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