Computer Hardware

Categories of Computers
Mainframes- Very large powerful computers. These computers are very expensive. They are usually connected to terminals which resemble small computers and allow many people to use the same computer (the mainframe). This is called timesharing The largest and fastest mainframes are known as supercomputers.

Minicomputers- Computers in introduced in 1965. They were smaller and less costly. It was not as powerful as the mainframe but was affordable to small businesses and school districts. They operate 3 to 10 times faster than microcomputers. They are not as popular as they used to be.
Microcomputers- Also known as personal computers (PCs) became popular in the mid 1970s. They were less costly, more powerful and smaller than the minicomputers. Their price allowed everyday people to own a computer. Today, the microcomputer is small enough to fit on a desk or in a briefcase. A notebook or laptop microcomputer is smaller and lighter than desktop microcomputers and can use power from an electrical outlet or rechargeable batteries. Although they are smaller in size notebook computers generally cost more than am equivalent desktop microcomputer because miniature components cost more. Notebook, laptop, and desktop computers are all in this category.

Hardware
Consists of the actual physical, tangible components of the computer. They are divided in 3 basic components: Central Processing Unit, Primary Storage Unit, and Peripheral Devices.

(input) Peripheral Devices ----> (processing) CPU and Primary Storage Unit ---->(output) Peripheral Devices

CPU - The "brain" of the computer. It performs processing functions. It is composed of 2 major components: the control unit and the arithmetic logic unit (ALU). The control unit is in charge of the activities of the CPU. It basically directs or tells the computer what to do. The ALU performs mathematical computations such as *, /, +, -, and < , >, =. The CPU is contained on a single chip called a microprocessor also known as an integrated circuit. The speed of a microprocessor is determined in clock speed, which is measured in megahertz ( MHz), millions of cycles per second. The higher the faster. ex. 100 MHz Pentium

 PRIMARY STORAGE UNIT- is also known as main memory, primary memory, or internal storage. There are two types of chips that take care of a computers internal memory, ROM and RAM chips. ROM is read only memory. ROM is nonvolatile memory because it does not disappear when the computer is turned off. ROM chips store information permanently in the computer's memory, this information supplies the computer with a list of operating instructions. RAM is random access memory. Information on RAM chips can be modified. Users can read, write, and erase its information. RAM is known as volatile memory because it is temporary. When an individual turns off the computer, he loses the information in RAM. RAM, also known as main memory, is the "working memory" of the computer. Its temporary storage. It stores program data while we are working with that program. A program must be loaded into RAM in order to work. Computers typically now come with 16 MB (megabytes) of RAM. The more MB, the better.

PERIPHERAL DEVICES- Devices that are connected to the computer under the microprocessor's control. It includes input devices, output devices, secondary storage, and input/output devices.

INPUT DEVICES
keyboard- is an input device which is similar to a standard typewriter, but it includes extra keys such as the function keys and a numeric keypad.
mouse- an input device. It is usually a palm sized box with two buttons. As you push and pull the mouse along a surface, a pointer moves on the computer screen. It is effective for selecting items.

lightpen- an input device. It looks like a pen attached to the computer by a cable. The user enters input by drawing on the screen.
trackball- an input device. a device which looks like a box with buttons and a ball inside it. The user rolls the ball to move a pointer on the screen.
trackpad- an input device. a pressure sensitive pad with buttons. When you press your finger on it and move it, the pointer moves in the direction you move your finger.
joystick- an input device. a small box with a moving stick and buttons.
touch screen- an input device which lets you touch the screen to make selections.
bar-code reader- input device allows the user to run an optical wand over a barcode and interpret information.
scanner- an input device which transforms images into electronic images.
OUTPUT DEVICES

Printer- an output device. It allows the user to keep permanent records by producing a printout or hardcopy of information. A dotmatrix printer uses tiny dots of ink to produce text and graphics. An inkjet printer sprays tiny dots of ink on a paper to produce text and graphics. A laser printer uses laser beams to bond a substance called toner to the paper.

Monitor- an output device. a tv like screen which uses cathode ray tube technology. Notebook computers use liquid crystal displays. Monitors have higher resolutions than regular television sets.

SECONDARY STORAGE- receives data from RAM and writes it on a storage medium such as a disk. This data can then be read later and sent back to RAM. It provides long term memory. Remember a computer can not execute a program unless it is first copied into RAM.

Hard disk drive- secondary storage. A hard disk drive is usually built in to most computers. It allows for the fastest access to data. It also allows for huge amounts of data to be saved. A hard disk consists of a sealed chamber inside the computer which holds several disk platters. Information is stored, read, and written on these disk platters.

floppy disk drive- secondary storage. A floppy disk drive is usually built into most computers. The actual disk to be used is not built in to the computer. Most computers now use a 3 1/2 inch floppy disk. The floppy disk is inserted in the disk drive and information can then be stored, written, and read from it. A floppy disk actually holds a disk inside its protective plastic jacket. The disk rotates inside the disk drive when it is being accessed.CD-ROM disk drive- secondary storage. compact disc read only memory disk drive. A cd-rom disc can only be read. you cannot write on them. cd-rom discs can hold a great deal of data. They can hold 400 times as much data as a 3 1/2 inch floppy disk.

There are two types of memories: Primary and Secondary. The primary memory or the main memory is part of the main computer system. The processor or the cpu directly stores and retrieves information from it. This memory is accessed by CPU, in random fashion. That means any location of this memory can be accessed by the CPU to either read information from it, or store information in it. The primary memory itself is implemented by two types of memory technologies. The first is called random access memory(RAM) and the other is read only memory(ROM). A more appropriate name for RAM is RWM(read write memory), the CPU can write and read information from any primary memory location implemented using RAM. The other part of primary memory is implemented using ROM which stands for Read Only Memory.

SDRAM
Although the concept of synchronous DRAM has been known since at least the 1970s and was used with early Intel processors, it was only in 1993 that SDRAM began its path to universal acceptance in the electronics industry. In 1993, Samsung introduced its KM48SL2000 synchronous DRAM, and by 2000, SDRAM had replaced virtually all other types of DRAM in modern computers, because of its greater speed.

SDRAM latency is not inherently lower (faster) than asychronous DRAM. Indeed, early SDRAM was somewhat slower than contemporaneous burst EDO DRAM due to the additional logic. The benefits of SDRAM's internal buffering come from its ability to interleave operations to multiple banks of memory, thereby increasing effective bandwidth.

Today, virtually all SDRAM is manufactured in compliance with standards established by JEDEC, an electronics industry association that adopts open standards to facilitate interoperability of electronic components. JEDEC formally adopted its first SDRAM standard in 1993 and subsequently adopted other SDRAM standards, including those for DDR2 and DDR3 SDRAM.

SDRAM is also available in registered memory varieties, for systems that need greater scalability.

As of 2007, 168-pin SDRAM DIMMs are not used in new PC systems, and 184-pin DDR memory has been mostly superseded. DDR2 SDRAM is the most common type used with new PCs, and DDR3 motherboards and memory are widely available, but more expensive than still-popular DDR2 products.

Today, the world's largest manufacturers of SDRAM include: Samsung Electronics Micron Technology Qimonda (formerly Infineon Technologies) and Hynix.

SDRAM timing

The fundamental limit on DRAM speed is the read cycle time, the time between successive read operations to an open row. This time decreased from 10 ns for 100 MHz SDRAM to 5 ns for DDR-400, but has remained relatively unchanged through DDR2-800 and DDR3-1600 generations. However, by operating the interface circuitry at increasingly higher multiples of the fundamental read rate, the achievable CAS latency, the time between supplying a column address and receiving the corresponding data. Again, this has remained relatively constant at 10–15 ns through that last few generations of DDR SDRAM.

In operation, CAS latency is a specific number of clock cycles programmed into the SDRAM's mode register and expected by the DRAM controller. Any value may be programmed, but the SDRAM will not operate correctly if it is too low. At higher clock rates, the useful CAS latency in clock cycles naturally increases. 10–15 ns is 2–3 cycles (CL2–3) of the 200 MHz clock of DDR-400 SDRAM, CL4-6 for DDR2-800, and CL8-12 for DDR3-1600. Slower clock cycles will naturally allow lower numbers of CAS latency cycles.

SDRAM modules have their own timing specifications, which may be slower than those of the chips on the module. When 100 MHz SDRAM chips first appeared, some manufacturers sold "100 MHz" modules that could not reliably operate at that speed. In response, Intel published the PC100 standard, which outlines requirements and guidelines for producing a memory module that can operate reliably at 100 MHz. This standard was widely influential, and the term "PC100" quickly became a common identifier for 100 MHz SDRAM modules, and modules are now commonly designated with "PC"-prefixed numbers (although the actual meaning of the numbers has changed).

SDR SDRAM

64 MB sound memory of Sound Blaster X-Fi Fatal1ty Pro uses two Micron 48LC32M8A2-75 C SDRAM chips working at 133MHz/7.5 ns 8-bit wide
Originally simply known as "SDRAM", Single Data Rate SDRAM can accept one command and transfer one word of data per clock cycle. Typical clock frequencies are 100 and 133 MHz. Chips are made with a variety of data bus sizes (most commonly 4, 8 or 16 bits), but chips are generally assembled into 168-pin DIMMs that read or write 64 (non-ECC) or 72 (ECC) bits at a time.

Use of the data bus is intricate and requires a complex DRAM controller. This is because data written to the DRAM must be presented in the same cycle as a write command, but reads produce output 2 or 3 cycles after the read command. The DRAM controller must ensure that the data bus is never required for a read and a write at the same time.

Typical SDR SDRAM clock speeds are 66, 100, and 133 MHz (15, 10, and 7.5 ns/cycle). Speeds up to 150 MHz were available for overclockers.

SDRAM control signals

All commands are timed relative to the rising edge of a clock signal. In addition to the clock, there are 6 control signals, mostly active low, which are sampled on the rising edge of the clock:

• CKE Clock Enable. When this signal is low, the chip behaves as if the clock has stopped. No commands are interpreted and command latency times do not elapse. The state of other control lines is not relevant. The effect of this signal is actually delayed by one clock cycle. That is, the current clock cycle proceeds as usual, but the following clock cycle is ignored, except for testing the CKE input again. Normal operations resume on the rising edge of the clock after the one where CKE is sampled high.
  Put another way, all other chip operations are timed relative to the rising edge of a masked clock. The masked clock is the logical AND of the input clock and the state of the CKE signal during the previous rising edge of the input clock.
• /CS Chip Select. When this signal is high, the chip ignores all other inputs (except for CKE), and acts as if a NOP command is received.
• DQM Data Mask. (The letter Q appears because, following digital logic conventions, the data lines are known as "DQ" lines.) When high, these signals suppress data I/O. When accompanying write data, the data is not actually written to the DRAM. When asserted high two cycles before a read cycle, the read data is not output from the chip. There is one DQM line per 8 bits on a x16 memory chip or DIMM.
• /RAS Row Address Strobe. Despite the name, this is not a strobe, but rather simply a command bit. Along with /CAS and /WE, this selects one of 8 commands.
• /CAS Column Address Strobe. Despite the name, this is not a strobe, but rather simply a command bit. Along with /RAS and /WE, this selects one of 8 commands.
• /WE Write enable. Along with /RAS and /CAS, this selects one of 8 commands. This generally distinguishes read-like commands from write-like commands.

SDRAM devices are internally divided into 2 or 4 independent internal data banks. One or two bank address inputs (BA0 and BA1) select which bank a command is directed toward.

Many commands also use an address presented on the address input pins. Some commands, which either do not use an address, or present a column address, also use A10 to select variants.