CSCE451/851

1. CSCE451/851 • Introduction to Operating Systems • http://cse.unl.edu/~ylu/csce451 • https://piazza.com/unl/fall2017/csce451/home

2. Chapter 1 -Introduction • What is an operating system? • Interface between application programs and hardware • Basic purpose of an operating system? • Provide services to application programs • Control functioning of hardware • From a user’s point of view • Ease of use <-> maximize work (or play) that user performs • From a system’s point of view • Resource allocator

3. user1 user2 user n editor ie calc appx Components of a Computer System hardware Operating System applications

4. 1.2 Computer System Organization • Most (if not all) modern CPU design is based on interrupts • The CPU has various lines thru which a signal can be sent (interrupt) • Interrupts can originate from hardware, cpu(trap), software(system call) • Interrupts are provided as a way to improve processor performance • Hardware interrupts can happen very frequently and must be handled promptly and efficiently

5. Handling an Interrupt • Interrupt vector – an array of pointers to interrupt service routines indexed by the interrupt number • When the CPU is interrupted: • CPU saves address of interrupted instruction • Invokes interrupt service routine whose address is stored in the Interrupt vector • After the interrupt is serviced, the saved instruction address (and state) is restored and CPU resumes

6. Program Flow of Control

7. I/O Structures (1.2.3) • A large part of operating system code is dedicated to managing I/O • Reliability • Performance • Various types of devices available • I/O is achieved through synchronization between • Operating system code called device drivers • Device controllers which transfer data between hardware and internal buffers • Interrupts which invoke the correct device drivers

8. Device Controller Interface Primary Memory .. busy done Error codes .. Instruction Data Status Cpu Data Logic Dev. Ctrlr. Data n-1 Device

9. I/O Structures (Character I/O) • Example of slow input device-keyboard • DC loads data register and interrupts CPU • CPU invokes DD - Interrupt Vector • DD fetches data from DC data register and transfers data to memory location • Example of serial output device-printer • OS Invokes DD • DD loads data/instruction register on DC • DC interrupts CPU when its done • DD checks status registers, raises error or loads next data/instruction on DC

10. I/O Structure – Block Devices • Character I/O is not effective enough for bulk data movement, Disks etc. • An alternative to character I/O is block I/O (DMA) • O/S sets up registers (instruction, buffer address, size etc.) in the DC • DC transfers data directly to memory, interrupts CPU only to signal completion/error

11. XFER DATA Interrupt CPU on Completion LOAD COM., ADD. & SIZE XFER DATA GET STATUS INTERRUPT CPU LOAD COMMAND GET STATUS Interrupt I/O (Character I/O) vs. Direct Memory Access (Block I/0) Primary Memory Primary Memory Input LOAD COMMAND* INTERRUPT CPU GET STATUS Input/Output XFER DATA Cpu Cpu Output Dev. Ctrlr. Dev. Ctrlr. Device Device

12. 1.3 Computer System Architecture • Self Read • Must know the difference between Symmetric and Asymmetric systems.

13. The evolution of OS

14. 1.4 Operating System Structure • Multiprogramming - keep multiple jobs in memory, switching between them when the CPU is idle. Improve utilization of computer system • Time sharing – switching between jobs based on timer • Switches between jobs very frequently • Each user/process is given the impression that they have sole access to the system OS Job 1 Job 2 Job 3 Job n

15. Multitasking and Timesharing • First multitasking operating systems such as DOS were introduced to improve CPU efficiency • Then Time shared systems such as Unix and Windows to allow multiple “simultaneous” program execution • The O/S need to save the state of each program during interrupts, it needs to schedule the programs etc. • To support the above and other issues, the concept of a process was created (Chapter 3)

16. 1.5 Modes of Operation • How do we prevent user processes from writing to Device Controller registers? • Most systems support at least two modes of operation: the user mode and the kernel mode

17. Dual-Mode Operation Only User code is allowed to execute when CPU is in user mode All Operating System code is available when CPU is in Kernel mode What if a user process needs some OS service? System calls (S/C) provide the means for a user program to ask the OS to perform tasks reserved for the OS on the user program’s behalf Process Scheduler USB Device Driver Create File S/C Create Process S/C Virtual memory Mgr PCI Device Driver Read File S/C Kill Process S/C Inter Process Communication Write File S/C Signal Process S/C Locks & Synch.

18. Dual Mode Operation • To support a stable and secure system, a dual-mode CPU was introduced • Various instructions and memory addresses could only be accessed in Kernel mode • To access service routines only available in the kernel (open a file, write a character to terminal, etc), a user process makes system calls • Interrupt handlers are invoked to handle these system calls

19. Dual Mode Operation • A system call allows a user process to request a protected service form the operating system • The system call number along with arguments are stored in well defined registers and a special instruction is executed • The cpu is placed in kernel mode and the system call service routine is executed • The operating system determines the service the user process is requesting and verifies the arguments • When complete, results are placed in the users process space and the cpu is placed back in user mode • Execution continues with the next instruction

20. Dual Mode Operation • The CPU is placed in kernel mode when an interrupt/trap/system call is received user mode mode bit=1 User process resume user process call sys call return mode bit = 1 mode bit = 0 kernel mode mode bit=0 Execute Sys Call

21. The Timer • The CPU timer interrupts the CPU during predetermined intervals • Most timers are variables and can be set to interrupt the CPU after the user process has executed for a predetermined amount of time • Instructions that update the timer must be privileged instructions • Ensures that the OS maintains control of the CPU • OS always sets time before turning over control to user

22. Other O/S Responsibilities • Discuss tasks the operating system would be responsible for each of the subsystems listed below • Memory management (Chap 1.7) • Process management (Chap 1.6) • Storage management (Chap 1.8) • Protection and Security (Chap 1.9)

23. Reading Assignment. • By this Friday • Read Chapter 3