System Calls

Question 1

What is a system call?

Answer 1

Interface for user programs to request OS services.

Question 2

What happens when a system call occurs?

Answer 2

Causes a trap into the OS. Dedicated ISR for handling system call interrupts.

The sys call identifier is wrote into a particular register.

Question 3

How are parameters passed to sys calls?

Answer 3

Can pass parameters in registers, by pushing them onto the stack. or as a table.

Stacks and tables do not limit the number of parameters that can be passed.

Question 4

How does pushing parameters to the stack work?

Answer 4

System call implementation knows to pop date from the stack

Question 5

How do parameter tables works?

Answer 5

Address of table passed to OS. Within the table are the passed parameters.

Question 6

On Linux, what happens after the system call?

Answer 6

Integer representing the success/failure of syscall stored in EAX.

Return value written to some specified address.

CPU state is restored.

User mode is restored before returning.

Question 7

How can Syscalls be invoked?

Answer 7

High-level functions and other system library functions (printf, open etc..)

syscall() C function

Invoking interrupt 128