Limited Direct Execution: Operating System Control Techniques

This lecture discusses the concept of limited direct execution, a fundamental technique in computer systems that allows programs to run efficiently while maintaining control by the operating system. The instructor begins by explaining direct execution, where programs run directly on the hardware without protection, leading to potential issues such as illegal instruction execution and loss of control over the CPU. To address these problems, limited direct execution introduces a dual-mode architecture, separating user mode and kernel mode. In user mode, processes can only execute regular instructions, while kernel mode allows the operating system to execute privileged instructions. The lecture also covers various architectures and their terminology for these modes, such as x86-64, ARM, and RISC-V. Additionally, the instructor highlights common privileged instructions and the consequences of executing them in user mode, emphasizing the importance of maintaining system integrity and control. The lecture concludes with a discussion on how the operating system regains control when a process attempts to execute a privileged instruction.