Basic Technique: Limited Direct Execution

Direct execution protocol

To make a program run as fast as one might expect, not surprisingly OS developers came up with a technique, which we call limited direct execution. The “direct execution” part of the idea is simple: just run the program directly on the CPU. Thus, when the OS wishes to start a program running, it creates a process entry for it in a process list, allocates some memory for it, loads the program code into memory (from disk), locates its entry point (i.e., the main() routine or something similar), jumps to it, and starts running the user’s code. The figure below shows this basic direct execution protocol (without any limits, yet), using a normal call and return to jump to the program’s main() and later back into the kernel.