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156 lines
5.4 KiB
Markdown
156 lines
5.4 KiB
Markdown
---
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title: "14-processes-and-system-calls"
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aliases:
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tags:
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- cosc204
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- lecture
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---
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- heirarchical structure in software systems
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- system calls and interrupts
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- representing process in OSs
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- overview of process scheduling
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# Heirarchical structure
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software systems have modularity
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- os kernel
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- system programs
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- applicatioin programs
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## application programs
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Defn: a program that ordinary usrers interact with, running in user space
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e.g.,
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- word processor
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- video player
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- web browser
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- games
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- etc
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## system programs/libraries
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Defn: a program which provides general purpose power level function, packeages in a way which is convenient for a user (e.g., sys admin or programmer) to employ
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system program types include.,
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- file manipulation progarms
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- status info programs
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- network and device management programs
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- compilers assemblers and interpreters
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- various library code
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- command shell programs (e.g., command interpreters)
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also runs in user space
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# system calls
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processes issue requests to the kernel by making system calls
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they are requesets to the OS kernel made through interrupt handlers
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the set of system calls is termed the programmer interface to OS
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System calls in xv6
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- fork() Create a new process
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- exit() Terminate the current process
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- wait() Wait for a child process to exit
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- kill(pid) Terminate a process identified with pid
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- getpid() Return the ID number of the current process
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- sleep(n) Put the current process into sleep for n seconds
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- exec(filename, *argv) Load a file named filename and execute it with arguments in argv
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- sbrk(n) Update the process memory space by n bytes
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- open(filename, flags) Open a file named filename with flags indicating read/write
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- read(fd, buf, n) Read n bytes from an open file fd into buf
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- write(fd, buf, n) Write n bytes from buf to an open file fd
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- close(fd) Close the open file fd
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- dup(fd) Duplicate the open file fd into another file
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- descriptor (the return value)
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- pipe(p) Create a pipe and return the file descriptors in p
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- chdir(dirname) Change the current working directory of the process to dirname
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- mkdir(dirname) Create a new directory dirname
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- mknod(name, major, minor) Create a device file name with major and minor numbers
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- fstat(fd) Return status information about an open file fd
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- link(f1, f2) Create another name f2 for the file f1
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- unlink(filename) Remove a file named filename
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## interrupts
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### detecting interrupt
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CPU hardware containes a wire called the interrupt requirest line
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when a cpu is running an instruction-execute cycle
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- fetch instruction from memory and store it in instruction register
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- decode/execute instruction
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an extra step is included
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- sense the interrupt request line.
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- it it has a signal, then respond to the interrupt
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### responding interrupt
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when interrupt detected, CPU switches to kernel space, and a handler function provided by the OS is invoked
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- the context of the current instructionis saved
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- control is transferred to a fixed memory location holding an interrupt-handling routine
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- when the routine is finished, it restores the context of the interrupted process
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the interrupt vector is an array of lications thta hold the addresses of these interrupt-handling routines. (usually held in low memory)
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this is hard to do
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- how to restore context?
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- nested handlers?
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### what causes them
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generated by:
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- errors: e.g., division by zero, invalid memory access
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- I/O device signals:; e.g., completion of IO operation or arrival of network packet
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- system calls (also called trap or soft interrupt) with special instruction (int or svc)
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a system call generates an interrupt and the OS transfers control to a system call handler
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# Processes
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a process is dynamic
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a given program can be executing many times of a given machines: each instance is a separate process
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process components
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- text section/code section: the program code itself
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- data section: any global variables used by the program
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- process stack: any local variables currently being used
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- program counter: a pointer to some place in the program code
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- contents of CPU registers
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- memory management information
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- device/file allocation information
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- accounting information
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## process control blocks
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the OS keeps a record of each process in the system, with a data structure called *process control block* (PCB)
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## scheduling processes
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processes take turns to run on the CPU. processes wait to run in one of two queues:
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- ready queue: process ready to execute
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- device queue: processes waiting to use a particular device. (one queue per device)
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generally represented as linked lists
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## flow of processes
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Rounded rectangles are events.
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Rectangles are queues where processes wait.
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Circles are processing resources that serve the events/queues.
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## process creation
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created by another process: parent process creates a child process
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- process can copy itself using fork
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- child process can then call exec() to load a new program
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- a child process can run concurrently with parent
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- child process can share the resources of its parent if not restricted
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system start with one process called *init*
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## procees heirarchy
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processes for a tree
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