The Computer Systems
By : Prabir Nandi
Computer InstructorKV Lumding
Major Components of a Computer System
Processor (CPU) Runs program instructions
Main Memory Storage for running programs and current data
Secondary Storage Long-term program & data storage (hard disk, CD, etc)
Input Devices Communication from the user to the computer(e.g. keyboard,
mouse) Output Devices
Communication from the computer to the user (e.g. monitor, printer, speakers)
Parts of a PC
System Unit - the metal box that houses the processor, main memory, and secondary storage devices.
Input and output devices - attached to the system unit via a device controller.
The terms "input" and "output" tell you if data flow is into or out of the system unit.
Abbreviated to “I/O”
Computer SystemsSystem Unit
Input device
Input device
Output device
Output device
Output device
The Processor
The processor is the "brain" of the computer system.
Main processor is called the Central Processing Unit (CPU).
A particular computer will have a particular type of processor, such as a Pentium or a SPARC chip.
Co-processors assist the CPU with some of the processing functions. Examples:
Math co-processors handle heavy duty math processing
Graphics coprocessors speed up the display of graphics onto the monitor
Component Interaction
The CPU controls all of the other resources within the system, in order to accomplish a task.
The CPU
The CPU is a silicon chip that contains millions of tiny electrical components.
The CPU’s three main parts are:
Control Unit
Arithmetic Logic Unit (ALU)
Registers
Central Processing Unit (CPU)
Arithmetic / Logic Unit
Registers
Control Unit
Small, fast storage areas for instructions and data
Performs calculations and decisions
Coordinates processing steps
Registers
Registers are small, fast memory within the CPU
Different registers hold different things
instructions and addresses of instructions
data (operands)
results of operations
Special Purpose Registers
Special Purpose Registers contain specific information the CPU needs.
Instruction Register (IR) contains the actual instruction which is currently being executed by the CPU.
The Program Counter (PC) contains the address of the next instruction to be executed by the program.
General Purpose Registers
General Purpose Registers hold:
the operands for arithmetic and logical operations (ie. the values on which the operation will be performed)
the results of such operations
So General Purpose Registers are used for holding and manipulating data used by the CPU
Computer SystemsNot All Processors Are Created EqualNewer machines use specialized processor chips that
have been developed to speed up the processing of data.
CISC ‑ Complex instruction set computer Large instruction set, many formats
RISC ‑ Reduced instruction set computer Small instruction set, single or small variations in
format MPP ‑ Massive parallel processing
Many CPUs working in parallel
MemoryComputer Memory -
millions/billions of on/off charges
Divided into:
Bits 0 or 1
BytesGroups of 8 bitsA byte is the smallest unit of storage.(Can hold one text character)
Words Groups of bits/bytes (8, 16, 32, 64-bits)
Memory
Storage is usually too large to be expressed in bytes or words. Instead we use:
Kilobyte (KB) = 1024 bytes (210 bytes)
Megabyte (MB) = 1024 x 1024 bytes orone million bytes (220 bytes)
Gigabyte (GB) = 1024 x 1024 x 1024 bytes orone trillion bytes (230 bytes)
Terabyte (TB) = 1024 x 1024 x 1024 x 1024 bytesone quadrillion bytes (240 bytes)
Main Memory
Each computer has a specific word size
Word sizes vary from computer to computer.
Word size is an even multiple of a bytes.
Each word within memory can hold either
data or
program instructions
Main Memory
524852495250525152525253525452555256
A word is stored in consecutive memory bytes.
10011010Each memory cell stores a set number of bits (some computers use 8 bits/one byte, others use words)
Each memory cell has a numeric address, which uniquely identifies its location
CPU and Memory CPU can interact with main memory in two ways:
It can write a byte/word to a given memory location.
The previous bits that were in that location are destroyed
The new bits are saved for future use.
It can read a byte/word from a given memory location.
The CPU copies the bits stored at that location and stores them in a CPU register
The contents of the memory location are NOT changed.
Main Memory Characteristics
Very closely connected to the CPU.
Contents are quickly and easily changed.
Holds the programs and data that the processor is actively working with.
Interacts with the processor millions of times per second.
Nothing permanent is kept in main memory.
Secondary Storage Characteristics
Connected to main memory through a bus and a device controller.
Contents are easily changed, but access is very slow compared to main memory.
Only occasionally interacts with CPU.
Used for long-term storage of programs and data.
Much larger than main memory (GBs vs. MBs).
Program Instructions Programs instructions are stored in secondary storage
(hard disks, CD-ROM, DVD).
To process data, the CPU requires a working area
Uses Main Memory
Also called: RAM (random access memory), primary storage, and internal memory.
Before a program is run, instructions must first be copied from the slow secondary storage into fast main memory
Provides the CPU with fast access to instructions to execute.
Instructions An instruction is a sequence of bits. A simple instruction format may consist of an
operation code (op code) and an address or operands.
Op Code Operands / Address
Instructions tell the computer’s CPU what to do.
Instructions The operation code specifies the operation the
computer is to carry out (add, compare, etc)
The operand/address area can store an operand or an address
An operand is a specific value or a register number
An address allows the instruction to refer to a location in main memory
The CPU runs each instruction in the program, starting with instruction 0, using the fetch-decode-execute cycle.
Access to Instructions
The hard disk is too slow to provide instructions to the CPU. So programs are first loaded into main memory, which is much faster.The CPU can then access the instructions more quickly.
Cache Memory
But as CPU speeds became faster, the main memory couldn’t provide the CPU with the instructions at a fast enough rate.
So even faster memory ( cache memory) is now placed between the CPU and main memory to provide the instructions at an quicker rate to the CPU.
Cache Memory
When an instruction or data is accessed from main memory, it is placed in the cache. Second and subsequent use of the same instruction/data will then be faster, since it is accessed directly from the cache.
Primary and Secondary Cache Memory
Most modern CPUs now have a cache memory (L1), on the same silicon wafer as the CPU, to provide the CPU with instructions at the same clock speed as the CPU.
An additional off-the-chip secondary cache (L2) may also interact with the CPU at a slower speed.
User view of Computer Systems
Software that controls the hardware devices
Operating System – - the user interface
Applications e.g. Word, Netscape, etc
How Programs Are Run The operating system presents an interface to the user
(e.g. Windows Desktop) The user double clicks on an icon to run a program (e.g.
Microsoft Word) The operating system copies the program (or at least the first part
of it) from the hard disk into main memory The CPU runs the instructions in the program, and presents the
initial Word screen
Within Word, the user uses the menu to open a document The application software (Word) asks the Operating System to
open the file. The Operating System communicates with the hardware to open
the file on the hard disk.