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The processor is the main brain of the computer. It rests on the motherboard and consists of millions of transistors arranged ina logical circuit. The processor cycle simplified is reading an instruction from a program loaded into memory, interpreting it, doing the required calculations, and then incrementing the program counter and then starting over again. It also controls all the parts of the PC by processing the data from them and sending command to them via buses.


[ a very simplified circuit diagram of the processor and memory on the motherboard ]

The processor used to do all the number cruching operations, but modern procesors come with a math co-processor or ALU that process all the floating point operations separately from the processor, which greatly frees up the processor resources. This greatly improves the system performance.

Another idea critical to understanding the processor is the system bus. The system bus is a bus that spans the entire motherboard. The processor, the memory, the devices slotted into the motherboard sockets (such as the graphics card) are connected via the system bus. These components send data to and from each other via the processor. All these things are also connected by the system clock. The signal from the quartz crystal sets the tempo for all these devices to run at so that they can synchronise their signalling (all the devices run at or at multiples of this base frequency).

The time it takes for the processor to excecute one cycle (i.e. carry out one instruction) is it's cycle time. The speed of the processor is usually measured in terms of cycles per second, measured in megahertz, and now gigahertz. The speed of proccesors have been increasing exponentially (predictably doubling every 18 months , Gordon Moore). The Pentium II manufactured by Intel, which is about 10 years old now, ran at a speed of 233MHz. Now the fastest Pentium 4 processor runs at 3.4GHz. However, we must understand that clockspeed is only an accurate comparison of processing power between different models from the same manufacturer (like pentium 3 and pentium 4). However one cannot compare let's say an AMD processor to an Intel processor. For example my AMD Athlon 2800XP runs at around 2.1GHz but its processing power is equivalent to a 2.8GHz Pentium 4. That's the reason why AMD cleverly names its processors numerically that suggest their equivalence to Intel processors.

System memory comes on separate chips that slot onto the motherboard. They are sometimes referred to as DRAM. All programs, operating systems and system files are loaded into memory and the processor reads individual instructions from this memory, and outputs it data to the memory too. All the different caches are also memory, but these are SRAM. They are way more expensive than DRAM, because they no need to be constantly "refreshed" like DRAM does, but are much faster, and thus are used in samll amounts at the critical bits. They work by being the "hotlist" of instructions in a program that the processor executes. Caching works on the basis that 20% of the instrucions are accessed 80% of the time. By predicting the frequent instructions, the execiution of them can be critically sped up by the caches. Cache also comes in different levels. The highest instruction on the hotlist are stored in the Level 1 cache, and the lowest in L2 and L3 cache. The speed of accessing the cache is limited by the bus length, and this is why the most frequently accessed is closest to the proceesor.

The data sroed in RAM is lost each time the computer is turned off. Thus, certain information like the BIOS settings need to be stored even while the power is off. This results in a need for Read-Only Memory.
Evolution of read-only memory:
ROM --> PROM --> EPROM --> EEPROM --> flash
ROM: This memory cannot be modified at all. Its contents are static from point of manufacture.
PROM: This is a special type of ROM that can be written after manufacture, but only once, using a PROM burner, which effectivley blows the fuses in it.
EPROM: Same as PROM, but its contents can be erased by exposure to ultraviolet light. Easily recognisable by a shiny window present on its surface.
EEPROM: Like EEPROM, but does not need to be removed to be reprogrammed. However, like PROM and EPROM, it also need to be erased entirely before being rewritten. Also, the lifetime of EEPROM has to be considered as the number of times it can be reprogrammed is limited.
Flash: Like EEPROM, but can be reprogrammed at lower voltages and unlimitedly. It is also much faster, as its contents are written in blocks at a time, so not all it contents need to be written if only some of it changes. Flash memory was typically used in BIOS chips. Now, improved versions of it are being used in other devices such as thumb drives and digital cameras.

What kind of a proccesor you use is really dependant on your needs. Processor speed is about proportional to your system performance. So getting a faster processor would make your computer faster. But getting the fastest processor does not guarantee you the fastest machine. You must also consider the limiting factors such as the motherboard's maximum capacity and the speed and latenmcy of the system memory.

Another factor for choice of processor is the actual thing you intend to do with your computer. If you are going to use your computer for word processing only, a slow processor would suffice. For those who want to process large amounts of data (such SETI, which processes data from satellite dishes), you need supercomputers with very powerful ALUs and large amounts of L1 and L2 cache. For a serious gamer, he may consider it more worthwhile to invest more in a more powerful graphics card than a more powerful processor.

 

Hard Disks

Hard drives have a controller and a hard drive BIOS built right on the drive. These control the read/write operations and perform other functions like interpreting the positioning of data to the system BIOS.

[diagrammatic representation of a hard disk]

5 main components

Platters - disks upon which data is stored. The surface is formatted into sectors and tracks where date is written.
Spindle - spins the platters
Read/Write Heads - they move across the platters to write and read data from the platters. There's a read/write head for each side of each platter with random access. When drive is powered off, the heads rest on the landing zone (LZ) which never contain any data
Head Actuator - Controls the read/write heads.
Circuit board - Receives commands from the hard drive controller and translates them in order to move the head actuator, which moves the read/write head across the platters to the required position.

Organization

Tracks - concentric circles that spins directly under the head. There is the same
number of tracks on both sides of each platter.
Cylinders - read/write heads all move on the actuator arm together, thus they are
positioned over the same track simultaneously. The set of tracks moving under the heads makes up a cylinder.
Sectors - Each track is divided up into 512 byte blocks called sectors. Data written to the drive is stored in these sectors a cluster at a time.
Clusters - A defined number of sectors make up a cluster. The number of sectors varies depending on the size of the HD and how it's partitioned.

Compact Disc

The basic CD-ROM is 12cm in diameter and is basically a 1.2mm sandwich of three coatings: a back layer of clear polycarbonate plastic, a thin sheet of aluminium and a lacquer coating to protect the disc. Nowadays CDs can hold up 750 MB worth of data.
To read the disc, the drive shines a laser onto the CD-ROM's surface by interpreting the way in which the laser light is reflected.CD manufacturing companies have been researching on new substrates to use on the surface of CDs, which the data can be modified by the consumer.

This first resulted in CD-Rs with which data can be written once and read many times. Later on, more complex substrates were manufactured, leading to the production of CD-RWs, which can be written many times. To ensure compatibility among different CD manufacturers, in the interest of consumers, a protocol called the Orange Book II standard was implemented industry-wide in 1990. CD-RW drives are now commonplace among today's computers as this technology has proved very popular with user.

Further research resulted in the creation of the DVD. The substrate in the DVD contains more bytes per square centimetre than CDs. Targeted at the home entertainment market, they are better than the VHS tapes as they are less bulky and cheaper to produce. Also, they have more storage capacity than VCDs, allowing them to have higher resolution images. However, the DVD industry has not adopted a common standard (unlike CDs), leading to different formats This is very consumer-unfriendly.
Nonetheless the DVDs ability to store entire movies on just 1 or 2 discs proved to be very useful. Recently developed, double-layered DVDs allow approximately 10GB of data to be stored, increasing the popularity of DVD technology.

Removable Storage

3.5 inches floppy disks have a capacity of 1.44 MB in storage, and uses open loop tracking. They simply order the head to move to the "correct" position. The low cost of the floppy disk and the ease of deleting and storing files that are small in size ensured its continuity amidst the rise of CDs and other storage devices.

Hard disks complements, with 500 MB to 1 GB storage capacity, have a good performance functioning as secondary, if slow, hard disk. An exception is the Castle ORB drive, which uses a new technology, allowing 2.2GB to be stored with a data transfer rate of 12.2 mbps.

Zip disks still use a magnetic medium, and have a capacity between 100-300 MB, providing greater storage than floppy disks. However, when the disk fails to read, it makes repetitive clicking noises, hence they are not very useful, and they faded out as thumb drives came along.

Thumb drives uses flash memory, allowing data to be copied n erased with ease. Furthermore, they only require a USB port to be read. With storage capacities of 64MB, 128MB and 256 MB. The cost per megabyte of storage is constantly decreasing and is readily becoming the preferred mode of storage media due to its compactness and low cost.