teh strcutures of computer systm

1. Chapter 2: Computer-System Structures
 Computer System Architecture
 Computer System Operation
 I/O Structure, Interrupts
 Storage Structure
 Storage Hierarchy, Caching
 Hardware Protection

2. Computer-System Architecture
memory controller system bus
 A general purpose computer-system consists of one or more
CPUs and a number of device controllers that are connected
through a common bus that provides access to a shared
memory.
 Each device controller is in charge of a specific type of device
(for example, disk drives, audio devices or video displays).

3. Computer-System Operation
 Device controllers and the CPU can execute concurrently
(in parallel), competing for memory.
 Memory controller synchronize access to the memory.
 Each device controller is responsibe from a particular
device type.
 Each device controller has a local buffer.
 Input/Output is from the device to local buffer of device
controller.
 After transfering device controller informs CPU that it has
finished its operation(i.e. read request) by causing an
interrupt (a signal that shows occurance of an event).
 CPU moves data from main memory to local buffers or
from local buffers to main memory.
Note: Operating Systems have a device driver for each
device controller. Device driver provides an interface
between rest of the OS and the device.

4. Common Functions of Interrupts
 Interrupt, transfers control to the appropriate interrupt
service routine (responsible for dealing with the interrupt),
through the interrupt vector, which contains the starting
addresses of all the service routines for the various
devices.
 Interrupt architecture must save the address of the
interrupted instruction (loaded into the program counter).
 Incoming interrupts are disabled while another interrupt is
being processed to prevent a lost interrupt, so any incoming
interrupts are delayed until operating system is done with
the current one; then, interrupts are enabled.
 A trap or exception is a software-generated interrupt
caused either by an error in the program or a request from
a user program.
 An operating system is managed by interrupts (interrupt
driven).

5. Common Functions of Interrupts(cont.)
 Interrupt driven (hardware and software)
 Hardware interrupt by one of the devices
 Software interrupt (exception or trap):
 Software error (e.g., division by zero)
 Invalid memory access
 Request for operating system service (using system
calls)
 Other process problems include infinite loop,
processes modifying each other or the operating
system

6. Interrupt Handling
 When an interrupt (or trap) occurs, the hardware transfers control
to the operating system.
 The operating system preserves the state of the CPU by storing
registers and the program counter.
 Then OS determines which type of interrupt has occured.
 Separate segments of code (in the OS) determine what action
should be taken for each type of interrupt.
 An interrupt service routine is provided to deal with the interrupt
and control is passed to the corresponding interrupt service
routine.
Note: Operating system waits for some events to occur. The
occurrence of an event is usually signaled by an interrupt from
either the hardware or the software.
When the CPU is interrupted, it stops what it is doing and
immediately transfers execution to a fixed location(contains the
starting address of interrupt service routine). Then interrupt
service routine executes; on completion, the CPU is resuming the
interrupted computation.

7. Interrupt Time Line For a Single Process Doing Output
Completion of
an I/O triggers
an interrupt
Duration of the
time spent in
interrupt service
routine

8. Storage Definitions and Notation Review
The basic unit of computer storage is the bit. A bit can contain one of two values, 0 and 1. All other
storage in a computer is based on collections of bits. Given enough bits, it is amazing how many
things a computer can represent: numbers, letters, images, movies, sounds, documents, and
programs, to name a few.
A byte is 8 bits, and on most computers it is the smallest convenient chunk of storage. For example,
most computers don’t have an instruction to move a bit but do have one to move a byte.
A less common term is word, which is a given computer architecture’s native unit of data. A word is
made up of one or more bytes. For example, a computer that has 64-bit registers and 64-bit memory
addressing typically has 64-bit (8-byte) words. A computer executes many operations in its native
word size rather than a byte at a time.
Computer storage, along with most computer throughput, is generally measured and manipulated in
bytes and collections of bytes.
A kilobyte, or KB, is 1,024 bytes
a megabyte, or MB, is 1,0242
bytes
a gigabyte, or GB, is 1,0243
bytes
a terabyte, or TB, is 1,0244
bytes
a petabyte, or PB, is 1,0245
bytes
Computer manufacturers often round off these numbers and say that a megabyte is 1 million bytes
and a gigabyte is 1 billion bytes. Networking measurements are an exception to this general rule; they
are given in bits (because networks move data a bit at a time).

9. Metric Units
Operating System Concepts

10. Storage Structure
 Main memory (primary storage, RAM) – only large storage
media that the CPU can access directly
 Random access
 Typically volatile
 Magnetic disks(secondary storage) – extension of main
memory that provides large nonvolatile storage capacity
 Hard disks – rigid metal or glass platters covered with
magnetic recording material
 Disk surface is logically divided into tracks, which are subdivided into
sectors
 The disk controller determines the logical interaction between the device
and the computer
 Solid-state disks – faster than hard disks, nonvolatile
 Various technologies
 Becoming more popular

11. Storage Structure(cont.)
Notes:
The programs must be in the main memory to be
executed.
 CPU automatically loads instruction from main
memory for execution.
Main memory is an array of words or bytes ranging
in size. Each word has it own size.
Secondary storage provides storage for programs and
data
Most programs(web browsers, compilers, word
processors) are stored on the disk until they are loaded
into memory.
Instruction-execution cycle: fetch the instruction
from memory, decode instruction, execute instruction,
store instruction back in memory.

12. Storage Hierarchy
 Storage systems organized in hierarchy according to
 Speed
 Cost
 Volatility
 Caching – copying information into faster storage
system; main memory can be viewed as a cache for
secondary storage

13. Storage-Device Hierarchy
Increasing access time Cheap, slow
Expensive, fast

14. Caching
 Important principle, performed at many levels in a
computer (in hardware, operating system, software)
 Use of high-speed memory to hold recently-accessed
data.
 Information in use copied from slower to faster storage
temporarily
 Faster storage (cache) checked first to determine if
information is there
 If it is, information used directly from the cache (fast)
 If not, data copied to cache and used there
 Requires a cache management policy, since caches have
limited memory size.
 Caching introduces another level in storage hierarchy.
This requires data that is simultaneously stored in more
than one level to be consistent.
 Ex: Value of a variable can be different in register and in the
various storage systems.

15. Direct Memory Access (DMA)
Structure
 Used for high-speed I/O devices, able to transmit
information at close to memory speeds.
 Device controller transfers blocks of data to or from
its local buffer directly to main memory without CPU
intervention.
 Only one interrupt is generated per block by DMA
controller, rather than an interrupt per byte.

16. How a Modern Computer Works
A von Neumann architecture

17. Memory Protection
 We must protect the OS from access by user programs
and protect user programs from one another.
 We must provide memory protection at least for the
interrupt vector and the interrupt service routines of
the operating system.
 In order to have memory protection, add two registers
that determine the range of legal addresses a program
may access:
 Base register – holds the smallest legal physical memory
address.
 Limit register – contains the size of the range
 Memory outside the defined range is protected.

18. Use of A Base and Limit Registers
User mode
Kernel mode
Supervisor mode
System mode
Privilaged mode
Monitor mode

19. Hardware Address Protection

20. Operating-System Operations
 Dual-mode operation allows OS to protect itself and other
system components
 User mode and kernel mode
 Mode bit provided by hardware
 Provides ability to distinguish when system is running user
code or kernel code
 Some instructions designated as privileged, only
executable in kernel mode (I/O instructions, instruction to
modify the memory-management registers or the timers, halt
instructions).
 System call changes mode to kernel, return from call
resets it to user mode