14 - System calls Flashcards
What is a system call?
A system call is an interface between a program running in user space and the operating system, allowing the process to request actions that cannot be performed by normal functions. It provides services via the OS’s API.
What is the difference between user mode and kernel mode?
User mode is where applications run with restricted access to system resources, while kernel mode grants the CPU unrestricted access to hardware, allowing the OS to execute privileged instructions.
How does a system call work in a typical operating system?
A system call is invoked by a process, triggering a context switch from user mode to kernel mode via a software interrupt. The OS then executes the corresponding kernel function and returns the result to the user application.
What are some key features of system calls?
Key features include security (preventing direct hardware access), abstraction (hiding hardware specifics), access control (ensuring permission checks), and consistent interaction between applications and the OS.
What are the main types of system calls?
The five main types are
* Process Control (creation, deletion)
* File Management (read, write, open, close)
* Device Management (requesting and releasing devices),
* Information Maintenance (managing system info like time and date),
* Communication (inter-process communication and message passing).
How is the read() system call executed?
For the read() system call, the process pushes parameters onto the stack, calls the library routine which places the system call code into a register, and then issues a trap instruction to switch to kernel mode where the read operation is performed before returning control to user mode.
How are system calls implemented on Intel x86 architectures?
On Intel x86, system calls use registers (with EAX holding the system call number and other registers for parameters) and the INT 0x80 instruction is used to trigger an interrupt that hands control over to the kernel.
What is the purpose of the exit system call?
The exit system call terminates the calling process, returning an exit status code. The kernel uses registers (such as %eax for the system call number and %ebx for additional parameters) to process this request.
What role do libraries play in the use of system calls?
Libraries like GNU C’s libc provide wrapper functions that simplify invoking system calls. These wrappers enhance code portability and provide a consistent interface across different systems, even if the underlying implementations differ.
Why are system calls essential for operating systems?
They serve as the only entry points into the kernel, ensuring that user applications access hardware resources securely and in a controlled manner, while also facilitating essential functions like process management, file handling, and inter-process communication.
