vault backup: 2022-09-21 13:54:30

content/notes/14-processes-and-system-calls.md

@@ -42,4 +42,69 @@ system program types include.,
 also runs in user space
 
 ## system calls
+processes issue requests to the kernel by making system calls
+
+![system call diagram|300](https://i.imgur.com/GAktF7t.png)
+
+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
+
+### data structure of kernel process
+