
A Non-Cooperative Approach:
The OS Takes Control
• Timer interrupt
• Boot time:
• Set interrupt handler
• Start timer
Context Switch
• When CPU switches to another process
• System must
• Save the state of the old process (suspend) and
• Load the saved state for the new process (resume)
• Context-switch time is overhead
• System does no useful work while switching
• Time dependent on hardware support
Saving and Restoring Context
• Save state of currently executing process
• Copy all “live” registers to process control block
• Restore state of process to run next
• Copy values of live registers from process control block
to registers
• How to get into and out of the context switching
• Process scheduling
• CPU scheduling
• Multi-level feedback
• Lottery scheduling

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