Unformatted text preview: ks are active. Suppose the scheduler arbitrarily assigned 100 milliseconds to each task, and
switching between tasks consumed 8 milliseconds. One full circuit of the task list would require
972 milliseconds (9 100) (9 8) to complete.
Figure 2–4 Round-Robin Scheduler.
Task 9 Task 8 Task 2 scheduler Task 3 Task 4 Task 7 Task 6 Task 5 A multitasking OS runs on a processor (such as the x86) that supports task switching.
The processor saves the state of each task before switching to a new one. A task’s state consists
of the contents of the processor registers, program counter, and status ﬂags, along with references to the task’s memory segments. A multitasking OS will usually assign varying priorities to
tasks, giving them relatively larger or smaller time slices. A preemptive multitasking OS (such as
Windows XP or Linux) permits a higher-priority task to interrupt a lower-priority one, leading to
better system stability. Suppose an application program is locked in loop and has stopped
responding to input. The keyboard handler (a high-priority OS task) can respond to the user’s
Ctrl-Alt-Del command and shut down the buggy application program.
2.1.5 Section Review
1. The central processor unit (CPU) contains registers and what other basic elements?
2. The central processor unit is connected to the rest of the computer system using what three
3. Why does memory access take more machine cycles than register access?
4. What are the three basic steps in the instruction execution cycle?
5. Which two additional steps are required in the instruction execution cycle when a memory
operand is used? For More BS-IT Books, Notes & Assignments visit: www.bsit.zxq.net 36 Chapter 2 • x86
Downloaded From: www.bsit.zxq.net Processor Architecture 6. During which stage of the instruction execution cycle is the program counter incremented?
7. When a program runs, what information does the OS read from the ﬁlename’s disk directory entry?
8. After a program has been loaded into memory, how does it begin execution?
9. Deﬁne multitasking...
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- Winter '13
- Central processing unit, X86, protected mode, www.bsit.zxq.net Processor Architecture