Dual-Mode Operation

To ensure proper operation, we must protect the operating system and all other programs and their data from any malfunctioning program. Protection is needed for any shared resource. The approach taken is to provide hardware support to allow us to differentiate among various modes of executions. At the very least, we need two separate modes of operation: user mode and monitor 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: monitor (0) or user (1). With the mode bit, we are able to distinguish between an execution that is done on behalf of the operating system, and one that is done on behalf of the user; As we shall see, this architectural enhancement is useful for many other aspects of system operation.

At system boot time, the hardware starts in monitor mode. The operating system is then loaded, and starts user processes in user mode. Whenever a trap or interrupt occurs, the hardware switches from user mode to monitor mode (that is, changes the state of the mode bit to be 0). Thus, whenever the operating system gains control of the computer, it is in monitor 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 monitor mode. If an attempt is made to execute a privileged instruction in user mode, the hardware does not execute the instruction, but rather treats the instruction as illegal and traps to the operating system.

The lack of a hardware-supported dual mode can cause serious shortcomings in an operating system. For instance, MS-DOS was written for the Intel 8088 architecture, which has no mode bit, and therefore no dual mode. A user program running awry can wipe out the operating system by writing over it with data, and multiple programs are able to write to a device at the same time, with possibly disastrous results. More recent and advanced versions of the Intel CPU, such as the 80486, do provide dual, mode operation As a result, more recent operating systems, such as Microsoft Windows/NT, and IBM OS/2, take advantage of this feature and provide greater protection for the operating system.

Last Updated Jul.29/99