1. Chapter 5 Computer Systems Organization
國立雲林科技大學 資訊工程研究所
張傳育(Chuan-Yu Chang ) 博士
Office: ES 709
TEL: ext. 4337

2. Outline
Introduction
The Von Neumann Architecture Memory and cache I/O and Mass Storage ALU Control Unit Putting All the Pieces Together
Historical Overview

3. Computer System Organization
Gates and circuits operate on the most elemental of data items, binary 0s and 1s
We need to look at higher-level components
Study computers as collections of functional units or subsystems.
Computer Organization
-- Study computers in terms of their major functional units and how they work
-- The concept of abstraction

4. The Concept of Abstraction
Figure 5.1
Hierarchy of abstraction
將圖(a)中的元件依功能群組成ABC三個block(圖b)。
我們只關心這些block執行的功能及如何連接。

5. Hierarchy of abstraction

6. The Von Neumann Architecture
Every computer in use today is based on a single design, they are based on the same fundamental principles.
The structure and organization of virtually all modern computers are based on a single theoretical model of computer design called the Von Neumann architecture

7. The Von Neumann Architecture (cont’d)
A model for designing and building computers that is based on the following three characteristics:
A computer constructed from four major subsystems called : memory, input/output, the arithmetic/logic unit (ALU), and the control unit(Fig. 5.2)
The stored program concept
The instructions to be executed by the computer are represented as binary values and stored in memory.
The sequential execution of instructions
One instruction at a time is fetched from memory to the control unit, where it is decoded and executed.

8. The Von Neumann Architecture

9. Memory and Cache
Memory is the functional unit of a computer that stores and retrieves the instructions and the data being executed.
Random Access Memory (RAM) has the following three characteristics: 1. Divided into fixed-size units called cells, each cell is associated with a unique identified called address All access to memory are to a specified address, must fetch or store a complete cell 3. The time it takes to fetch or store a cell is the same for all cells in memory.
Read Only Memory (ROM)
Simply a random access memory unit.
The ability to store information has been disabled.

10. Structure of Random Access Memory

11. RAM Cell size -- The number of bits per cell -- Or called memory width
-- Non standard width – 6,8,12,16,24,30,32,36,48,60
-- Standard width – 8-bit unit, called a byte
-- The largest unsigned integer value in a single cell is , that is 255.
-- Multiple memory cells to store a single data value
-- 2 or 4 bytes for one integer number
-- 4 or 8 bytes for real number

12. Address of Cell
Each memory cell in a RAM is identified by a unique unsigned integer address 0,1,2,3,…
N bits to represent the address of a cell
So the range of addresses available from 0 ….. 2n-1
2n is called the maximum memory size or the address space of the computer
Address vs. the content of that address

13. Symbols T
240 兆 G
230 十億 M
220 百萬 K
210
1024 千 m
10-3 毫
10-6 微 n
10-9 奈 p
10-12 微微

14. Memory Basic Operations
Two basic memory operations:
Fetch (address)
Fetch a copy of the content of the memory cell with the specified address and return those contents as the result of the operation
Original contents are unchanged
Nondestructive fetch
Store (address, value)
Store the specified value into the memory cell specified by address. The previous contents of the cell are lost.
Destructive store
Memory access time
The time to carry out either a fetch or a store operation.
Fetching and storing are allowed only to an entire cell.

15. Memory Registers
Two registers are used to implement the fetch and store operations, both operations require two operands:
Address
The address of the cell being accessed.
Value
The value stored operation /returned by the fetch operation.
MAR – Memory Address Register
Holds the address of the cell to be fetched or stored.
MDR – Memory Data Register
Contains the data value being fetched or stored.

16. Memory Registers(cont’d)
Fetch and store operations in a RAM
Fetch (address)
Load the address into the MAR
Decode the address in the MAR
Copy the contents of that memory location into the MDR
Store (address, value)
Load the value into the MDR
Store the contents of the MDR into the memory location

17. Organization of Memory and Decoding Logic
We can decode the address in the MAR using a decoder circuit.

18. Scalability
The problem with the memory organization shown in Fig. 5.5 is that it does not scale vary well, it could not be used to build a large memory unit.
Solution – two dimensional organization
The memory locations are stored in row major order
Each memory cell is connected to two selection lines, one called the row selection line, the other called the column selection line
Using two decoders instead of one decoder to choose the correct row and column selection lines.

19. Two-Dimensional Memory Organization
Only the memory cell located at the intersection of these two selection lines will carry out a memory fetch or store operation.
The two high-order bits are sent to the row decoder, the two low-order bits are sent to the column decoder

20. Fetch / Store Controller
To control whether memory does a fetch or a store operation, our memory unit needs one additional device called a Fetch/Store Control.
This unit determines whether we will put the contents of a memory cell into the MDR (a fetch operation) or put the contents of the MDR into a memory cell (a store operation)

21. Overall RAM Organization

22. Cache Memory In Von Neumann model, only a single type of memory
Processor was sitting idle waiting for data or instructions to arrive
The memory access was becoming a bottleneck

23. Cache Memory (cont’d) Principle of Locality
When the computer uses something, it will probably use it again very soon
The computer references a piece of data, it should also move that data from RAM to a special, high-speed memory unit called cache memory
A cache is 5 to 10 times faster than RAM but much smaller

24. The organization of the two-level memory hierarchy
When the computer needs a piece of information
Look first in cache to see whether the information needed is there
If it is not there, then access the desired information from RAM at the slower speed
Copy the data just fetched into the cache.

25. Cache Hit Rate Example:
The information we need is in the cache 60% of the time, 60% is called the cache hit rate
10 nsec for cache, 50 nsec for RAM
Average access time =
(0.6*10) + 0.4*(10+50) = 30 nsec

26. Input/Output and Mass Storage
Input/Output (I/O) unit
The devices that allow a computer system to communicate and interact with the outside world and store information.
Human interfaces: monitor, keyboard, mouse
Archival storage: not dependent on constant power
Volatile memory
Information disappears when the power is turned off. e.g., RAM
Non-volatile storage
Information could be saved between shutdowns of the machine. e.g., disks,tapes.
There exist many different types of I/O systems.

27. I/O Devices Two basic I/O device type
Human-readable
keyboards, screens, laser printers and so on.
Machine-readable (mass storage system)
floppy drives, hard disks, optical disks, CD-ROMs, and tapes.
Mass storage device comes in two distinct forms
Direct Access Storage Devices (DASD)
Sequential Access Storage Devices (SASD)

28. Input/Output and Mass Storage (continued)
Mass storage devices
Direct access storage device
Hard drive, CD-ROM, DVD, etc.
Uses its own addressing scheme to access data
Sequential access storage device
Tape drive, etc.
Stores data sequentially
Used for backup storage these days

29. Random Access vs. Direct Access
Remember the fundamental characteristics of random access:
Every memory cell has a unique address.
It takes the same amount of time to access every cell.
Direct access satisfies condition 1 but not condition 2.
Every unit of information has a unique address, but the time needed to access that unit depends on its physical location and the current state of the device.

30. Sectors and Tracks A disk stores information in units called sectors.
Each sector contains an address and a data block with a fixed number of characters.
A fixed number of sectors are placed on a concentric circle on the surface of the disk, called a track.

31. Organization of a typical disk

32. Disk Access Time Disk access time consists of
Seek time is the time needed to position the read/write head over the correct track.
Latency is the time for the beginning of the desire sector to rotate under the read/write head.
Transfer time is time for the entire sector to pass under the read/write head and have its contents read into or written from memory.

33. Disk of Physical Characteristics
Rotation Speed=7200 rev/min =120 rev/sec = 8.33 msec/rev
Arm movement time=0.02 msec to move to an adjacent track
Number of tracks/surface =1000 (0 to 999)
Number of sectors/track =50
Numbers of bytes/sector =512

34. Disk Access Time (table summarizes)
Best
Worst
Average
Seek Time
19.98 6
Latency
8.33
4.17
Transfer
0.17
Total
28.48
10.34

35. Sequential Access Storage Devices
Another type of mass storage device uses an access technique called sequential access.
A sequential access storage device behaves just like an audio cassette tape.
This type of backup operation fits the SASD model very well, and tape backup units are common storage devices on computer systems.

36. Input/Output Speed
Many I/O is very slow when compared to other components of a computer.
I/O controller : handles I/O event.
I/O buffer
A small amount of memory
I/O control and logic
To handle the mechanical functions of the I/O devices
Generate interrupt signal when an I/O operation is done.

37. Organization of the I/O Controller

38. The Arithmetic/Logic Unit
ALU(Arithmetic/Logic Unit) is the subsystem that performs such mathematical and logical operations as addition, subtraction, and comparison for equality.
All modern machines the ALU and control units are integrated into a single component called the processor.

39. ALU Organization The ALU is made up of three parts: Registers
Register is a storage cell that holds the operands of an arithmetic operation or result.
Interconnections between the components
Bus: Data path
ALU circuitry

40. Registers
A register is a storage cell that hold the value of operands/results.
Similar to random access memory with the following differences:
Accessed by register designator
Faster than regular memory cell
They are not used for general-purpose storage but for specific purposes.

41. Multi-register ALU Organization
可用來儲存operand和result。

42. Using a Multiplexor Circuit to select the proper ALU result

43. Overall ALU Organization

44. Control Unit
The most fundamental characteristic of the von Neumann architecture is the stored program
A sequence of machine language instructions stored as binary values in memory.
The task of the control unit :
Fetch from memory the next instruction to be executed.
Decode instructions.
Execute the instruction by issuing the appropriate command to the ALU, memory and I/O controllers.

45. Machine Language Instructions
The operation code(shorthand op code )is a unique unsigned-integer code assigned to each machine language operation recognized by the hardware.
The address fields are the memory addresses of the values in which this operation will work.
N bits address has a maximum of 2N memory cell.
The number of address field in an instruction typically varies from 0 to 3.

46. Instruction Set
The set of all operations that can be executed by a processor is called its instruction set.
Reduced Instruction Set Computers (RISC)
Minimizes the amount of hardware circuitry needed to build a processor.
A program for a RISC machine may require more instruction more than CISC.
Complex Instruction Set Computers (CISC)

47. Machine Language (Assembly)
Machine language instructions can be grouped into four basic classes:
Data transfer
These are operations that move information between or within the different components of the computer.
Arithmetic/ logical
These are operations that cause the ALU to perform a computation.
Compare
These operations compare two values and set an indicator on the basis of the results of the compare.
Branch
Altering the normal sequential flow of control.
The decision whether to branch is based on the current settings of the condition codes.

48. Example of Simple Machine Language Instruction Sequences

49. Control Unit Registers and Circuits
The CU consists of three components:
Program counter
The program counter (PC) holds the address of the next instruction to be executed.
Instruction register
The instruction register(IR) holds a copy of the instruction fetched from memory.
Instruction decoder
To determine what instruction is in the IR, the op code portion of the IR must be decoded using an instruction decoder.

50. Organization of the Control Unit Registers and Circuits

51. The Instruction Decoder

52. The Organization of a Von Neumann Computer

53. Instruction Set for Von Neumann Machine

54. Historical Overview of Computer System Development

55. Historical Overview of Computer System Development (cont.)
They did not have a memory where information could be stored in machine-readable form
They were not programmable.
Pascaline
One of the Earliest Mechanical Calculators.
Jacquard loom

56. Historical Overview of Computer System Development (cont.)
ENIAC Computer
The world’s first microcomputer
-Altair 8800

57. Graph of Computer Speeds, 1945 to the Present

58. Historical Overview of Computer System Development (cont.)
The Future: Non-Von Neumann Architectures
Von Neumann bottleneck
The inability of the sequential one-instruction-at-a-time.
If you cannot build something to work twice as fast, do two things at once. The results will be identical.
Parallel processing: building computers with more than one processor.
SIMD (Single Instruction stream /Multiple Data stream)
MIMD (Multiple Instruction stream/ Multiple Data stream)

59. The Future: Non-Von Neumann Architectures (continued)
SIMD architecture
Single instruction/Multiple data
Multiple processors running in parallel
All processors execute same operation at one time
Each processor operates on its own data
Suitable for “vector” operations

60. A SIMD Parallel Processing System

61. The Future: Non-Von Neumann Architectures (continued)
MIMD architecture
Multiple instruction/Multiple data
Multiple processors running in parallel
Each processor performs its own operations on its own data
Processors communicate with each other

62. Model of MIMD Parallel Processing