Voltage drop

 

1. Alternatively referred to as a brownout, dip, sag, voltage reduction, and voltage sag, a voltage drop is any decrease in voltage, typically under one second. Longer reduction in voltage is called undervoltage.

Power line sags often occur during the summer and when too much equipment is plugged into the same circuit. When this occurs your computer could experience such symptoms as the computer locking up, lost data, system crashes, and can also cause hardware in your computer such as the hard drive to fail.

Tip: If your area experience brownouts, we recommend purchasing a SPS or UPS for your computer and other essential equipment to help prevent any damage and corruption.

• Computer power supply help and support.

2. Linux command, see the Linux sag command page for further information about this term.

some common error codes and how to fix them

 Windows Error Codes and How to Fix them

While working with an operating system, it is not uncommon to encounter errors. Some errors that occur frequently on a Windows system are compiled in a common Windows error codes list. It may not be difficult to fix the common Windows errors but for certain errors, some level of expertise or an in-depth knowledge of the Windows OS may be required. A consolidated list of common windows problems and solutions is useful for a user when they encounter an error. A common Windows error codes list provides a quick and a easy reference and also a possible solution to fix a common Windows error.

Common Windows Error Codes and How to Fix them!

Below is a list of the most common error codes that you may face while using Windows and its basic components. Some solutions have been provided for you to try to solve them. You should also note that some require the assistance of a computer technician and you should not try to fix them yourself as they could seriously damage your computer system.

• Error #0

Error Message: Connection Successful!
Solution: Does not require any action.

• Error #2

Error Message: System cannot find specified file
Solution: You should reinstall NCP & DUN

• Error #5

Error Message: Access Denied
Solution:
1. Ensure that you have entered the right username and password in the field labeled "domain" for NT/2k
2. Check that the option "accept any authentication including clear text" is activated

• Error #5

Error Message: Invalid Procedure Call
Solution: Dialup and Network components must be installed again

• Error #6

Error Message: Stack Overflow
Solution:
1. Try cold booting your computer
2. If this does not work, the problem might come from your RAM or the Swap file

• Error #20

Error Message: The system cannot find the specified device
Solution:
1. Ensure that the right modem has been selected
2. If it does not respond, try to reinstall it
3. You can also try to reinstall NCP/DUN/RAS

• Error #71

Error Message: No more connections are allowed
Solution:
1. Someone else might have connected under your account
2. If you have been ghosted on the server, your Internet Service Provider can bump the ghost program off

• Error #380

Error Message: Invalid Proper Value
Solution:
1. Autodial should be pointed to the correct dialler
2. Else, set up the dialler again

• Error #600

Error Message: An operation is pending
Solution:
1. You should first try rebooting your computer
2. Ensure that no other program is using your modem
3. Else, press Ctrl-Alt-Delete and if RNAAPP loads, contact a technician to fix this problem
4. Make sure that your computer is Spyware free

• Error #601

Error Message: The port handle is invalid
Solution:
1. Select the modem's COM port and under the Diagnosis section, open "More Info". You should reboot your computer if you are able to get ATI responses, remove the erroneous init string or enter ATZ as the init
2. You can also reinstall your modem
3. Else press Ctrl-Alt-Delete and if RNAAPP loads, contact a technician to fix this problem
4. Your Dialup Networking should be reinstalled together with its components
5. Your modem might not be working properly. You should then consult your computer manufacturer

• Error #602

Error Message: The port is already open
Solution:
1. Try to reboot your computer
2. Ensure that no other program is using the modem
3. Else press Ctrl-Alt-Delete and if RNAAPP loads, contact a technician to fix this problem
4. Try uninstalling the AOL Adapters in the Network Control panel and reboot your computer. Install your Dialup Adapter again
5. Remove the erroneous init string or enter ATZ as the init
6. If you have Quicken 2000, deactivate the Quicken Download Manager
7. Try installing your modem again
8. Make sure that your computer is Spyware free

• Error #603

Error Message: Caller's buffer is too small
Solution:
1. Ensure that your port is not configured to a low speed
2. In your "Port Settings", ensure that FIFO is not set to low
3. Try reinstalling your Dialup Networking together with its components
4. If you are still having problems, contact your computer manufacturer

• Error #604

Error Message: Wrong information specified.
Solution:
1. Ensure that the are no outages in your area
2. Try to enter the username and password again
3. Set up the dialler again
4. Your Dialup Networking should be installed again together with its components
5. You can also try to reinstall your modem

• Error #605

Error Message: Cannot set port information.
Solution:
1. Ensure that the are no outages in your area
2. Try to enter the username and password again
3. Set up the dialler again
4. Your Dialup Networking should be installed again together with its components
5. You can also try to reinstall your modem

• Error #606

Error Message: The port is not connected.
Solution:
1. Select the modem's COM port and under the Diagnosis section, open "More Info". You should reboot your computer if you are able to get ATI responses, remove the erroneous init string or enter ATZ as the init
2. You can also reinstall your modem
3. Else, press Ctrl-Alt-Delete and if RNAAPP loads, contact a technician to fix this problem
4. Your modem might not be working properly. You should then consult your computer manufacturer if the problem persists.

• Error #607

Error Message: The event is invalid.
Solution:
1. If this problem arises while dialling up to the internet, verify that the settings of the event log have been correctly set
2. Try rebooting your system and ensure that the dialler/network settings have been correctly configured
3. Else, install RAS/DUN again

• Error #608

Error Message: The device does not exist.
Solution:
1. Check that the right modem is set in the dialler
2. Ensure that you are using the right modem drivers by checking the modem diagnostics. If it's not the case, set up the correct drivers.
3. Reinstall the dialler
4. Your modem might not be working properly. Try reinstalling it
5. You should consult your computer manufacturer if the problem persists

• Error #609

Error Message: The device type does not exist.
Solution:
1. Check that the right modem is selected in the dialler
2. Ensure that you are using the right modem drivers by checking the modem diagnostics. If it's not the case, set up the correct drivers
3. Your modem might not be working properly. Try reinstalling it
4. You should consult your computer manufacturer if the problem persists

• Error #610

Error Message: The buffer is invalid.
Solution:
1. Ensure that your port speed is set to low in the modem properties
2. In your "Port Settings", check that FIFO is not set to low
3. Reinstall the dialer again
4. Else, reinstall RAS/DUN
5. You should consult your computer manufacturer if the problem persists

• Error #611

Error Message: The route is not available.
Solution:
1. Try to reboot your computer
2. Ensure that you set the right configurations for the dialer and network settings
3. Else, install the network components again
4. If you are using a LAN connection, consult your system administrator

• Error #612

Error Message: The route is not allocated.
Solution:
1. Reboot your computer
2. Ensure that you set the right configurations for the dialler and network settings

• Error #613

Error Message: Invalid compression specified.
Solution:
1. Try to reboot your computer
2. Else, add &K3 to the extra settings.
3. If this still does not solve the problem, add &K3
4. You can also try to reinstall the modem

• Error #614

Error Message: Out of buffers.
Solution:
1. Ensure that our port speed is not too low
2. Try to install your modem again

• Error #615

Error Message: The port was not found.
Solution: Try to install your modem drivers

• Error #616

Error Message: An asynchronous request is pending.
Solution:
1. Reboot your computer
2. Try to install your modem drivers again

• Error #617

Error Message: The port or device is already disconnecting
Solution:
1. Wait until your modem disconnects properly
2. If this operation is taking too long, reboot your computer

• Error #618

Error Message: The port is not open.
Solution:
1. Reboot your computer
2. Ensure that no other program is making use of the modem

• Error #619

Error Message: The port is disconnected.
Solution:
1. Reboot your computer
2. Ensure that no other program is making use of the modem

• Error #621

Error Message: Cannot open the phone book file.
Solution: Install Blue Frog connection software again

• Error #622

Error Message: Cannot load the phone book file.
Solution: Install Blue Frog connection software again

• Error #623

Error Message: Cannot find the book entry.
Solution: Install Blue Frog connection software again

• Error #626

Error Message: Cannot load a string
Solution:
1. Try to reboot your system
2. Install Blue Frog connection software again

• Error #627

Error Message: Cannot find key
Solution: Install Blue Frog connection software again

• Error #628

Error Message: The port was disconnected
Solution:
1. Install Blue Frog connection software again
2. Try to install your modem drivers again

• Error #629

Error Message: The port was disconnected by the remote machine.
Solution:
1. Install Blue Frog connection software again
2. Try to enter an alternate Access Number
3. Try to install your modem drivers again

• Error #630

Error Message: The port was disconnected due to hardware failure.
Solution:
1. Install Blue Frog connection software again
2. Try to reinstall your modem drivers
3. Try to use an alternate modem
4. Else, replace your current modem with a new one

• Error #631

Error Message: The port was disconnected by the user.
Solution:
1. Ensure that the right Access Number is being dialled
2. Try to reboot your computer

• Error #632

Error Message: The structure size is incorrect.
Solution:
1. Try to reboot your computer
2. Install Blue Frog connection software again

• Error #633

Error Message: The port is already in use or is not configured for Remote Access dialout.
Solution:
1. Try to reboot your computer
2. Else, try to reinstall your modem drivers

• Error #634

Error Message: Cannot register your computer on the remote network.
Solution:
1. Try to check your network settings and modify your computer name
2. Install the network control panel again

• Error #635

Error Message: Unknown Error
Solution:
1. Try to reboot your computer
2. Install Blue Frog connection software again
3. Else try to reinstall your modem drivers

• Error #636

Error Message: The wrong device is attached to the port
Solution: Check in your settings if you have chosen the right modem

• Error #637

Error Message: The string could not be converted
Solution:
1. Try to reboot your computer
2. Install Blue Frog connection software again

• Error #638

Error Message: The request has timed out.
Solution:
1. Try to reboot your computer
2. Install Blue Frog connection software again

• Error #645

Error Message: Internal authentication error.
Solution:
1. Try to reboot your computer
2. Install Blue Frog connection software again
3. Else, try to reinstall your modem drivers

• Error #650

Error Message: The Remote Access server is not responding.
Solution:
1. Check that your connection and network settings have correctly been configured
2. Ensure that the modem has been correctly set up and look for updated drivers
3. Set FIFO buffers to a lower speed and ensure that you have unchecked the option "Only connect at this speed"

• Error #676

Error Message: The line is busy
Solution: Ensure that your Access Number is still active and is correctly dialled

• Error #678

Error Message: There is no answer.
Solution:
1. First of all check that your phone lines are correctly plugged in
2. Ensure that the Access Number is correct
3. Else, try another Access number
4. If you are trying to make use of your connection in a bad weather, this might be the source of the problem. Wait until it calms down and try again
5. Else, install your modem drivers again
6. If the problem persists, you might need to contact your ISP

• Error #680

Error Message: There is no dial tone.
Solution:
1. First of all check that your phone lines are correctly plugged in
2. The cables might be damaged. Replace them and try again
3. Else, install your modem drivers again
4. If the problem persists, you might need to contact your ISP or a technician

• Error #691

Error Message: Access denied because username and/or password is invalid on the domain.
Solution:
1. Ensure that you have entered the right username and password
2. Make sure that your account is active and turned on
3. You might also try to enter another Access Number
4. If the problem persists, you might need to contact your ISP

• Error #692

Error Message: Hardware failure in port or attached device.
Solution:
1. Browse for and remove any Blue Frog init strings
2. Install Blue Frog connection software again
3. Else, install your modem drivers again

• Error #718

Error Message: PPP Timeout
Solution:
1. Try to reboot your computer
2. Ensure that you have entered the right username and password
3. You might try another Access Number
4. Install Blue Frog connection software again
5. Else, install your modem drivers again

• Error #721

Error Message: Remote PPP peer is not responding.
Solution: For Windows 2000/XP/Vista operating systems, ensure that the security setting for the dialler reads "Allow unsecured Password"

• Error #734

Error Message: The PPP link control protocol terminated.
Solution: For Windows 2000/XP/Vista operating systems, ensure that the security setting for the dialer reads "Allow unsecured Password"

• Error #777

Error Message: The connection attempt failed because the modem on the remote computer is out of order.
Solution:
1. Install Blue Frog connection software again
2. Install your modem drivers again
3. Else, install a new modem

• Error #778

Error Message: It was not possible to verify the identity of the server.
Solution:
1. Ensure that you are dialling the correct phone number
2. Verify the connection and Network settings
3. Ensure that you do not have an outage on the phone line
4. Ensure that you have correctly installed your modem and check for updated drivers
5. Set FIFO buffers to a lower speed and ensure that you have unchecked the option "Only connect at this speed"

• Error #779

Error Message: To dial out using this connection, you must use a smart card.
Solution:
1. Ensure that all authorisation is allowed in your connection settings in the Security section
2. Verify that you have correctly set the other dialler settings
3. Else, create the dialler again
4. You might need to use the Smart Card Reader as initially setup by the Network Administrator

• Error #780

Error Message: An attempted function is not valid for this connection.
Solution: Ensure that the dialler, not the one set up for LAN use though, has been properly installed and has the right settings

• Error #781

Error Message: The encryption attempt failed because no valid certificate was found.
Solution: You have to get a valid certificate by registering to the website http://windows.microsoft.com/...

Note: The solution for the following problems is already provided in the error messages. If you are still facing problems with them, either contact your ISP or a technician to fix the problem.

• Error #782

Error Message: Network Address Translation (NAT) is currently installed as a routing protocol, and must be removed before enabling Internet Connection Sharing

• Error #783

Error Message: Internet Connection Sharing cannot be enabled. The LAN connection selected as the private network is either not present, or is disconnected from the network. Please ensure that the LAN adapter is connected before enabling Internet Connection Sharing.

• Error #784

Error Message: You cannot dial using this connection at logon time, because it is configured to use a user name different than the one on the smart card. If you want to use it at logon time, you must configure it to use the user name on the smart card.

• Error #785

Error Message: You cannot dial using this connection at logon time, because it is not configured to use a smart card. If you want to use it at logon time, you must edit the properties of this connection so that it uses a smart card.

• Error #786

Error Message: The L2TP connection attempt failed because there is no valid machine certificate on your computer for security authentication.

• Error #787

Error Message: The L2TP connection attempt failed because the security layer could not authenticate the remote computer.

• Error #788

Error Message: The L2TP connection attempt failed because the security layer could not negotiate compatible parameters with the remote computer.

• Error #789

Error Message: The L2TP connection attempt failed because the security layer encountered a processing error during initial negotiations with the remote computer.

• Error #790

Error Message: The L2TP connection attempt failed because certificate validation on the remote computer failed.

• Error #791

Error Message: The L2TP connection attempt failed because security policy for the connection was not found.

• Error #792

Error Message: The L2TP connection attempt failed because security negotiation timed out.

• Error #793

Error Message: The L2TP connection attempt failed because an error occurred while negotiating security.

• Error #794

Error Message: The Framed Protocol RADIUS attribute for this user is not PPP.

• Error #795

Error Message: The Tunnel Type RADIUS attribute for this user is not correct.

• Error #796

Error Message: The Service Type RADIUS attribute for this user is neither Framed nor Callback Framed.

• Error #797

Error Message: A connection to the remote computer could not be established because the modem was not found or was busy.

• Error #798

Error Message: A certificate could not be found that can be used with this Extensible Authentication Protocol.

• Error #799

Error Message: Internet Connection Sharing (ICS) cannot be enabled due to an IP address conflict on the network. ICS requires the host be configured to use 192.168.0.1. Please ensure that no other client on the network is configured to use 192.168.0.1

• Error #800

Error Message: Unable to establish the VPN connection. The VPN server may be un-reachable, or security parameters may not be configured properly for this connection.
Solution:
1. Check your ADSL line as it is probably not synchronized
2. Verify that your Security Options are correctly set in your connection options
3. Either your Network Card or your modem might not work properly. Contact a technician if the problem persists

• Error #995

Error Message: Operation aborted.

• Error #1382

Error Message: Maximum length exceeded.

• Error #1793

Error Message: Account expired.

• Error #2250

Error Message: Network connection does not exist.

NETWORK

                                                         

(n.) A network is a group of two or more computer systems linked together. There are many types of computer networks, including:

local-area networks (LANs) : The computers are geographically close together (that is, in the same building).

wide-area networks (WANs) : The computers are farther apart and are connected by telephone lines or radio waves.

campus-area networks (CANs): The computers are within a limited geographic area, such as a campus or military base.

metropolitan-area networks MANs): A data network designed for a town or city.

home-area networks (HANs): A network contained within a user's home that connects a person's digital devices.

In addition to these types, the following characteristics are also used to categorize different types of networks:

topology : The geometric arrangement of a computer system. Common topologies include a bus, star, and ring. See the Network topology diagrams in the Quick Reference section of Webopedia.

protocol : The protocol defines a common set of rules and signals that computers on the network use to communicate. One of the most popular protocols for LANs is called Ethernet. Another popular LAN protocol for PCs is the IBM token-ring network .

architecture : Networks can be broadly classified as using either apeer-to-peer or client/server architecture.

Computers on a network are sometimes called nodes. Computers and devices that allocate resources for a network are called servers.

(v.) To connect two or more computers together with the ability to communicate with each other.

A SESSION BORDER CONTROLLER (SBC)

A session border controller (SBC) is a dedicated hardware device or software applicationthat governs the manner in which phone calls are initiated, conducted and terminated on a Voice over Internet Protocol (VoIP) network. Phone calls are referred to as sessions.

An SBC acts a router between the enterprise and carrier service, allowing only authorized sessions to pass through the connection point (border). The SBC defines and monitors the quality of service (QoS) status for all sessions, ensuring that the callers can actually communicate with each other and that emergency calls are delivered correctly and prioritized above all other calls. An SBC can also serve as a firewall for session traffic, applying its own quality of service (QoS) rules and identifying specific incoming threats to the communications environment.

For security reasons, session border controllers are likely to be deployed on both the carrier and enterprise sides of the connection. An SBC on the enterprise side is referred to as an E-SBC.

how RAM(random access memory) works

Random access memory (RAM) is the best known form of computer memory. RAM is considered "random access" because you can access any memory cell directly if you know the row and column that intersect at that cell.

The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially (like a cassette tape). If the data is not in the current location, each memory cell is checked until the needed data is found. SAM works very well for memory buffers, where the data is normally stored in the order in which it will be used (a good example is the texture buffer memory on a video card). RAM data, on the other hand, can be accessed in any order.

Similar to a microprocessor, a memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information -- a 0 or a 1. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.


The capacitor in a dynamic RAM memory cell is like a leaky bucket. It needs to be refreshed periodically or it will discharge to 0. This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.

Memory is made up of bits arranged in a two-dimensional grid.

In this figure, red cells represent 1s and white cells represent 0s. In the animation, a column is selected and then rows are charged to write data into the specific column.

Memory cells are etched onto a silicon wafer in an array of columns (bitlines) and rows (wordlines). The intersection of a bitline and wordline constitutes the address of the memory cell.

DRAM works by sending a charge through the appropriate column (CAS) to activate the transistor at each bit in the column. When writing, the row lines contain the state the capacitor should take on. When reading, the sense-amplifier determines the level of charge in the capacitor. If it is more than 50 percent, it reads it as a 1; otherwise it reads it as a 0. The counter tracks the refresh sequence based on which rows have been accessed in what order. The length of time necessary to do all this is so short that it is expressed in nanoseconds (billionths of a second). A memory chip rating of 70ns means that it takes 70 nanoseconds to completely read and recharge each cell.

Memory cells alone would be worthless without some way to get information in and out of them. So the memory cells have a whole support infrastructure of other specialized circuits. These circuits perform functions such as:
Identifying each row and column (row address select and column address select)
Keeping track of the refresh sequence (counter)
Reading and restoring the signal from a cell (sense amplifier)
Telling a cell whether it should take a charge or not (write enable)

Other functions of the memory controller include a series of tasks that include identifying the type, speed and amount of memory and checking for errors.


Static RAM

Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory . A flip-flop for a memory cell takes four or six transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes up a lot more space on a chip than a dynamic memory cell. Therefore, you get less memory per chip, and that makes static RAM a lot more expensive.

Static RAM is fast and expensive, and dynamic RAM is less expensive and slower. So static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space.

Memory chips in desktop computers originally used a pin configuration called dual inline package (DIP). This pin configuration could be soldered into holes on the computer's motherboard or plugged into a socket that was soldered on the motherboard. This method worked fine when computers typically operated on a couple of megabytes or less of RAM, but as the need for memory grew, the number of chips needing space on the motherboard increased.

The solution was to place the memory chips, along with all of the support components, on a separate printed circuit board (PCB) that could then be plugged into a special connector (memory bank) on the motherboard. Most of these chips use a small outline J-lead (SOJ) pin configuration, but quite a few manufacturers use the thin small outline package (TSOP) configuration as well. The key difference between these newer pin types and the original DIP configuration is that SOJ and TSOP chips are surface-mounted to the PCB. In other words, the pins are soldered directly to the surface of the board, not inserted in holes or sockets.

Memory chips are normally only available as part of a card called a module. You've probably seen memory listed as 8x32 or 4x16. These numbers represent the number of the chips multiplied by the capacity of each individual chip, which is measured in megabits (Mb), or one million bits. Take the result and divide it by eight to get the number of megabytes on that module. For example, 4x32 means that the module has four 32-megabit chips. Multiply 4 by 32 and you get 128 megabits. Since we know that a byte has 8 bits, we need to divide our result of 128 by 8. Our result is 16 megabytes!


Types of RAM

The following are some common types of RAM:




SRAM


Static random access memory uses multiple transistors, typically four to six, for each memory cell but doesn't have a capacitor in each cell. It is used primarily for cache.


DRAM


Dynamic random access memory has memory cells with a paired transistor and capacitor requiring constant refreshing.

FPM DRAM

Fast page mode dynamic random access memory was the original form of DRAM. It waits through the entire process of locating a bit of data by column and row and then reading the bit before it starts on the next bit. Maximum transfer rate to L2 cache is approximately 176 MBps.


EDO DRAM

Extended data-out dynamic random access memory does not wait for all of the processing of the first bit before continuing to the next one. As soon as the address of the first bit is located, EDO DRAM begins looking for the next bit. It is about five percent faster than FPM. Maximum transfer rate to L2 cache is approximately 264 MBps.


SDRAM

Synchronous dynamic random access memory takes advantage of the burst mode concept to greatly improve performance. It does this by staying on the row containing the requested bit and moving rapidly through the columns, reading each bit as it goes. The idea is that most of the time the data needed by the CPU will be in sequence. SDRAM is about five percent faster than EDO RAM and is the most common form in desktops today. Maximum transfer rate to L2 cache is approximately 528 MBps.

DDR SDRAM

Double data rate synchronous dynamic RAM is just like SDRAM except that is has higher bandwidth, meaning greater speed. Maximum transfer rate to L2 cache is approximately 1,064 MBps (for DDR SDRAM 133 MHZ).

RDRAM

Rambus dynamic random access memory is a radical departure from the previous DRAM architecture. Designed by Rambus, RDRAM uses a Rambus in-line memory module (RIMM), which is similar in size and pin configuration to a standard DIMM. What makes RDRAM so different is its use of a special high-speed data bus called the Rambus channel. RDRAM memory chips work in parallel to achieve a data rate of 800 MHz, or 1,600 MBps. Since they operate at such high speeds, they generate much more heat than other types of chips. To help dissipate the excess heat Rambus chips are fitted with a heat spreader, which looks like a long thin wafer. Just like there are smaller versions of DIMMs, there are also SO-RIMMs, designed for notebook computers.
Credit Card Memory: Credit card memory is a proprietary self-contained DRAM memory module that plugs into a special slot for use in notebook computers.


PCMCIA Memory Card

Another self-contained DRAM module for notebooks, cards of this type are not proprietary and should work with any notebook computer whose system bus matches the memory card's configuration.

CMOS RAM

CMOS RAM is a term for the small amount of memory used by your computer and some other devices to remember things like hard disk settings -- see Why does my computer need a battery? for details. This memory uses a small battery to provide it with the power it needs to maintain the memory contents.


VRAM


VideoRAM, also known as multiport dynamic random access memory (MPDRAM), is a type of RAM used specifically for video adapters or 3-D accelerators. The "multiport" part comes from the fact that VRAM normally has two independent access ports instead of one, allowing the CPU and graphics processor to access the RAM simultaneously. VRAM is located on the graphics card and comes in a variety of formats, many of which are proprietary. The amount of VRAM is a determining factor in the resolution and color depth of the display. VRAM is also used to hold graphics-specific information such as 3-D geometry data and texture maps. True multiport VRAM tends to be expensive, so today, many graphics cards use SGRAM (synchronous graphics RAM) instead. Performance is nearly the same, but SGRAM is cheaper.



Memory Modules

The type of board and connector used for RAM in desktop computers has evolved over the past few years. The first types were proprietary, meaning that different computer manufacturers developed memory boards that would only work with their specific systems. Then came SIMM, which stands for single in-line memory module. This memory board used a 30-pin connector and was about 3.5 x .75 inches in size (about 9 x 2 cm). In most computers, you had to install SIMMs in pairs of equal capacity and speed. This is because the width of the bus is more than a single SIMM. For example, you would install two 8-megabyte (MB) SIMMs to get 16 megabytes total RAM. Each SIMM could send 8 bits of data at one time, while the system bus could handle 16 bits at a time. Later SIMM boards, slightly larger at 4.25 x 1 inch (about 11 x 2.5 cm), used a 72-pin connector for increased bandwidth and allowed for up to 256 MB of RAM.

As processors grew in speed and bandwidth capability, the industry adopted a new standard in dual in-line memory module (DIMM). With a whopping 168-pin or 184-pin connector and a size of 5.4 x 1 inch (about 14 x 2.5 cm), DIMMs range in capacity from 8 MB to 1 GB per module and can be installed singly instead of in pairs. Most PC memory modules and the modules for the Mac G5 systems operate at 2.5 volts, while older Mac G4 systems typically use 3.3 volts. Another standard, Rambus in-line memory module (RIMM), is comparable in size and pin configuration to DIMM but uses a special memory bus to greatly increase speed.

Many brands of notebook computers use proprietary memory modules, but several manufacturers use RAM based on the small outline dual in-line memory module (SODIMM) configuration. SODIMM cards are small, about 2 x 1 inch (5 x 2.5 cm), and have 144 or 200 pins. Capacity ranges from 16 MB to 1 GB per module. To conserve space, the Apple iMac desktop computer uses SODIMMs instead of the traditional DIMMs. Sub-notebook computers use even smaller DIMMs, known as MicroDIMMs, which have either 144 pins or 172 pins.

Most memory available today is highly reliable. Most systems simply have the memory controller check for errors at start-up and rely on that. Memory chips with built-in error-checking typically use a method known as parity to check for errors. Parity chips have an extra bit for every 8 bits of data. The way parity works is simple. Let's look at even parity first.

When the 8 bits in a byte receive data, the chip adds up the total number of 1s. If the total number of 1s is odd, the parity bit is set to 1. If the total is even, the parity bit is set to 0. When the data is read back out of the bits, the total is added up again and compared to the parity bit. If the total is odd and the parity bit is 1, then the data is assumed to be valid and is sent to the CPU. But if the total is odd and the parity bit is 0, the chip knows that there is an error somewhere in the 8 bits and dumps the data. Odd parity works the same way, but the parity bit is set to 1 when the total number of 1s in the byte are even.

The problem with parity is that it discovers errors but does nothing to correct them. If a byte of data does not match its parity bit, then the data are discarded and the system tries again. Computers in critical positions need a higher level of fault tolerance. High-end servers often have a form of error-checking known as error-correction code (ECC). Like parity, ECC uses additional bits to monitor the data in each byte. The difference is that ECC uses several bits for error checking -- how many depends on the width of the bus -- instead of one. ECC memory uses a special algorithm not only to detect single bit errors, but actually correct them as well. ECC memory will also detect instances when more than one bit of data in a byte fails. Such failures are very rare, and they are not correctable, even with ECC.


 

How Much RAM Do You Need?



It's been said that you can never have enough money, and the same holds true for RAM, especially if you do a lot of graphics-intensive work or gaming. Next to the CPU itself, RAM is the most important factor in computer performance. If you don't have enough, adding RAM can make more of a difference than getting a new CPU!

If your system responds slowly or accesses the hard drive constantly, then you need to add more RAM. If you are running Windows XP, Microsoft recommends 128MB as the minimum RAM requirement. At 64MB, you may experience frequent application problems. For optimal performance with standard desktop applications, 256MB is recommended. If you are running Windows 95/98, you need a bare minimum of 32 MB, and your computer will work much better with 64 MB. Windows NT/2000 needs at least 64 MB, and it will take everything you can throw at it, so you'll probably want 128 MB or more.

Linux works happily on a system with only 4 MB of RAM. If you plan to add X-Windows or do much serious work, however, you'll probably want 64 MB. Mac OS X systems should have a minimum of 128 MB, or for optimal performance, 512 MB.

The amount of RAM listed for each system above is estimated for normal usage -- accessing the Internet, word processing, standard home/office applications and light entertainment. If you do computer-aided design (CAD), 3-D modeling/animation or heavy data processing, or if you are a serious gamer, then you will most likely need more RAM. You may also need more RAM if your computer acts as a server of some sort (Web pages, database, application, FTP or network).

Another question is how much VRAM you want on your video card. Almost all cards that you can buy today have at least 16 MB of RAM. This is normally enough to operate in a typical office environment. You should probably invest in a 32-MB or better graphics card if you want to do any of the following:

• Play realistic games

• Capture and edit video

• Create 3-D graphics

• Work in a high-resolution, full-color environment

• Design full-color illustrations

application modernization

Application modernization is the refactoring
, re-purposing or consolidation of legacy software programming to align it more closely with current business needs.
The goal of an application modernization project is to create new business value from existing applications. An application is a program designed to perform a specific function directly for the user or, in some cases, for another application program. Keeping legacy applications running smoothly can be a time-consuming, resource-intensive process, especially when the software becomes so outdated that it becomes incompatible with newer versions of the underlying operating system (OS) or system hardware.
Traditional methods for modernizing applications include rewriting existing application code written in COBOL to a more modern, Web-friendly programming language or placing a Web interface in front of an outdated application to salvage parts of the application that might still have value. The challenges in modernizing legacy applications come mostly from the fact that, in many legacy applications, the business process workflow is hardcoded and tightly coupled with other aspects of the legacy code

PARAVIRTUALIZATION

 Paravirtualization is an enhancement of virtualization technology in which a guest OS is recompiled prior to installation inside a virtual machine. Paravirtualization allows for an interface to the virtual machine that can differ somewhat from that of the underlying hardware. This capacity minimizes overhead and optimizes system performance by supporting the use of virtual machines that would be underutilized in conventional or full virtualization.
The main limitation of paravirtualization is the fact that the guest OS must be tailored specifically to run on top of the virtual machine monitor (VMM), the host program that allows a single computer to support multiple, identical execution environments. However, paravirtualization eliminates the need for the virtual machine to trap privileged instructions. Trapping, a means of handling unexpected or unallowable conditions, can be time-consuming and can adversely impact performance in systems that employ full virtualization.
Paravirtualization is an expansion of a technology that has existed for years in the IBM OS known as VM. Xen, an open-source software project, incorporates paravirtualization.