able to distinguish between the execution of operating-system code and userdefined
code. The approach taken by most computer systems is to provide
hardware support that allows us to differentiate among various modes of
execution.
At the very least, we need two separate modes of operation: user mode
and kernel mode (also called supervisor mode, system mode, or privileged
mode). A bit, called the mode bit, is added to the hardware of the computer to
indicate the current mode: kernel (0) or user (1). With the mode bit, we are able
to distinguish between a task that is executed on behalf of the operating system
and one that is executed on behalf of the user. When the computer system is
executing on behalf of a user application, the system is in user mode. However,
when a user application requests a service from the operating system (via a
system call), it must transition from user to kernel mode to fulfill the request.
This is shown in Figure 1.8. As we shall see, this architectural enhancement is
useful for many other aspects of system operation as well.
At system boot time, the hardware starts in kernel mode. The operating
system is then loaded and starts user applications in user mode. Whenever a
trap or interrupt occurs, the hardware switches from user mode to kernel mode
(that is, changes the state of the mode bit to 0). Thus, whenever the operating
system gains control of the computer, it is in kernel mode. The system always
switches to user mode (by setting the mode bit to 1) before passing control to
a user program.
The dual mode of operation provides us with the means for protecting the
operating system from errant users—and errant users from one another. We
accomplish this protection by designating some of the machine instructions that may cause harm as privileged instructions. The hardware allows privileged
instructions to be executed only in kernel mode. If an attempt is made to
execute a privileged instruction in user mode, the hardware does not execute
the instruction but rather treats it as illegal and traps it to the operating system.
The instruction to switch to user mode is an example of a privileged
instruction. Some other examples include I/O control, timer management, and
interrupt management. As we shall see throughout the text, there are many
additional privileged instructions.
We can now see the life cycle of instruction execution in a computer system.
Initial control is within the operating system, where instructions are executed
in kernel mode. When control is given to a user application, the mode is set to
user mode. Eventually, control is switched back to the operating system via an
interrupt, a trap, or a system call.
System calls provide the means for a user program to ask the operating
system to perform tasks reserved for the operating system on the user
program's behalf. A system call is invoked in a variety of ways, depending
on the functionality provided by the underlying processor. In all forms, it is the
method used by a process to request action by the operating system. A system
call usually takes the form of a trap to a specific location in the interrupt vector.
This trap can be executed by a generic t r a p instruction, although some systems
(such as the MIPS R2000 family) have a specific syscall instruction.
When a system call is executed, it is treated by the hardware as a software
interrupt. Control passes through the interrupt vector to a service routine in
the operating system, and the mode bit is set to kernel mode. The systemcall
service routine is a part of the operating system. The kernel examines
the interrupting instruction to determine what system call has occurred; a
parameter indicates what type of service the user program is requesting.
Additional information needed for the request may be passed in registers,
on the stack, or in memory (with pointers to the memory locations passed in
registers). The kernel verifies that the parameters are correct and legal, executes
the request, and returns control to the instruction following the system call.
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