Arithmetic overflow in os161 mips trap uses these

Arithmetic overflow
*
/
In OS/161, mips trap uses these codes to decide whether it has been in-
voked because of an interrupt, a system call, or an exception.
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Processes and the Kernel 27
Interrupts (Revisited)
• Interrupts are a third mechanism by which control may be transferred to the
kernel
• Interrupts are similar to exceptions. However, they are caused by hardware
devices, not by the execution of a program. For example:
– a network interface may generate an interrupt when a network packet arrives
– a disk controller may generate an interrupt to indicate that it has finished
writing data to the disk
– a timer may generate an interrupt to indicate that time has passed
• Interrupt handling is similar to exception handling - current execution context is
saved, and control is transferred to a kernel interrupt handler at a fixed address.
CS350 Operating Systems Winter 2016
Processes and the Kernel 28
Interrupts, Exceptions, and System Calls: Summary
• Interrupts, exceptions and system calls are three mechanisms by which control
is transferred from an application program to the kernel
• When these events occur, the hardware switches the CPU into privileged mode
and transfers control to a predefined location, at which a kernel handler should
be located.
• The kernel handler creates a trap frame and uses it to saves the application
thread context so that the handler code can be executed on the CPU.
• Just before the kernel handler finishes executing, it restores the application
thread context from the trap frame, before returning control to the application.
In OS/161, trap frames are placed on the kernel stack of the thread performed
the system call, or of the thread that was running when the interrupt or ex-
ception occurred
CS350 Operating Systems Winter 2016

Processes and the Kernel 29
System Calls for Process Management
Linux OS/161
Creation fork,execv fork,execv
Destruction exit,kill exit
Synchronization wait,waitpid,pause, ... waitpid
Attribute Mgmt getpid,getuid,nice,getrusage, ... getpid
CS350 Operating Systems Winter 2016
Processes and the Kernel 30
The fork, exit, getpid and waitpid system calls
main() {
rc = fork(); /
*
returns 0 to child, pid to parent
*
/
if (rc == 0) { /
*
child executes this code
*
/
my_pid = getpid();
x = child_code();
_exit(x);
} else { /
*
parent executes this code
*
/
child_pid = rc;
parent_pid = getpid();
parent_code();
p = waitpid(child_pid,&child_exit,0);
if (WIFEXITED(child_exit))
printf("child exit status was %d\n",
WEXITSTATUS(child_exit))
}
}
CS350 Operating Systems Winter 2016

Processes and the Kernel 31
Process Creation Example (Part 1)
Kernel Process A
Parent process (Process A) requests creation of a new process.
fork
CS350 Operating Systems Winter 2016
Processes and the Kernel 32
Process Creation Example (Part 2)
Kernel Process A Process B
B’s thread is
ready, not running
system call return
Kernel creates new process (Process B)
fork
CS350 Operating Systems Winter 2016

Processes and the Kernel 33
The execv system call
int main()
{
int rc = 0;
char
*
args[4];
args[0] = (char
*
) "/testbin/argtest";
args[1] = (char
*
) "first";
args[2] = (char
*
) "second";
args[3] = 0;
rc = execv("/testbin/argtest", args);