Memory

MEMORY

MEMORY:

Memory is one of the functions of the brain that enables to store and remember the past events. Similarly, in computers the term memory refers to a chip that stores data. It also enables us to retrieve the stored data. The processor retrieves information stored in the memory for processing.

TYPES OF MEMORY:

Memory can be divided into mainly two types:

Volatile memory:

It stores the data temporarily. It loses the data as soon as the system supply is turned off. Most forms of modern Random Access Memory (RAM) are volatile storage, including Dynamic Random Access Memory (DRAM) and Static Random Access Memory (SRAM).

Non-volatile memory:

It stores the data permanently. It does not lose the data even if the system supply is turned off. Non-volatile memory is typically used for the task of secondary storage, or long-term persistent storage. Non-volatile data storage can be categorized in electrically addressed systems (Read Only Memory) and mechanically addressed systems (Hard disks, Optical discs, Magnetic Tapes).

Further, memory is physical, flash and cache memory.

PHYSICAL MEMORY:

Physical memory is the total amount of memory installed in the computer. It is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. For example, if the computer has two 1GB memory modules installed, it has a total of 2GB of physical memory.

THE DIFFERENT TYPES OF PHYSICAL MEMORY ARE:

RAM:

RAM stands for Random Access Memory. It is a semiconductor-based memory where the CPU or the other hardware devices can read and write the data. It temporarily stores and it is a volatile memory. Once the system is turned off, it loses the data. As a result, RAM is used as temporary data storage.

ROM:

ROM stands for Read Only Memory. It stores the data permanently and it is a non-volatile memory. It does not lose the data even after the system turns off. As a result, ROM is a permanent data storage area.

The different types of ROM are:

PROM: It stands for Programmable Random-Only Memory. It is also known as One Time Programmable (OTP) chips. It stores programs permanently and is a non-volatile memory. Programming the ROM is sometimes referred as burning and it requires a special machine called a Device Programmer or ROM Burner.

EPROM: It stands for Erasable Programmable Read-Only Memory. Ultra-violet (UV) rays can remove the programs from this memory. It can be easily recognized by the clear quartz crystal window set on the top of chip. An EPROM eraser is a device that contains a UV light source that erases the chip by causing a chemical reaction, which essentially melts the fuses back together.

EEPROM/Flash ROM: It stands for the Electrically Erasable Programmable Read-Only Memory. Electrical signal removes the programs from this memory. EEPROM can be erased by an electric field, rather than exposed to UV. Also, the data can be erased bit by bit allowing only selected portions of the code to be replaced. This is also called Hybrid memory as it reads and writes data similar to the RAM but, maintains data similar to the ROM. It is a mixture of RAM and ROM.

Fig (i) PHYSICAL MEMORY

FLASH MEMORY:

Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It is primarily used in memory cards, USB Flash Drives, MP3 players and solid-state drives for general storage and transfer of data between computers and other digital products.

It is a specific type of EEPROM (electrically erasable programmable read-only memory) that is erased and programmed in large blocks; in early flash the entire chip had to be erased at once. Flash memory costs far less than byte-programmable EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid state storage is needed.

Example applications include PDAs (personal digital assistants), laptop computers, digital audio players, digital cameras and mobile phones. It has also gained popularity in console video game hardware, where it is often used instead of EEPROMs or battery-powered static RAM (SRAM) for game save data.

Flash memory is non-volatile, meaning no power is needed to maintain the information stored in the chip. In addition, flash memory offers fast read access times (although not as fast as volatile DRAM memory used for main memory in PCs) and better kinetic shock resistance than hard disks. These characteristics explain the popularity of flash memory in portable devices. Another feature of flash memory is that when packaged in a "memory card," it is extremely durable, being able to withstand intense pressure, extremes of temperature, and even immersion in water.

Although technically a type of EEPROM, the term "EEPROM" is generally used to refer specifically to non-flash EEPROM which is erasable in small blocks, typically bytes. Because erase cycles are slow, the large block sizes used in flash memory erasing give it a significant speed advantage over old-style EEPROM when writing large amounts of data.

Some of the latest memory cards include Multimedia Card (MMC), Secure Digital (SD), Compact Flash, Memory Stick, xD-Picture Card and Smart Media

Fig (ii) FLASH MEMORY

CACHE MEMORY:

Cache memory is a small and fast memory which is placed between the CPU and main memory. It is accessed at a very high speed than the system memory. As a result, the programs which access the same data or instructions over and over run faster. The CPU does not have to transverse to the main memory to get the data. It will access the cache to find out the data.

Fig (iii) Working of Cache Memory

The Table 1 shows the different types of memories:

Physical memory	RAM	Volatile memory from where you can read and write data.
	ROM	PROM	Stores programs permanently and is a non-volatile memory.
		EPROM	Programs can be erased from this memory using UV rays.
		EEPROM	Programs can be erased from this memory using electrical signals.
Flash memory	An electrically re-programmable, non-volatile high density device.
Cache memory	A small and fast memory placed between the CPU and main memory.

Table 1: Types of memory

RAM (RANDOM ACCESS MEMORY):

• RAM is the main memory of the computer. It holds the data until the system is turned off. Once the system is switched off, the data is lost. As a result, it is known as the temporary data storage area. There are two types of RAM, Static RAM and Dynamic RAM.
• STATIC RAM (SRAM): SRAM is a type of semiconductor memory. It stores the data as long as the power is supplied to the system.
• Once the power is turned off or is lost temporarily, data stored in SRAM is lost. SRAM uses six transistors for each memory cell. Due to more number of transistors present in the cell, the cells do not refresh frequently. Hence the data is stored for longer period.
• Refreshing a cell means re-writing data in a cell. SRAM is faster in accessing the data. The data accessing speed of SRAM makes it behaves like a cache memory.
• SRAM is as expensive as compared to DRAM. Example of SRAM is all types of cache memory.

DYNAMIC RAM (DRAM):

• The life time of DRAM is very short. It is approximately for 4ms. The data in DRAM are stored in memory cells. Each memory cell contains a pair of a transistor and a capacitor. Each memory cell is referred as a bit of data, the smallest amount of information that the system can work with.
• The memory cells of DRAM are refreshed by the DRAM controller after every few milliseconds to retain the data in the memory.
• The cells in DRAM are arranged in rows and columns. Each cell has a row and column reference number. DRAM access the data using the cell reference number.
• DRAM is less expensive than SRAM.

TYPES OF DRAM:

Extended Data Out DRAM (EDO DRAM):

Extended Data-Out (EDO) DRAM is a type of asynchronous DRAM. It is also known as Hyper Page Mode DRAM. It is faster than normal DRAM. This is because EDO DRAM starts fetching the data from the next cell before the previous process completes. Its approximate data transfer speed is 264 Mega Bytes per second.

Fig (iv): EDO DRAM

SYNCHRONOUS DYNAMIC RANDOM ACCESS MEMORY (SDRAM):

SDRAM synchronizes the memory speed with the CPU clock speed. The speed of the SDRAM depends on the speed of the CPU Bus. It is faster than SRAM, DRAM, EDO RAM and VRAM memories. The data transfer speed of SDRAM is measured in nanoseconds (ns) and megahertz units. It runs with an average speed of 133 MHz. SDRAM used in P-II/PIII, 100/133 MHz of 168 pins with 3.3V operating voltage.SDRAM is widely used in computers; from the original SDRAM, further generations of DDR (or DDR1) and then DDR2 and DDR3 have entered the mass market, with DDR4 currently being designed and anticipated to be available in 2012.

Types	Frequency (MHz)	No. of pins	Operating voltage (volts)
PC100	100	168	3.3
PC133	133	168	3.3

Fig (v): SDRAM

Rambus Dynamic Random Access Memory (RAMBUS):

The first PC motherboards with support for RDRAM debuted in 1999. They supported PC-800 RDRAM, which operated at 400 MHz and delivered 1600 MB/s of bandwidth over a 16-bit bus. It was packaged as a 184-pin RIMM (Rambus in-line memory module) form factor, similar to a DIMM (dual in-line memory module). Data is transferred on both the rising and falling edges of the clock signal, a technique known as double data rate. For marketing reasons the physical clock rate was multiplied by two (because of the DDR operation); therefore, the 400 MHz Rambus standard was named PC-800. This was significantly faster than the previous standard, PC-133 SDRAM, which operated at 133 MHz and delivered 1066 MB/s of bandwidth over a 64-bit bus using a 168-pin DIMM form factor.

Fig (vi): RDRAM

Double Data Rate Synchronous Random Access Memory (DDR-SDRAM):

It is a class of memory integrated circuits used in computers. Compared to single data rate (SDR) SDRAM, the DDR SDRAM interface makes higher transfer rates possible by more strict control of the timing of the electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to reach the required timing accuracy. The interface uses double pumping (transferring data on both the rising and falling edges of the clock signal) to lower the clock frequency. One advantage of keeping the clock frequency down is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller. The name "double data rate" refers to the fact that a DDR SDRAM with a certain clock frequency achieves nearly twice the bandwidth of a single data rate (SDR) SDRAM running at the same clock frequency, due to this double pumping.

With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus, with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.

Fig (vii): DDR-SDRAM

Double Data Rate (DDR2) SDRAM (DDR2-SDRAM):

It supersedes the original DDR SDRAM specification and the two are not compatible. In addition to double pumping the data bus as in DDR SDRAM (transferring data on the rising and falling edges of the bus clock signal), DDR2 allows higher bus speed and requires lower power by running the internal clock at half the speed of the data bus. The two factors combine to require a total of four data transfers per internal clock cycle.

With data being transferred 64 bits at a time, DDR2 SDRAM gives a transfer rate of (memory clock rate) × 2 (for bus clock multiplier) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a memory clock frequency of 100 MHz, DDR2 SDRAM gives a maximum transfer rate of 3200 MB/s.

Fig (viii): DDR2 SDRAM

Double Data Rate (DDR) 3 SDRAM (DDR3-SDRAM):

In computing, DDR3 SDRAM or double-data-rate three SDRAM is a modern kind of dynamic random access memory with a high bandwidth interface. It is one of several variants of dynamic RAM and/or associated interface techniques that have been used since the early 1970s, and it is not directly compatible with any of the earlier types, not even with DDR2 SDRAM. This is due to different signaling voltages, timings, and other factors. The primary benefit of DDR3 SDRAM over its predecessor, DDR2 SDRAM, is the ability to transfer at twice the data rate (8× the speed of its internal memory arrays), enabling higher bandwidth or peak data rates. In addition, the DDR3 standard allows for chip capacities of up to 8 gigabits, thus enabling a memory module size of 16 gigabytes (using 16 chips).

Standard name	Memory clock (MHz)	I/O bus clock (MHz)	Data rate (MT/s)	Module name	Peak transfer rate (MB/s)
DDR3-800D DDR3-800E	100	400	800	PC3-6400	6400
DDR3-1066E DDR3-1066F DDR3-1066G	133	533	1066	PC3-8500	8533
DDR3-1333F DDR3-1333G DDR3-1333H DDR3-1333J	166	667	1333	PC3-10600	10667
DDR3-1600G DDR3-1600H DDR3-1600J DDR3-1600K	200	800	1600	PC3-12800	12800
DDR3-1866J DDR3-1866K DDR3-1866L DDR3-1866M	233	933	1866	PC3-14900	14933
DDR3-2133K DDR3-2133L DDR3-2133M DDR3-2133N	266	1066	2133	PC3-17000	17066

TABLE 2 :DDR3 STANDARD MODULES

Fig (ix): DDR3 SDRAM

Quad Data Rate (QDR) SDRAM:

It is a type of computer memory, more specifically a type of synchronous dynamic random access memory that can transfer four words of data in each clock cycle. Like DDR SDRAM, QDR SDRAM transfers data on both rising and falling edges of the clock signals. QDR has separate read and write ports that can operate simultaneously. This requires a larger number of wires from the memory device to the memory controller, but results in double the theoretical maximum data transfer rate. QDR SDRAM uses two clocks: one for read data and one for write data.

Fig (x): QDR SDRAM

Fully Buffered DIMM (FBD):

A FB-DIMM is a new memory interconnect technology standard for high-end memory connections. FB-DIMM transitions the memory channel to a serial interfaces and replaces the DIMM register with a memory buffer. FB-DIMM connections are expected to enable systems to scale the number of memory channels available to a server systems. Current DIMMs will be unable to handle the memory requirements of the type of high-speed servers that will use DDR2 at speeds of 667 to 800 Mbps.

The interface is point to point fast serial allowing more modules to be attached for a comparable pin count. FBD is 70 pins/channel: DDR2 is 240 pins/channel. In turn, fewer pins mean more channels. More DIMMs/ channel is a result of going serial. The interface does not have a shared bus between modules.

Fig (xi): Fully Buffered DIMM

Video RAM (VRAM):

Video adapter or video system uses VRAM. It stores the images that are to be displayed on the computer screen. Before displaying the image on the computer screen, CPU first reads the image from the memory and then writes it in VRAM. This memory acts as a buffer between the CPU and the video card. It provides more bandwidth than DRAM and EDO DRAM. VRAM is costly as compared to SRAM. VRAM reads and writes the data simultaneously in one process. So that VRAM is known as monolithic RAM. It is a multitasking memory. As a result, VRAM chips are dual-ported.

TYPES OF MEMORY PACKAGES:

Memory package is a small circuit board that contains memory chips. The types of packages are

Single In-line Memory Module (SIMM)

Dual In-line Memory Module (DIMM)

Small Outline Dual In-line Memory Module (SODIMM)

Micro DIMM

Rambus In-line Memory Module (RIMM)

These packages refer the form factor of a RAM chip. The installation of a memory depends on the form factors of RAM. A form factor is the size and shape of the memory packages.

Single In-line Memory Module (SIMM):

SIMM (Single Inline Memory Modules) were first made in 8 bit editions. They were small cards with 1, 2 or 4 MB RAM. They were connected to the motherboard with a 30 pin edge connector. The modules were 8 bit wide. This meant that 16 bit processors (286 and 386SX) needed 2 SIMMs in a pair. Thus, there was room for two modules in what is called a bank.  32 bit processors (386DX and 486) need 4 of the small 8 bit SIMMs in a bank, since their banks are 32 bit wide. So, on a typical 1st generation 486 motherboard, you could install 4 X 1 MB, 4 X 2 MB, or 4 X 4 MB in each bank. If you only had one bank (with room for 4 modules), it was expensive to increase the RAM, because you had to discard the old modules. With the advent of the 486 processor, demand increased for more RAM. Then the larger 32 bit modules came into use. A 486 motherboard could still have 4 SIMM sockets, but when the modules were 32 bit wide, they could be installed one at a time. This was quite ingenious. You could add different types of modules and still use the old ones. Also, since the 486 motherboard ran at only 33 MHz on the system bus, the RAM module quality was not so critical. You could mix 60 ns and 70 ns modules of different brands without problems. Here you see a couple of SIMM modules. On top is a 64 bit module (168 pins - don't try to count them). Next is a 32 bit module with a 72 pin connector. Below is an 8 bit module with a 30 pin connector.

Dual In-line Memory Module (DIMM):

DIMM package is also a small circuit board that contains the memory chips. The difference between the SIMM and DIMM is that DIMM is 168-pin package. The data widths of the DIMM packages are 64-bit, 72-bit or 80-bit.

A 168-pin DIMM package is available in the SDRAM, EDO or FPM DRAM chips. A 168-pin DIMM package has 84 pins on each side of the package. This type of pin configuration helps in placing the DIMM package on the memory socket. The latest computers use a DIMM package. Its pin configuration does not support the motherboard of old computers. A DIMM package is available from 32MB to 2GB sizes. It supports 3.3V of electricity.

Small Outline Dual In-line Memory Module (SODIMM):

Laptops and notebook systems use this package. It is the smallest version of the DIMM. The SODIMM package has a notch at the bottom of the circuit board. This notch helps in inserting the SODIMM package in the memory socket. SODIMM packages are available with 144 and 200 pins.

A 144-pin SODIMM package has 64-bit data path. The FPM DRAM and EDO DRAM use this package. The 72-pins on both the sides of the package divide a 144-pin package. A 200-pin SODIMM package has 64-bit data path. PC2100 memory and PC2700 memory use this package. The 100 pins on both the sides of the package divide a 200-pin package.

                         FIG A FIG B

FIG A: NOTCH LOCATION OF SODIMM DDR1, DDR2 AND DDR3

FIG B: NOTCH LOCATION OF DDR1, DDR2 AND DDR3

Micro DIMM:

Micro DIMM stands for Micro Dual Inline Memory Module. This package is smaller than DIMM and SODIMM packages. The sub-notebook systems use these memory packages. The Micro DIMM package pins connect the memory module with the memory socket. These pins provide two communication lines for the module and the system. This package does not notch at the bottom. Micro DIMM packages are available with 144 and 172 pins. A 144-pin Micro DIMM package has 64-bit data path. PC100 SDRAM uses this package. The 144-pin Micro DIMM package has two sides. It contains 72 pins on each side of the package. The height of this chip is 1.545 inch long and 1 inch high.

144-pin Micro DIMM Package

Rambus Inline Memory Module (RIMM):

RDRAM chip uses the RIMM memory package. This package is same as the DIMM package. It only differs in the pin configuration. The high bandwidth and the low latency applications use this memory package. The RIMM package has the data storage of speed f 600 MHz, 711 MHz, 800 MHz and 1066 MHz. It has 184 connecting pins. This package starts operating from 2.5 V supply. The RIMM packages are available in 16-bit data buses, 32-bit data buses, and 64-bit data buses. The memory bandwidth of the RIMM package is up to 9.6 GB per second.

FIG: RIMM

TROUBLESHOOTING RAM

How to Spot Problems and Find Solutions:

After installing the new RAM, if the RAM is not recognized by your system, or if you get a long beep or sequence of beeps at startup, you'll have to do some troubleshooting. Most problems are caused by purchasing the wrong memory, installing it incorrectly, or damaging the memory module by handling it improperly.

More advanced problems exist too, especially with older computers and when memory banks are involved. When a computer's motherboard uses memory banks, it may require that two identical modules be installed in the two slots of a single memory bank, and installing two different models will cause problems. Sometimes these banks can hold different size (in MB) modules though, as long as they are from the same manufacturer, but often they can't. To uncover and resolve these problems (both common and advanced), work through the following sections in the order presented.

Check the Installation:

When a problem occurs with newly-installed RAM, you should first make sure it's installed correctly. The modules should be secure in their slots, lined up properly with the notches in the slot, should fit properly, and the retaining clips or ejector clips, if they exist, should be firmly secured. When this is achieved, the memory is said to be seated correctly.

If these items are not the problem, reread your computer manual and the installation instructions for the memory you purchased. Some memory has to be installed directly next to existing memory, and an open space between modules will cause the memory to not be recognized. Other times, memory must be from the same manufacturer, and the memory modules must all be the same size (all must be 512 MB, for example). The instructions that came with the RAM should include this information.

Miscellaneous Installation Troubleshooting:

If problems still exist, and you've verified you selected and properly installed the correct RAM, you'll need to try a few more obscure procedures. Reseating the memory modules often works; simply remove and replace them. If multiple modules are installed, you can also try switching their places on the motherboard. Put module 1 in slot 2, and module 2 in slot 1. Finally, try installing a single module in slot 1 (making sure it is enough RAM to successfully start the computer, at least 128 MB), restart your computer, and verify that the memory is recognized, remove it, and do the same with the other modules. You may find you have a defective memory module.

Visit the Manufacturer's Web Site:

If you're still having problems, you should visit the manufacturer's Web site. Most sites have troubleshooting Web pages, articles on resolving known issues, and information about defective RAM and returning it if it doesn't work. Most also have free technical support, often by phone, and many have online diagnostic tools to help you in the process. These are good resources and should be part of the troubleshooting sequence.

Use the Windows Memory Diagnostic Tool:

Microsoft has released a software memory diagnostic tool, the Windows Memory Diagnostic that tests RAM on the computer for errors. This site also has a user's guide and list of system requirements that you can read before downloading the tool.

To run the diagnostic, you download the tool, save it to a disk or CD, and restart your computer using that disk as the startup disk. The diagnostic tool loads and an interface opens that helps you determine if your installed RAM has errors. This is quite useful if talking to the manufacturer's technical support people did not help, or if the company does not think the RAM is defective. If you can prove it is with this report, you're more likely to get your money returned or new RAM sent out.

Advanced Troubleshooting:

If you're convinced you purchased and installed the correct memory modules, you've spoken with the manufacturer's technical support people, and you're positive the memory is not damaged or defective, you may be facing some known issues that others have already experienced, issues that involve Windows XP. Most of the time, you won't be the first person to encounter a particular problem, and finding an answer is as simple as reading a Knowledge Base or TechNet article.

For example, if after installing new RAM, you get this error message: Hardware Malfunction: Call your hardware vendor for support. The system has halted NMI: Parity Check/Memory Parity Error, you can search the Knowledge Base for that message and read about the most likely problem and solution. The KB article, Hardware Malfunction Results in System Error Message, details this error and others that may accompany it. If you receive errors such as these, locating the problem and solution using the Knowledge Base is a good bet.

Purchasing, installing, and getting RAM to work is generally a simple process. However, occasionally things go awry and if this happens to you, hopefully this article leads you in the right direction.

Conclusion :-

Memory is one of the functions of the brain that enables to store and remember the past events. Similarly, in computers the terms memory refers to a chip that stores data. It also enables us to retrieve the stored data. The processor retrieves information stored in for processing data. The storage capacity of a memory depends on the type of memory package used.

     Memory is categorized into Volatile and Non- volatile Memory

• Volatile Memory – Stores data temporarily. For example ROM.
• Non -Volatile Memory – Stores data permanently even if system supply is turned off. For example RAM.