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P. 5.1
Digital Technology and Computer Fundamentals
Chapter 5
Computer Organization
P. 5.2
Objectives
At the end of this chapter, you should be able to: describe the basic units of a
computer system;
describe the stored program concept in computer system;
describe the functions of each component in a CPU;
P. 5.3
Objectives (Cont’d)
describe the instruction execution process: fetch cycle and execution cycle;
distinguish the different categories of memory;
describe the operations of a memory;
describe the organization of a memory; and
P. 5.4
Objectives (Cont’d)
describe the principles of operations of the different secondary storage devices.
P. 5.5
References
M. Marris Mano, "Computer System Architecture," third edition, Prentice Hall.
V. Carl Hamacher, Zvonko G. Vranesic, Safwat G. Zaky, "Computer Organization," McGraw-Hill, 3rd ed. 1990.
Andrew S. Tanenbaum, "Structured Computer Organization," 3rd ed. Prentice Hall, 1990.
P. 5.6
References
Herman Lam, John O'Malley, "Fundamentals of Computer Engineering,” Wiley, 1988.
G. Shelly, T. Cashman, G. Waggoner, W. Waggoner, “Discovering Computers 98, A link to the Future,” International Thomson, 1998.
P. 5.7
General Principles
Digital computers are electronic machines capable of performing arithmetic and logical operations.
A typical microcomputer system consists of a Central Processing Unit (CPU), Main Memory, and Input / output devices.
They are connected by different buses.
P. 5.8
General Principles (Cont’d)
Address bus
Data bus
Control bus
CPU
I/O
Devices
Main
Memory
P. 5.9
General Principles (Cont’d) The physical parts of the computer
system are called hardware. A computer performs different
tasks under the command of programs.
A program is a sequence of instructions.
P. 5.10
General Principles (Cont’d) A computer instruction is a binary code
that specifies a sequence of micro-operations for the computer.
The instruction codes together with data are stored in memory.
The CPU reads each instruction from memory, decodes it, and executes it by issuing a sequence of micro-operations to different hardware components.
P. 5.11
Instruction
Every computer has its own unique instruction set.
Each instruction will be assigned a binary code, called the machine code, consisting of two parts, opcode and address.
P. 5.12
Instruction (Cont’d)
The specification of the operation to be performed by the instruction is called the operation code or the opcode.
The address part specifies the source or the destination of the operand.
P. 5.13
Instruction (Cont’d)
Every operation must have the opcode part, the address part is not always needed.
Consequently, it is unavoidable to have instructions of different lengths.
P. 5.14
Central Processing Unit
The function of the CPU is to execute programs stored in the main memory by fetching and decoding them, then executing them one after another.
P. 5.15
Components of CPU
A simplified CPU
ALU
External buses
Address
buffer
Data
buffer
PC SP Acc
PSR
Timing and
Control Unit
IR
Register
Register
Register
Internal buses
P. 5.16
Timing and Control Unit
Responsible for synchronization of the system and generating control signals
Generates the clock signal. Decoding an instruction Providing a reset function to
initialize the CPU along with other components in the microprocessor system.
P. 5.17
Arithmetic and Logic Unit (ALU) Perform various arithmetic and
logical operations. Placing the results into the
accumulator. Changes the flags in the Processor
Status Registers.
P. 5.18
Accumulator
Used extensively for data manipulations.
It is commonly used to store the operands and results of different operations
P. 5.19
Instruction Register (IR)
Controlled only by the Timing and Control Unit
A temporary store for an instruction transferred from the main memory.
When instructed, IR passes the instruction to the Timing and Control Unit for instruction decoding.
P. 5.20
Program Counter (PC)
Place to store the address information of the next instruction to be executed.
The content of the Program Counter is updated by the Timing and Control Unit after the instruction decoding.
P. 5.21
Stack Pointer (SP)
Stack is a special implementation of memories. (Last In First Out operation).
The Stack pointer (SP) contains the address of next free memory location.
P. 5.22
Stack Operations
Push: Data are stored in the location pointed to by the Stack Pointer. After the operation, the content of the SP will be incremented.
Pop: Content of the SP is decremented and the data in the memory location pointed by the SP will be retrieved.
P. 5.23
Processor Status Register (PSR) Contains various bits of status
information. Common flags include: zero, carry,
overflow, interrupt, and negative. These flags are set or cleared by
the various components in CPU, after an operation.
Programmers know from these flags the result of an operation.
P. 5.24
General Purpose Registers Store temporary data, usually the
result from an operation and will be used again in a later operation.
Fast storage, inside the CPU without selecting external chips.
Sometimes used for indexing purposes.
P. 5.25
Instruction fetch-execute cycle The instruction cycle consists of
the fetch cycle and the execute cycle.
The fetch cycle is identical for all instructions.
The execute cycle is different for different instructions.
P. 5.26
The fetch cycle
The address of the next instruction is stored in PC.
A Memory Data Register (MDR) resides in the CPU which holds data items for communication.
The address information to the memory is held in the Memory Address Register (MAR).
P. 5.27
The fetch cycle (Cont’d)
Fetching: transfer of the address from the PC
to MAR
generation of the READ control signal,
waiting for the memory to transfer the instruction or data into MDR.
P. 5.28
The fetch cycle (Cont’d)
The instruction code is then stored temporarily in IR for decoding.
After the instruction is decoded, the content of the program counter will be updated and the fetch cycle completes.
P. 5.29
The execute cycle
The CPU performs the different tasks according to the instruction.
Different resources will be required for different types of instructions.
At the end of the execute cycle, the contents of the Processor Status Register will be updated
P. 5.30
Semiconductor Memories An integrated circuit capable of
storing a binary number and recalling it when addressed or selected.
One of the storage devices used in computers.
Store either programs or data in a computer.
P. 5.31
Categorizing Memories
Based on the way a memory is accessed.
Time required to locate and access a memory is called access time.
P. 5.32
Categorizing Memories (Cont’d) Random access devices: in which
any location may be selected at random, equal access time for each location.
Sequential access devices: in which the locations are sequentially arranged, access time varies according to the location.
P. 5.33
Categorizing Memories (Cont’d) Volatile devices: in which the data
stored will be lost when power supply is off.
Non-volatile devices: in which the data stored will not be lost when power is off.
P. 5.34
Random access memory
Random Access Memory (RAM) is a read / write memory.
Random access and volatile device.
There are two basic types of RAM in use today, dynamic RAM and Static RAM.
P. 5.35
Dynamic RAM
Designed for high capacity, moderate speeds, and low power consumption.
Memory cells are charge-storage capacitors with driver transistors.
The presence or absence of charge in a capacitor interpreted as 1 or 0.
P. 5.36
Dynamic RAM (Cont’d)
The charge in a capacitor has tendency to discharge itself.
Required periodic charging to maintain the data stored - refreshing.
P. 5.37
Static RAM
Made of flip-flops and logic gates. The two stable states in flip-flops
represent binary values. Do not require refreshing. Low capacity, high power
consumption and relatively high cost per unit storage.
P. 5.38
Read Only Memory (ROM) Read only refers to the
applications at the end user’s view. Data is permanently programmed
either at the time of manufacture or by the user prior to the memory being installed.
Non-volatile and random access. Several types.
P. 5.39
Standard ROMs
Programmed by the manufacturer. Users can only read the data or
execute programs in the ROM. Store certain standard applications
for general user applications.
P. 5.40
Programmable ROMs (PROM) Can be programmed permanently
by the user or distributor using special equipment.
Only programmed once.
P. 5.41
Erasable Programmable ROMs (EPROM) Can be programmed and erased
by the user for many times. Erasure is carried out by shining
high intensity ultra-violet light through a special transparent window at the top of the memory IC.
Special device called EPROM writer is needed.
P. 5.42
Electrically Erasable Programmable ROMs (EEPROM) Similar to EPROM. User can erase a single bit
electrically in one operation. Operations require a special
equipment.
P. 5.43
Memory Organization
Usually organized into N locations where N is generally some power of 2.
The content in each location is called a word.
Each word consists of same number of bits, word length.
Each word is assigned an address.
P. 5.44
Operations on a memory
Reading from a memory: retrieving data, and writing to a memory: storing data.
Assisted by two external circuits, the memory address register (MAR) and memory data register (MDR)
P. 5.45
MDR and MAR
P. 5.46
MDR and MAR (Cont’d)
In general, for an n-bit address information, a total of 2n locations can be addressed.
MAR consists of n binary devices, so that a total number of 2n words can be addressed.
The size of the MDR is the same as the word length.
P. 5.47
Reading a memory
Reading a memory location will usually not destroy or change the contents in the location - non-destructive read.
The address is placed in the MAR Then a read signal is passed to the
memory. Data in the specified location will be
passed to MDR.
P. 5.48
Writing a memory
The data is first placed in the MDR, and the address in the MAR.
Then a write signal is passed to the memory.
The memory will store the data in MDR into the specified location.
After the operation, the data originally stored in the location will be lost.
P. 5.49
Locality of Reference
References to memory at any given interval of time confined within a few localized areas in memory.
Over short interval, addresses generated by a typical program refer to a few localized areas of memory repeatedly.
Remainder of memory is accessed relatively infrequently.
P. 5.50
Cache Memory
Keeping the most frequently accessed instructions and data in fast small memory.
The average memory access time will be less.
Such a fast small memory is referred to as a cache memory.
P. 5.51
Cache Memory (Cont’d)
Because of the locality of reference property of programs, although cache is small, most memory requests will be found in the fast cache memory.
Between CPU and main memory. Access time of cache less by a
factor of 5 to 10.
P. 5.52
Cache Memory (Operation) When the CPU needs to access
memory, the cache is examined. If the word is found in the cache, it
is read from the fast memory. If not, main memory is accessed to
read the word. A block (1 - 16) of words containing the one just accessed is then transferred to cache memory.
P. 5.53
Virtual Memory
Current active programs and data do not fit into the physical main memory space.
Secondary storage devices hold the overflow.
OS automatically moves programs between the main memory and secondary storage, called virtual memory technique.
P. 5.54
Virtual Memory (Cont’d)
The binary addresses issued by the processor are called virtual or logical addresses.
Translation from virtual addresses to physical addresses is implemented by a combination of hardware and software components.
P. 5.55
Virtual Memory (Cont’d)
If a virtual address refers to an address in the physical memory, then the contents of the appropriate location in the main memory are accessed immediately.
If not, its contents must be brought into a suitable location in the main memory before they can be used.
P. 5.56
Virtual Memory (Cont’d)
The simplest method: arrange files in fixed-length units called pages.
Each page consists of a block of contiguous locations in the main memory or in secondary storage.
When required, a page of information is moved back and forth between the main memory and secondary storage.
P. 5.57
Virtual Memory (Cont’d)
Bridging the size gap between the main memory and secondary storage
Usually implemented in part by software techniques.
P. 5.58
Secondary Storage
A kind of non-volatile memory. Relative low cost and large
storage. Directly addressed. External to the microprocessor. Contents must be loaded into the
main memory before they can be accessed.
P. 5.59
Secondary Storage
P. 5.60
Floppy disks
Also called a diskette, consists of a circular piece of thin plastic, coated with magnetic material.
Must be formatted before used. Formatting defines the tracks,
cylinders, and sectors on the disk surface.
P. 5.61
Formatting floppy disks
P. 5.62
Formatting floppy disks (Cont’d) A sector is a pie-shaped section of
the floppy disk. A track sector is a section of track
within a sector. Each track sector holds 512 bytes.
P. 5.63
Formatting floppy disks (Cont’d) A cluster consists of 2 – 8 track
sectors depending on the OS. The smallest unit to store data. One file can be stored in many
clusters.
P. 5.64
Formatting floppy disks (Cont’d) Formatting process also
establishes a directory, the file allocation table (FAT).
FAT records information about files stored on the floppy disk.
The file name, size, the time and date changed, and the cluster number where the file begins.
P. 5.65
Accessing floppy disks
Through a floppy disk drive. A drive motor rotates the disk. A read/write head can rest on the
top and bottom surface of the rotating floppy disk.
To write data, the read/write head generates electronic impulses to change the magnetic polarity, of magnetic areas along a track
P. 5.66
Accessing floppy disks (Cont’d)
P. 5.67
Hard disks
P. 5.68
Hard disks (Cont’d)
Consist of one or more rigid platters coated with magnetic material on the surface.
The platters, read/write heads, and access arms are all enclosed in an airtight sealed case.
P. 5.69
Accessing Hard disks
Much like storing data on floppy disks except hard disks have multiple platters
read/write heads do not touch disk surface.
Access time for a hard disk is faster because it spins faster and spinning constantly.
P. 5.70
Accessing Hard disks (Cont’d) Data is stored on both sides A hard disk must be formatted
before it can store data. Before a hard disk is formatted, it
can be divided into separate areas called partitions.
Each partition can function as if it were a separate disk.
P. 5.71
Disks cartridges
Disk cartridges, are removable. They provide both the storage
capacity and fast access times of hard disks and the portability of floppy disks.
P. 5.72
Magnetic tape
Thin ribbon of plastic, one side of which is coated with magnetic material.
Sequential access storage media. The most common types are
cartridge tape, but some older systems use reel-to-reel tape.
For backup and transferring data between systems.
P. 5.73
Accessing Magnetic tape Same as disk drives, tape drives
have an electromagnetic read/write head to access magnetic patterns.
On Quarter-Inch-Cartridge (QIC) tape devices, data is recorded in a single track. When the end is reached, the tape reverses direction and data is recorded in another track in opposite direction.
P. 5.74
Accessing Magnetic tape (Cont’d) Digital audio tape (DAT) uses
helical scan technology to record data in tracks at a six-degree angle to the tape.
Older, one-half-inch reel-to-reel tape drives record data in nine channels, eight channels for data bits and one channel for a parity bit.
P. 5.75
Accessing Magnetic tape (Cont’d)
P. 5.76
PC Cards
Credit card-sized cards that fit into PCMCIA (Personal Computer Memory Card International Association) expansion slots.
Mostly used on portable computers Only 10.5mm thick but can contain
more than 200 MB of data.
P. 5.77
Optical Disks
P. 5.78
Optical Disks (Cont’d)
To record data on an optical disk, a high-power laser heats the surface and makes a microscopic pit.
A low-power laser light is reflected from the smooth unpitted areas and is interpreted as 1.
The pitted areas do not reflect the laser beam and are interpreted as 0.
P. 5.79
Optical Disks (Cont’d)
A CD-ROM can store more than 800 MB data.
CD-ROM drives are often advertised as x-times speed drives.
The original standard established a minimum transfer rate of 150 kbps.
P. 5.80
Optical Disks (Cont’d)
Most optical disks are prerecorded and cannot be modified by users.
WORM devices, allow write once, read many on the disks.
Most common erasable optical drives use magneto-optical technology, in which a magnetic field changes the polarity of a spot that has been heated by a laser.
P. 5.81
Smart Card
Same size and thickness of a credit card
Contains a thin microprocessor capable of storing information.
When inserted into compatible equipment, the information on the smart card can be read and if necessary, updated.
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