Limited Direct Execution

The OS must virtualize the CPU in an efficient manner while retaining control over the system. To do so, both hardware and operating-system support will be required. The OS will often use a judicious bit of hardware support in order to accomplish its work effectively.

• The “direct execution” part of the idea is simple: just run the program directly on the CPU.
• The Limited part of the idea means the process only has a subset of the CPU instructions available to use when running

Details

Direct execution means the program will run fast (bare metal) compare this to Java or C# which run in a Virtual Machine.

Problem #1 Restricted Operations

• User Mode - code that runs in user mode is restricted in what it can do.
• Kernel Mode - In this mode, code that runs can do what it likes, including privileged operations such as issuing I/O requests and executing all types of restricted instructions.

Special instructions to trap into the kernel and return-from-trap back to user-mode programs are also provided, as well as instructions that allow the OS to tell the hardware where the trap table resides in memory.

Intel Trap Instruction

Trap Table

• The kernel sets up a trap table at boot time
• To specify the exact system call, a system-call number is usually assigned to each system call.
• What horrible things could you do to a system if you could install your own trap table? Could you take over the machine?

System Call

Standard Library

Problem #2 Switching Between Processes

• If a process is running on the CPU, this by definition means the OS is not running
• How can the operating system regain control of the CPU so that it can switch between processes?
• A Cooperative Approach - the OS trusts the processes of the system to behave reasonably
• The OS Takes Control - Needs hardware support
• Timer interrupt!

Saving and Restoring Context

• The OS scheduler decides what to run next
• A Context switch is what happens when on process is moved off the processor and another is moved on

Concurrency?

• Now we have to worry about what happens when we get interrupted right in the middle of doing something important
• Not to worry we will go over that in a future chapter!