COMPUTER MEMORY

COMPUTER MEMORY
device that is used to store data or programs (sequences of instructions) on a temporary or permanent basis for use in an electronic digital computer. Computers represent information in binary code, written as sequences of 0s and 1s. Each binary digit (or "bit") may be stored by any physical system that can be in either of two stable states, to represent 0 and 1. Such a system is called bistable. This could be an on-off switch, an electrical capacitor that can store or lose a charge, a magnet with its polarity up or down, or a surface that can have a pit or not. Today capacitors and transistors, functioning as tiny electrical switches, are used for temporary storage, and either disks or tape with a magnetic coating, or plastic discs with patterns of pits are used for long-term storage

There are several different types of memory:

·  RAM (random-access memory): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.

·  ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.

·  PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.

·  EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.

·  EEPROM (electrically erasable programmable read-only memory): An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge. 

      RAM (random access memory)

RAM (random access memory) is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be quickly reached by the computer's processor. RAM is much faster to read from and write to than the other kinds of storage in a computer, the hard disk, floppy disk, and CD-ROM. However, the data in RAM stays there only as long as your computer is running. When you turn the computer off, RAM loses its data. When you turn your computer on again, your operating system and other files are once again loaded into RAM, usually from your hard disk.

TYPES OF MEMORY

Dual Channel

Some memory controllers offer a dual channel for the memory. The memory modules are used in pairs to achieve higher bandwidth and thus make the best use of the system's capacity. When using the Dual Channel, it is vital to use identical modules in a pair (same frequency and capacity and preferably the same brand).

Memory Rating Guide

  One of the biggest things that confuse people these days is the good old PC rating on DDR memory. Even worse, most people don't realize that any memory over PC3200 hasn't been JEDEC (Joint Electron Device Engineering Council) approved. Even though PC3500 or PC4400 hasn't been official accepted, it is usually quite capable of running at those specs. However, because there is noting regulating this memory, you're Mom & Pop's PC4400 or other non standard memory may actually be overclocked PC3200. The bottom line is to always buy memory from a trusted source.
     Below is a chart that clearly shows a breakdown of the PC rating to DDR speed to actual bus speed. I hope this helps!

 

DDR Memory Chart

DDR Memory Table: specifications of DDR desktop memory
	Type	Speed	Bus Speed
DDR	PC2100	266Mhz	133MHz
DDR	PC2700	333MHz	166MHz
DDR	PC3200	400MHz	200MHz
DDR	PC3500	433MHz	216MHz
DDR	PC3700	466MHz	233MHz
DDR	PC4000	500MHz	250MHz
DDR	PC4200	533MHz	266MHz
DDR	PC4400	550MHz	275MHz
DDR	PC4500	566MHz	283MHz
DDR	PC4800	600MHz	300MHz
DDR	PC5000	625MHz	313MHz

 

DDR2 Memory Chart

DDR2 Memory Table: specifications of DDR2 desktop memory
	Type	Speed	Bus Speed
DDR2	PC2-4200	533MHz	266MHz
DDR2	PC2-5400	667MHz	333MHz
DDR2	PC2-6000	750MHz	375MHz
DDR2	PC2-6400	800MHz	400MHz
DDR2	PC2-8000	1000MHz	500MHz
DDR2	PC2-8800	1100MHz	550MHz
DDR2	PC2-9000	1120MHz	560MHz

DDR3 Memory Chart

DDR3 Memory Table: specifications of DDR3 desktop memory
	Type	Speed	Bus Speed
DDR3	PC3-6400	800MHz	400MHz
DDR3	PC3-8500	1066MHz	533MHz
DDR3	PC3-10666	1333MHz	667MHz
DDR3	PC3-12800	1600MHz	800MHz
DDR3	PC3-14400	1800MHz	900MHz
DDR3	PC3-16000	2000MHz	1000MHz

 

Form Factors: Computer memory modules and pin configurations

When referring to computer hardware, a form factor is a specification of physical dimensions, layouts, and other explicit information that helps ensure the hardware works with products that support that form factor. Form factors help prevent incompatibilities between multiple hardware manufacturers.

·  SIMM (single in-line computer memory module). This early form factor for computer memory is typically seen only in legacy systems. The first SIMM modules had 30 pins and transferred data 8 bits at a time. As 32-bit computing developed, wider 72-pin SIMMs were developed that could transfer data at 32 bits.

·  DIMM (dual in-line computer memory module). DIMMs look similar to SIMMs, except that the DIMMs are longer (168 pins), and the pins on the opposing sides of the computer memory module are electrically isolated from each other. The 168-pin DIMMs transfer data at 64 bits at a time and are commonly found in Pentium and Athlon systems. Newer DDR DIMMs have 184 pins. DDR2 DIMMs may have 184 or 240 pins.

·  RIMM (a trademarked name for the 184-pin Direct Rambus Memory Module). These modules look similar to DIMMs but generate more heat because they provide faster access and transfer speeds. RIMMs come with an aluminum heat spreader that covers the module. Despite having the same number of pins (184) as a DDR DIMM, a RIMM can only be used on motherboards or systems specifically designed for RIMMs.

·  SO DIMM (small outline) and MicroDIMM. Laptops commonly use

 AUXILARY

RAM module formats

There are many type of random access memory. They exist in the form of memory modules that can be plugged into the mother board.

Early memories existed in the form of chips called DIP (Dual Inline Package). Nowadays, memories generally exist in the form of modules, which are cards that can be plugged into connectors for this purpose. There are generally three types of RAM module:

• modules in SIMM format (Single Inline Memory Module): these are printed circuit boards with one side equipped with memory chips. There are two types of SIMM modules, according to the number of connectors:
• SIMM modules with 30 connectors (dimensions are 89x13mm) are 8-bit memories with which first-generation PCs were equipped (286, 386). 
  ·          
  o    SIMM modules with 72 connectors (dimensions are 108x25mm) are memories able to store 32 bits of data simultaneously. These memories are found on PCs from the 386DX to the first Pentiums. On the latter, the processor works with a 64-bit data bus; this is why these computers must be equipped with two SIMM modules. 30-pin modules cannot be installed on 72-connector positions because a notch (at the centre of the connectors) would prevent it from being plugged in.
  
  
  
  • modules in DIMM format (Dual Inline Memory Module) are 64-bit memories, which explains why they do not need pairing. DIMM modules have memory chips on both sides of the printed circuit board and also have 84 connectors on each side, giving them a total of 168 pins. In addition to having larger dimensions than SIMM modules (130x25mm), these modules have a second notch to avoid confusion.
    
     It may be interesting to note that the DIMM connectors have been enhanced to make insertion easier, thanks to levers located either side of the connector.
    Smaller modules also exist; they are known as SO DIMM (Small Outline DIMM), designed for portable computers. SO DIMM modules have only 144 pins for 64-bit memories and 72 pins for 32-bit memories.
    • modules in RIMM format (Rambus Inline Memory Module, also called RD-RAM or DRD-RAM) are 64-bit memories developed by Rambus. They have 184 pins. These modules have two locating notches to avoid risk of confusion with the previous modules.

     

    Pictures of Selected RAM Form Factors 

  • SIMM (Single-Inline MemoryModule). Used mainly in older computers; now considered obsolete. Its contacts were along only one face of the edge, hence the word "single."
    
    
    DIMM (Dual-Inline Memory Module). The most common RAM form factor in use today.
    SDRAM and DDR-SDRAM are examples of two types of RAM using the DIMM form factor.
    
    
    
    
    
    
    

    Parity and Ecc Memory

    A parity bit is a bit, with a value of 0 or 1, that is added to a block of data for error detection purposes. It gives the data either an odd or even parity, which is used to validate the integrity of the data.
    Parity bits are often used in data transmission to ensure that data is not corrupted during the transfer process. For example, every 7 bits of data may include a parity bit (for a total of 8 bits, or one byte). If the data transmission protocol is set to an odd parity, each data packet must have an odd parity. If it is set to even, each packet must have an even parity. If a packet is received with the wrong parity, an error will be produced and the data will need to be retransmitted.
    The parity bit for each data packet is computed before the data is transmitted. Below are examples of how a parity bit would be computed using both odd and even parity settings.
    

    Odd parity: Initial value: 1010101 (four 1s) Parity bit added: 1 Transmitted value: 10101011 Result: Odd parity (five 1s)

    Even parity: Initial value: 1010101 (four 1s) Parity bit added: 0 Transmitted value: 10101010 Result: Even parity (four 1s)

    The value of the parity bit depends on the initial parity of the data. For example, the binary value 10000000 has an odd parity. Therefore, a 0 would be added to keep the parity odd and a 1 would be added to give the value an even parity.
    While parity checking is a useful way validating data, it is not a foolproof method. For instance, the values 1010 and 1001 have the same parity. Therefore, if the value 1010 is transmitted and 1001 is received, no error will be detected. This means parity checks are not 100% reliable when validating data. Still, it is unlikely that more than one bit will be incorrect in a small packet of data. As long as only one bit is changed, an error will result. Therefore, parity checks are most reliable when using small packet sizes.
    

    ECC modules

    ECC (Error Correction Coding) memory modules are memories with several bits dedicated to error correction (they are known as control bits). These modules, used mainly in servers, allow detection and correction of errors.
    Installing, Upgrading or Replacing Memory
    1. When you upgrade your computer memory, make sure that you install the correct type of memory module for your computer. Your computer uses SIMM, DIMM, or RIMM memory.
    2. Preventing static electricity discharge
    3. Installing or replacing RIMM memory
    
    If your computer uses RIMM system memory, you need to use a combination of Rambus Interface Memory Modules (RIMMs) and Continuity-RIMMs (C-RIMMs). All memory banks must be filled with either a RIMM or a C-RIMM.
    4. Check Documentation and WEB
    5. BANK POPULATION
    6. MATCH
    - SPEED, CHARACTERISTICS, BRAND

    PC memory problems: how to diagnose them

    Memory leak

    A memory leak, in computer science (or leakage, in this context), occurs when a computer program consumes memory but is unable to release it back to the operating system. In object-oriented programming, a memory leak happens when an object is stored in memory but cannot be accessed by the running code.^[1] A memory leak has symptoms similar to a number of other problems (see below) and generally can only be diagnosed by a programmer with access to the program source code; however, many people refer to any unwanted increase in memory usage as a memory leak, though this is not strictly accurate from a technical perspective.
    Because they can exhaust available system memory as an application runs, memory leaks are often the cause of or a contributing factor to software aging.
    
    1. Computer does not boot, instead you get a beep code. See the POST and beep code page for additional information about beep codes.
    
    
    2. Random computer crashes causing BSOD, General Protection Fault error messages, Illegal Operations, Fatal Exceptions, etc.
    
    
    3. Computer random reboots.
    
    
    4. Installing Windows or another program fails.
    SO-DIMM (Small-Outline Dual-Inline Memory Module). Used primarily in laptop computers and other compact computing devices.
    
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