Memory

Primary Storage

Primary Storage is also referred as primary memory, internal memory or main memory. A primary storage device is a medium that holds memory for short periods of time while a computer is running although it has much lower access time and faster performance, it is generally costlier than secondary storage.

Both RAM and ROM are examples of primary storage devices. Key characteristic of primary storage devices are:

·        Directly accessible by the CPU

·        Volatile

·        Non-removable

Secondary Storage

Secondary Storage is also referred as external memory, secondary memory, or auxiliary storage. Secondary storage device is a non-volatile device that holds data until it is deleted or overwritten. Example of secondary storage devices are Floppy disk, Hard disk, Zip disk, etc.

For most computer and users secondary storage is required because of the need to permanently store some data to a computer. Without primary storage if your were working on a document and the power is turned OFF everything would be lost because primary storage is temporary. However, a computer with secondary storage that saved the file before the power turned OFF would be able to recover the last saved version of the file.

Primary Storage Devices

ROM ( Read Only Memory )

ROM is where data is stored permanently. Hence it is also called as Non-Volatile Memory. BIOS is stored on ROM because the user shouldn’t disrupt the information.

Types of ROM

PROM ( Programmable Read Only Memory )

This is basically a blank ROM chip that can be written to, but only once. They are more fragile than ROMs.

EPROM ( Erasable Programmable Read Only Memory )

This is just like PROM, except that you can erase the ROM by shining a special ultra-violet light into a sensor on top of the ROM chip for a certain amount of time. Doing this wipes the data out, allowing it to be rewritten. An EPROM eraser is not selective, it will erase the entire EPROM.

EEPROM ( Electrically Erasable Programmable Read Only Memory )

EEPROM chips remove the drawbacks of EPROMs. In EEPROMs the chip does not have to removed to be rewritten. In EEPROMs the entire chip does not have to be completely erased to change a specific portion of it. In EEPROMs changing the contents does not require additional dedicated equipment.

Instead of using UV light, you can return the electrons in the cells of an EEPROM to normal with the localized application of an electric field to each cell. This erases the targeted cells of the EEPROM, which can then be rewritten. Any byte within an EEPROM may be erased and rewritten.

Flash Memory

Flash Memory is actually a variation of electrically erasable programmable read-only memory ( EEPROM ). EEPROM chips are too slow to use in many products that make quick changes to the data stored on the chip. Flash memory devices are high density, low cost, nonvolatile, fast ( to read, but not write ), and electrically reprogrammable. From a software viewpoint, flash and EEPROM technologies are very similar.

The big difference between the two is that EEPROM can be erased and rewritten at the byte level; flash memory can erase or reprogram blocks of bytes, not individual bytes, hence it is faster. Example of flash memory devices are BIOS chip, Smart Media, Memory Stick, Flash USB Drive etc.

RAM ( Random Access Memory )

RAM is the best known form of computer memory. RAM is considered “random access” because one can access any memory cell, which is the basic unit of data storage, in the same amount of the. The opposite of RAM is serial access memory (SAM). SAM stores data as a series of memory cells that can only be accessed sequentially. If the data is not in the current location, each memory cell is checked until the needed data is found.

RAM is a volatile memory, meaning all data is lost when power is turned OFF. RAM is used for temporary storage of program data.

Types of RAM

There are two basic types of RAM

1.     Dynamic RAM

2.     Static RAM

Dynamic RAM (DRAM)

Dynamic RAM is a type of RAM that only holds its data if it is continuously access by special logic called a refresh circuit. Many hundreds of times each second, this circuity reads the content of each memory cell whether the memory cell is being used at that time by the computer or not. Due to the way in which the cells are constructed, the reading action itself refreshes the content of memory. If this is not done regularly, then the DRAM will lose its content, even if it continues to have power supplied to it. This refreshing action is why the memory is called dynamic.

Static RAM ( SRAM )

Static RAM is a type of RAM that holds it data without external refresh, for as long as power is supplied to the circuit. This is contrast to dynamic RAM ( DRAM ), which must be refreshed many times per second in order to hold its data contents. SRAM’s are used for specific applications within the PC, where their strengths outweigh their weaknesses compared to DRAM.

SRAM	DRAM
Made up of Transistors	Made up of Capacitors
Refreshing is not required	Periodic refreshing is required
Faster	Slower
More power consumption and Heat Dissipation	Less power consumption and head dissipation
Low data storage density	High data storage density
Costlier	Cheaper
Example – Used in cache memory	Example – used for main memory

Cache Memory

In general, a processor is much more likely to need information again it has recently used, compared to a random piece of information in memory. This is the principle behind caching. It’s a small amount of fast memory ( Static RAM ) placed between the processor and the slower main memory. Nowadays it is integrated within the CPU. Cache are used in various forms to reduce the effective time required by a processor to access addressed, instructions or data that are normally stored in main memory.

Working of Cache Memory

The processor request a piece of information. The first place it looks in the level 1 cache, since it is the fastest. If it finds it there ( called a hit on the cache ), great ; it uses it with no performance delay. If not, it’s a miss and the level 2 cache is searched. If it finds it there ( level 2 “hit” ). It is able to carry on with relatively little delay.

Otherwise, it must issue a request to read it from the system RAM. The system RAM may in turn either have the information available or have to get it from the still slower hard disk or CD-ROM. It is important to realize just how slow some of these devices are compared to the processor. Even the fastest hard disks have an access time measuring around 10 milliseconds. If it has to wait 10 milliseconds, a 200 MHz processor will waste 2 million clock cycles! And CD-ROMs are generally at least 10 times slower. This is why using caches to avoid accesses to these slow devices is so crucial.

Virtual Memory

Virtual Memory combines your computer’s RAM with temporary space n your hard disk. When RAM memory runs low, virtual memory moves data from RAM memory to a space called a paging file. Moving data to and from the paging file frees up RAM memory to complete its work. If your computer lacks the RAM memory needed to run a program or operation, windows uses virtual memory to compensate.

The more RAM memory your computer has, the faster your programs will generally run. If a lack of RAM memory is slowing your computer, you might be tempted to increase virtual memory to compensate. However, your computer can read data from RAM memory much more quickly than from a hard disk, so adding RAM memory is a better solution.

Changing the virtual Memory size.

1.     Right click on the start button and select Control Panel.

2.     Now go to System and Security.

3.     Click on System.

4.     Now tap on Advance System setting from the right panel.

5.     Click the Advance tab, and then, under performance, click Setting.

6.     Under the option Advance check the Virtual memory option and click change.

7.     Uncheck the option Automatically manage paging file size for all drives.

8.     Under Drive [ volume label ], click the drive that contains the paging file you want to change.

9.     Click Custom size, type a new size in megabytes in the Initial size ( MB ) or maximum size ( MB ) box, click Set, and then click ok.

Different types of DRAM

FPM ( Fast Page Mode ) RAM: A type of Ram that allows faster access if the data being called in the same row as the data previously requested, also called page mode memory.

EDO ( Extended Data Out ) RAM: EDO RAM was nothing more than a moderate improvement on old-style FPM DRAM. EDO needed to be refreshed much less often, thereby providing an extended period where data could be taken out of RAM. EDO DRAM enabled a system to access data more quickly than FPM RAM.

SDRAM ( synchronous Dynamic RAM ): SDRAM is still DRAM, but it is synchronous-tied to the system clock. As mentioned earlier, regular DRAM (EDO or FPM ) was not tied to any clock. If the CPU wanted some data from RAM, the chipset sent the necessary signals to the DRAM, waited a certain number of clock ticks and then accessed the RAM again to get the data.

SDRAM is tied to the system clock, just like the CPU and chipset, so the chipset knows when data is ready to be grabbed from SDRAM, resulting in little wasted time. SDRAM is quite a bit faster than DRAM.

Read more about Computer Memory here.

DDR ( Double Data Rate ) RAM: DDR utilizes both the rising and falling edge of the system clock, potentially doubling the speed of the memory. Today, DDR technology is found on high-end video cards and computer memory such as DDR RAM. DDR memory belongs to the dual in – line memory module ( DIMM ) architecture family and is a faster form of SDRAM.

There are presently four generation of DDR memories:

DDR1, DDR2, DDR3 and DDR4.

ECC ( Error Correction Code ) RAM: many higher-end systems use a special type of RAM called error correction code (ECC) DRAM, ECC is a major advance in error checking on DRAM.

SGRAM ( Synchronous Graphic RAM ): It operates in similar fashion to SD RAM but it is streamlined to work with graphic cards. This RAM enables fast read and write operation for the graphics processor when working with the information in the video frame buffer.

VRAM ( Video RAM ): It is memory that is optimized for video cards where each memory cell Is dual ported Therefore video data can be written to the RAM while the graphics adapter simultaneously read from it to refresh the display.

RDRAM ( RAMBUS DRAM ): Inter wanted faster RAM not just for higher-end performance machines including servers, but standard desktops. Its choice was a new type of DRAM called Rambus DRAM ( RDRAM ).

The three version of it are intended PC 600 ( clock speed: 300MHz, PC70 ( actually 711 or 3 5 6MHz ), and PC800 (400MHz).

Basic Fundamental of Computer

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