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

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