Saifurs Spoken Vocabulary.

Lesson 5 : Memory

5.1. Learning Objectives
On completion of this lesson you will be able to describe :
i) Different memory hardware
ii) Different types of memory organization (i.e., SRAM, DRAM, SDRAM).

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5.2. Introduction

The PC must have main memory. Main memory is high-speed memory that the CPU can read from or write to. "High speed" means, less than a microsecond to read/write. The other name for such memory is RAM, Random Access Memory-a particular kind of chip on circuit boards.

5.3. Different Memory Hardware

Memory is easy to pick out on a circuit board. It's packaged either as a "bank" of eight or nine small chips, or it's a mini-circuit board with several square chip mounted on it, called a SIMM (Single Inline Memory Module). Memory is always organized into banks either eight or nine discrete chips, or a SIMM. Most motherboards have room for four banks of memory. As each SIMM is the equivalent of nine chips built in single independent card, SIMMs make replacing bad memory easier.

RAM chips are distinguished by their following characteristics:

i) Package type, which can be:
ii) DIPs, which are normal looking chips. DIP means Dual
Inline Package. It is just a simple IC with two sets of legs inline. These ICs are directly inserted in sockets on the motherboard. Laptop and old 8088 to 386 computers use this package.
i) Single Inline Pin Packages, or SIPPs (a small circuit board with pins across its bottom).
ii) Single and Double Inline Memory Modules (SIMMs and DIMMs respectively), which are small circuit boards with an edge connector across the bottom.
iii) PCMCIA cards. This package is used in laptop computers. In this package the memory is built in a card which is inserted in the PCIMCIA slots of laptop computers.
iv) Various proprietary vendor-specific chip packages.
v) Access speeds, measured in nanoseconds. Common access times are 60, 70, 80, 90, and 100ns for normal memories, and as low as 8 ns for expensive high-speed memories. Lower numbers are faster.

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5.4. Static Versus Dynamic RAM

As computers are built faster and faster, all of the components must get faster and faster. Design a system around a 100 MHz chip, and you need a lot of 100 MHz components including RAM. Unfortunately, about the fastest type of common (that is, cheap) RAM doesn't come any faster than 20 MHz.

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5.4.1. Static RAM

The simplest kind of memory to understand is called static RAM (SRAM). It's called "static" RAM because when you put data into it, the data stays there.

To build that kind of memory, you've got to build about six transistors into each bit storage location. That kind of memory-SRAM-can bc quite fast, but also quite expensive. If we used SRAM for our PC memory, then there'd be no trouble with getting memories that kept up with our CPUs. But, on the other hand, we would be able to afford those computers, as SRAM is about 10 times more expensive than the DRAM that we're used to buying.

5.4.2. Dynamic RAM

The economical answer to SRAM was Dynamic RAM (DRAM). Each DRAM built of a single transistor and a capacitor, in contrast to SRAM's six transistor. DRAM has two problems from the point of view of a PC designer. First, the "dynamic" in its name means that it forgets the data that you give it almost as you can give it the data. That means that DRAM-based systems require refresh circuitry to get around this problem. Second, the way that DRAM is built to be cheaper is that DRAM is organized not simply into a set of addresses; rather, each bit in a DRAM has a row address and a column address.

5.4.3. SDRAM

Synchronous Dynamic Random Access Memory, (SDRAM) is a new variant of DRAM that includes an on-chip burst counter. This burst counter can be used to increment column addresses and helps increase SDRAM the speed of burst accesses. Aside from the facts that faster is always better and speed is pursued for its own sake, the reason behind the SDRAM is that CPUs are getting faster. With the increasing of speed, they demand faster memory, in order to function at Its maximum potential. With SDRAM, the CPU and RAM are locked together same clock. Thus, the speed of the RAM and the CPU are linked, or synchronized.

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Lesson 6 : Bus Structure

6.1. Learning Objectives

On completion of this lesson you will be able to describe:

i) What is bus structure

ii) Different types of bus structure

iii) Components of different bus.

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6.2. Introduction

Different expansion cards (Display card, Network card etc.) are connected with the motherboard. A card of one company may be connected with a motherboard of another company. So the connection point between the card and the motherboard must be predefined. The predefined structure of connection point is known as the bus standard.

These connectors are also called "expansion slots” as expansion boards must plug into these slots. Some PCs have no slots at all, and so weren't expandable; other machines have three, and most clone-type machines have eight slots. Some machines offer 10 slots.

There are different types of bus structures: PC, ISA, PCI, PCMCIA etc. A card with PCI connection points must be inserted in a PCI expansion slot of the motherboard. Similarly ISA cards are inserted in ISA slots.

6.3. PC Bus

This bus was used in IBM PC. It has 62 lines. These lines are offered to the outside world through a standard connector. Now we shall look at what these 62 lines do.

6.3.1. Data Path

The original PC and XT were based on the 8088 chip. The 8088 had a data path (the "front door") of just 8 bits, so the PC bus only includes eight data lines. That means this bus is "8 bits wide," and so data transfers can only occur in 8-bit chunks on this bus. Expansion slots on a computer with this bus are called "8-bit" slots. Eight of the 62 wires, then, transport data around the computer.

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Lesson 7 : Hard Disk Drive

7.1. Learning Objectives
On completion of this lesson you will be able to describe:
 Components of a hard disk drive
 Working principle of hard disk drive.

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7.2. Hard Disk Drive

Hard disk drive is used to store data and programs. It is one kind of memory. But the information stored in it doesn’t vanish when the computer is off.

7.3. Structure of Hard Disk
 A hard disk consists of many round shape disks called platters.
 A platter consists of many concentric circles called tracks.
 A track is divided in many semicircles called sectors.
 A sector has many bytes.
 A byte has eight bits.
 Data is stored in Bits.

The outer most track is numbered as track-0. On an 80 track platter the inner most track is numbered as track-79. In the figure each of the three platters has a track-0. If we combine all the track-0 of three platters in parallel then it forms a cylinder. Thecylinder formed by combining all track-0s is called cylinder-0. Similarly the cylinder formed by combining all track-1s is called cylinder-1.Heads are used to store and read data from disks. If data is stored on both sides of a platter then two heads are needed for a platter. If there are 3 disks then 6 platters and so on. Heads move from track to track and read data. But all the heads in a hard disk move jointly. That means, if head of first platter is at track-0, then all the other heads are also on track-0 on other platters. Thus all the heads are on cylindar-0. In such a way heads move from cylinder to cylinder.

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