OS Protects Territorial Integrity

The multitasking OS loads multiple tasks into different areas of memory and permits each to run for a slice of time. As described in the previous chapter, it permits a task to run for a timeslice, suspends it, permits another task to run for a timeslice, suspends it, etc. If the OS is executing on a fast processor with fast access to memory, this task switching can be accomplished so quickly that all of the tasks appear to be executing simultaneously.

While the processor is executing a task, the OS kernel and all of the other dormant tasks are resident in memory. As each of the tasks (and the OS kernel) were suspended earlier in time, the processor created a snapshot of the processor's register image in memory at the moment that task was suspended. In the IA32 environment, the OS sets up a separate Task State Segment (TSS) for each task to be used during task switches. When it's time to resume execution of a program, the processor can reload its register set from the task's TSS and pick up right where it left off.