PROCESSING DEVICE - Notes

This topic includes the following sections:
• How Computers Represent Data
• How Computers Process Data
• Factors Affecting Processing Speed

How Computers Represent Data

•Binary Numbers
•The Binary Number System
•Bits and Bytes
•Text Codes

How Computers Represent Data
– Binary Numbers

•Computer processing is performed by transistors, which are switches with only two possible states: on and off.
•All computer data is converted to a series of binary numbers– 1 and 0. For example, you see a sentence as a collection of letters, but the computer sees each letter as a collection of 1s and 0s.
•If a transistor is assigned a value of 1, it is on. If it has a value of 0, it is off. A computer's transistors can be switched on and off millions of times each second.

How Computers Represent Data - Bits and Bytes

•A single unit of data is called a bit, having a value of 1 or 0.
•Computers work with collections of bits, grouping them to represent larger pieces of data, such as letters of the alphabet.
•Eight bits make up one byte. A byte is the amount of memory needed to store one alphanumeric character.
•With one byte, the computer can represent one of 256 different symbols or characters.

How Computers Represent Data - Text Codes

A text code is a system that uses binary numbers (1s and 0s) to
represent characters understood by humans (letters and
numerals).

•An early text code system, called EBCDIC, uses eight-bit codes, but is used primarily in older mainframe systems.
•In the most common text-code set, ASCII, each character consists of eight bits (one byte) of data. It specifies characters for values from 0 to 127. Extended ASCII specifies characters for values from 128 to 255. ASCII is used in nearly all personal computers.
•In the Unicode text-code set, each character consists of 32 bits (four bytes) of data. Mostly all characters used in world like chinese, korean and japanese and mathematical symbols can be represented

How Computers Process Data

Where Processing Occurs:
•The Control Unit
•The Arithmetic Logic Unit
•Machine Cycles
•The Role of Memory in Processing
•Types of RAM

How Computers Process Data –
Where Processing Occurs

•Processing takes place in the PC's central processing unit (CPU).
•The system's memory also plays a crucial role in processing data.
•Both the CPU and memory are attached to the system's motherboard, which connects all the computer's devices together, enabling them to communicate.

The Control Unit

The two main parts of a CPU are the control unit and the arithmetic logic unit (ALU)
•The control unit directs the flow of data through the CPU, and to and from other devices.
•The control unit stores the CPU's instruction set, which contains the list of instructions for all the tasks the CPU can perform.
•Each instruction is expressed in microcode a series of basic directions that tell the CPU how to execute more complex operations.

The Arithmetic Logic Unit

•The actual manipulation of data takes place in the ALU.
•The ALU can perform arithmetic and logic operations.
•The ALU is connected to a set of registers—high speed memory locations built directly in to the CPU that are used to hold data and program instructions currently being processed.

Machine Cycles

To execute each instruction, the CPU follows a series of
steps-called a machine cycle. The machine cycle includes
two smaller cycles.
1.Instruction cycle
Fetching :Before the CPU can execute an instruction, the
control unit must retrieve a command or data from
memory
Decoding: Before execution, CU break down the command into instruction that corresponds to those in the instruction set.
2. Execution cycle
Executing: When the command is executed, the CPU
carries out the instruction by converting them into
microcodes.

Storing: CPU may be required to store the results of an instruction in memory.
•CPU performance is measured in MIPS and BIPS.
•By using a technique called pipelining, many CPUs can process more than one instruction at a time.
•Multitasking by hyperthreading.

There are two basic types of RAM: static and dynamic
• Dynamic RAM (DRAM) chips must be recharged
with electricity very frequently, or they will lose their
contents. One transistor per bit, slower, cheaper,
larger capacity
• Static RAM (SRAM) does not need to be recharged
as often as DRAM, and can hold its contents longer.
To store and retrieve data CPU uses Memory address a number that indicates a location on the memory chip.

The Role of Memory

vNonvolatile memory: Permanent. ROM
§PROM chip ->Chip that cannot be changed .Used in printer and hard drives.
§ROM contains a set of start up instruction->BIOS.
§BIOS contains POST to ensure that the system is functioning properly and all expected hardware devices are present.

Flash memory: Special type of nonvolatile memory used in portable digital devices like digital camera, portable MP3 player, USB.
Volatile memory: Non permanent. RAM
Single in line memory module, Dual in line memory module

Factors Affecting Processing Speed

• Registers
• RAM
• The System Clock
• The Bus
• Cache Memory

Factors Affecting Processing Speed – Registers

•The CPU contains a number of small memory areas, called registers, which store data and instructions while the CPU processes them.
•The size of the registers (also called word size) determines the amount of data with which the computer can work at a one time.
•Today, most PCs have 32-bit registers, mean the CPU can process four bytes of data at one time. Register sizes are rapidly growing to 64 bits.

Factors Affecting Processing Speed – RAM

•The amount of RAM in a PC has a direct affect on the system's speed.
•The more RAM a PC has, the more program instructions and data can be held in memory, which is faster than storage on disk.
•If a PC does not have enough memory to run a program, it must move data between RAM and the hard disk frequently. This process, called swapping, can greatly slow a PC's performance.
•Process of swapping unused content of RAM to hard disk is called Virtual memory

The System Clock

•A single "tick" of the clock is the time required to turn a transistor off and back on. This is called a clock cycle.
•Clock cycles are measured in Hertz (Hz), a measure of cycles per second. If a computer has a clock speed of 300 MHz, then its system clock "ticks" 300 million times every second.
•The faster a PC's clock runs, the more instructions the PC can execute each second.

The Bus

•A bus is group of parallel wires. Each wire can transfer one bit at a time.
•Data bus is an electrical path between the components of a computer. Data and instructions travel along this bus.
•The data bus' width determines how many bits can be transmitted between the CPU and other devices.

•The address bus runs only between the CPU and RAM, and carries nothing but memory addresses for the CPU to use.
•Peripheral devices are connected to the CPU by an expansion bus.

Cache Memory

•Cache memory is high-speed memory that holds the most recent data and instructions that have been loaded by the CPU.
•Cache is located directly on the CPU or between the CPU and RAM, making it faster than normal RAM.
•CPU-resident cache is called Level-1 (L1) cache. External cache is called Level-2 (L2) cache. Cache present resident as well as external to CPU is called Level-3(L3) cache.
•The amount of cache memory has a tremendous impact on the computer's speed.

X= (A+B) * (C+D)
RISC Processor- Reduced instruction set computing
Instruction set is small and the instruction is simple.
Y=A+B
Z=C+D
X=Y*Z
CISC Processor–Complex instruction set computing
Instruction set is large and the instruction is complex.
X= (A+B) * (C+D)