The system unit of a PC is the case that houses processing hardware and other hardware.
All of the hardware contained within the system unit is connected to the system board or motherboard.
Memory:
RAM Random Access Memory) is the computer’s main memory and is used to temporarily storage
programs and data with which it is working. RAM is volatile (erased when the power to the PC goes off).
RAM comes in a variety of types, speeds, and size. Memory speed is typically measured in megahertz.
Virtual Memory: look at RAM for areas that have not been used recently and copy them onto the hard disk.
This frees up space in RAM to load the new application
ROM: Read Only Memory. Non-volatile chips inside which data or programs are stored. It can be
programmed with programmed with specific data when it is manufactured
Data Processing:
CPU:
The CPU (central processing unit) does the vast majority of processing for a computer. Major operations
include: comparison, arithmetic operations etc.
CPUs contain a variety of circuitry and components and are connected to the motherboard.
Also called the processor or microprocessor.
Examples: Intel’s Pentium, Celeron and AMD’s Athlon etc.
Processing Speed:
It indicates how many ticks of the system clock occur per second. Processing speed (clock speed) is
measured in megahertz (MHz) or gigahertz (GHz) and measures the number of clock ticks per second.
Pentium4’s processing speed can be 3.06 GHZ.
Word Size:
A computer word is a group of bits or bytes that a CPU can manipulate at one time. 64-bit words means
data move around/within the CPU and from CPU to memory in 64-bit chunks.
Cache memory fast memory chips located on or close to the CPU chip.
If your computer's CPU had to constantly access the hard drive to retrieve every piece of data it needs, it
would operate very slowly. When the information is kept in memory, the CPU can access it much more
quickly. Most forms of memory are intended to store data temporarily.
As you can see in the figure bellow, the CPU accesses memory according to a distinct hierarchy. Whether it
comes from permanent storage (the hard drive) or input (the keyboard), most data goes in random access
memory (RAM) first. The CPU then stores pieces of data it will need to access, often in a cache, and
maintains certain special instructions in the register
CPURegisterCache (level1, level2)RAMPhysical Devices (Hard drive, floppy diskette)
Figure: CPU-Memory-Access
Registers: Registers are temporary memory units that store words. The registers are located in the
processor, instead of in RAM, so data can be accessed and stored faster.
Data Transmission:
Buses: Buses Connects the CPU to peripheral devices. There are verities of bus standards: ISA, PCI, AGP,
USB, FireWire etc.
CPU Chipset: It bridges various buses to the CPU.
Figure: Buses
Ports:
Connectors to which devices can be attached.
Common ports: Keyboard, Mouse, Monitor, Modem. Refer to page 109 figure.
Typical CPU Components:
ALU, Control Unit, Registers, Prefetch Unit, Decode Unit, Internal Cache
ALU: Arithmetic and Logic Unit: the smart part of a processor chip that performs commands like adding,
subtracting, multiplying and dividing
Control Unit: Based on instructions from the Decode Unit, it creates control signals that tell the Arithmetic
Logic Unit (ALU) and the Registers how to operate, what to operate on, and what to do with the result. The
Control Unit makes sure everything happens in the right place at the right time.
Decode Unit: it decodes or translates complex machine language instructions into a simple format
understood by the Arithmetic Logic Unit (ALU) and the Registers. This makes processing more efficient.
Refer to page 116 for CPU cycle.