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TYPES OF RAM AND ROM

TYPES OF RAM AND ROM 

Common RAM Types 
• 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. These types of memory are physically bulky and somewhat limited to its capacity. 

    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. Static RAM is 
used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space. 

• DRAM 
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. 
Dynamic random access memory (DRAM) was an improvement over SRAM. Require more power than SRAM for refresh signals and therefore, most commonly found in desktop computers. These chips can hold very large amounts of information. Most PCs today use DRAM of one type or another. 
    
    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 changes its state. 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. 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. 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. 

Types of DRAM 
• 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 it has higher 
bandwidth, meaning greater speed. Maximum transfer rate to L2 cache is approximately 1,064 MBps (for DDR SDRAM 133 MHZ).now a days various versions of SDRAM in the form of DDR1,DDR2 and DDR3 is available which has various speed for data transfer. 

• 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. 

• 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. This memory uses a small battery to provide it with the power it needs to maintain the memory contents. 

• VRAM
 Video RAM, also known as multi-port 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.

• Windows RAM (WRAM) 
     Windows RAM (WRAM) is specialized memory for Windows accelerator cards. Developed by Samsung, it is similar to video RAM, except that it’s much faster. While information is being read from one set of WRAM addresses to draw the screen, other information can be written to another set of addresses.This is faster than the normal VRAM, where all addresses can only be read from or written to. The ability of WRAM to be read from or written to simultaneously is called dual ported memory.

ROM 
    Read-only memory (ROM), also known as firmware, is an integrated circuit programmed with 
specific data when it is manufactured. ROM chips are used not only in computers but in most other 
electronic items as well. Here you will learn about the different types of ROM and how each works.

ROM Types 
There are five basic ROM types: 
1. ROM 
2. PROM 
3. EPROM 
4. EEPROM 
5. Flash memory 
Each type has unique characteristics, but they are all types of memory with two things in 
common: 
• Data stored in these chips is nonvolatile -- it is not lost when power is removed. 
• Data stored in these chips is either unchangeable or requires a special operation to change 
(unlike RAM, which can be changed as easily as it is read). 
This means that removing the power source from the chip will not cause it to lose any data.




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