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Monday, April 22, 2002
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Cram up various types of RAM
Raman Mohan

Cram up various types of RAM --- Illustration by Rajiv Kaul

AFTER the CPU, the other most critical component of your computer is the Random Access Memory of RAM in short. The PCs never seem to have enough of it. As new applications are released, PCs hunger for RAM increases proportionately. There were times in mid-í90s when in India at least eight to 16 MB of RAM was the industry standard. However, as newer versions of operating systems began to circulate it became necessary to go in for additional RAM. The trend continues till this day when even 256 MB of RAM is considered just right for your PC though you could do with some more.

Though most computer users realise a higher RAM is always better, not many have an idea of what it is and what type of RAM does what. We generally go by the thumb rule that the more the better. However, we must understand exactly what it is and why we need it. RAM is the storehouse in a computer where the operating system, applications and data in current use are housed so that it can be quickly accessed by the CPU. RAM is much faster to read from and write to than the other storage devices in a computer such as the hard disk, floppy disk, and CD-ROM. But as opposed to the other storage devices, the data in RAM stays there only as long as the computer is running. RAM sheds all data when the computer is shut down. However, the operating system and other files are loaded into RAM right from the hard disk when the computer is switched on again. The basic difference between the storage in a HDD and RAM is that between a manís short term and long term memory.

 


RAM is a short-term memory that concentrates on the current job in hand but the HDD is the long term memory which stores data for as long as you wish or until it crashes due to a fault. RAM is stored in microchips that plug into the computerís motherboard. It is connected to the processor through a set of electrical paths technically known as a bus. The hard drive, on the other hand, stores data on magnetised surface in sectors. We need RAM in large quantities because if it is filled up, the processor needs to continuously go to the hard disk to retrieve old data. While RAM access time is expressed in nanoseconds, hard disk access time is expressed in milliseconds. Hereís how RAM works in general.

When the CPU gets the next task instruction it is supposed to perform, the instructions may contain the address of some memory of RAM location from which data is to be read. This address is sent to the RAM controller. The RAM controller processes the request and sends it to the appropriate address so that transistors along the lines open up the cells so that each capacitor value can be read. A capacitor with a charge over a certain voltage level represents the binary value of 1 and a capacitor with less than that charge represents a 0. The data that is read is transmitted along the data lines to the processorís nearby data buffer known as level-1 cache and another copy may be held in level-2 cache. The amount of time that RAM takes to write data or to read it once, the request has been received from the processor is called the access time. Typical access times vary from 9 nanoseconds to 70 nanoseconds, depending on the kind of RAM.

Broadly, RAM is of two types. The main RAM is the one which stores every kind of data and makes it quickly accessible to a microprocessor. The other is Video RAM, which stores data intended for your display screen thus enabling images to get to your display faster. Main RAM is further divided into static RAM (SRAM) and dynamic RAM (DRAM). Static RAM is expensive. It requires four times the amount of space for a given amount of data than Dynamic RAM, but, unlike Dynamic RAM, it does not require to be power-refreshed and is therefore faster to access. The typical access time of SRAM is 25 nanoseconds as against 60 nanoseconds for Dynamic RAM. Then there is Burst SRAM (also known as SynchBurst SRAM). It is synchronised with the system clock or, in some cases, the cache bus clock. This allows it be more easily synchronised with any device that accesses it and reduces access waiting time. It is used as the external level-2 cache memory for the Pentium II microprocessor chipset.

Dynamic RAM has several incarnations. Prior to newer forms of DRAM, Fast Page Mode DRAM (FPM DRAM) was the most common kind of DRAM in personal computers. This type of DRAM essentially accesses a row of RAM without having to continually re-specify the row. A row access strobe (RAS) signal is held active while the column access strobe (CAS) signal changes to read a sequence of contiguous cells. This reduces access time and lower power requirements.

Enhanced DRAM (ED RAM) is the combination of SRAM and DRAM in a single package. Typically, 256 bytes of Static RAM is embedded along with the Dynamic RAM. Data is read first from the faster (typically 15 nanoseconds) SRAM and if it is not found there, it is read from the DRAM, typically at 35 nanoseconds. Extended Data Output RAM (EDO RAM) or Extended Data Output Dynamic RAM (EDO DRAM) is up to 25 per cent faster than standard DRAM. It reduces the need for level-2 cache memory.

BEDO DRAM improves page mode DRAM by "building in" three successive column address shifts after the first column address is specified so that four bits are read as a burst. Together with a dual-bank architecture, BEDO DRAM promised to offer 4-1-1-1 access times. However, because Intel and other manufacturers preferred SDRAM. BEDO DRAM was not widely used.

Nonvolatile RAM (NVRAM) is a special kind of RAM that retains data when the computer is turned off or there is a power failure. Like the computerís read-only memory (ROM), it is powered by a battery within the computer. It can also work by writing its contents to and restoring them from an EEPROM.

Synchronous DRAM(SDRAM) is a generic name for various kinds of DRAM that are synchronised with the clock speed that the microprocessor is optimised for. This tends to increase the number of instructions that the processor can perform in a given time. The speed of SDRAM is rated in MHz rather than in nanoseconds. This makes it easier to compare the bus speed and the RAM chip speed.

JEDEC (Joint Electron Device Engineering Council) SDRAM is an industry standard synchronous DRAM. It has a dual-bank architecture and several burst mode accesses that can be present. JEDEC SDRAM chips operate at either 83 MHz or 100 MHz.

JEDEC SDRAM is also known as PC66 SDRAM because it was originally rated for 66 MHz bus operation and to distinguish it from Intelís PC 100 architecture.

PC 100 SDRAM is SDRAM that states that it meets the PC 100 specification from Intel. Intel created the specification to enable RAM manufacturers to make chips that would work with Intelís i440BX processor chipset. The i440BX was designed to achieve a 100 MHz system bus speed. Ideally, PC100 SD RAM would work at the 100 MHz speed, using a 4-1-1-1 access cycle.

Double Data Rate SD RAM can theoretically improve RAM speed to at least 200 MHz. It activates output on both the rising and falling edge of the system clock rather than on just the rising edge, potentially doubling output.

Enhanced SD RAM (ESDRAM), made by Enhanced Memory Systems, includes a small static RAM (SRAM) in the SDRAM chip. This means that many accesses will be from the faster SRAM. In case, the SRAM doesnít have the data, there is a wide bus between the SRAM and the SDRAM because they are on the same chip. ESDRAM is apparently competing with DDR SDRAM as a faster SDRAM chip for Socket 7 processors.

DIRECT RAM BUS DRAM (DRDRAM) is a proprietary technology proposed by Rambus, Inc. in partnership with Intel. Like SLDRAM, it promises RAM speed up to 800 MHz. It has a smaller bus width (16 bits compared to 64 bits) than current SDRAM designs.

SyncLink DRAM is, along with Direct Rambus DRAM (DRDRAM), a protocol-based approach. Like Double Data Rate SDRAM, SLDRAM can operate at twice the system clock rate. SyncLink is an open industry standard that is expected to compete and perhaps prevail over Direct Rambus DRAM.

FRAM (ferroelectric RAM) is random access memory that combines the fast read and write access of Dynamic RAM (DRAM) ó the most common kind of personal computer memory ó with the ability to retain data when power is turned off (as do other non-volatile memory devices such as ROM and flash memory). Because FRAM is not as dense (cannot store as much data in the same space) as DRAM and SRAM, it is not likely replace these technologies. However, because it is fast memory with a very low power requirement, it is expected to have many applications in small consumer devices such as personal digital assistants (PDAs), handheld phones, power meters, and smart cards, and in security systems. FRAM is faster than flash memory. It is also expected to replace EEPROM and SRAM for some applications and to become a key component in future wireless products.

 

Video RAM

Video RAM is actually any kind of RAM used to store image data for the video display unit. However, the most common type of video RAM continues to be called Video RAM (VRAM). Video RAM in fact is a special arrangement of dynamic RAM (DRAM). Video RAM is actually a buffer between the processor and the monitor and is often called the frame buffer. When images are to be sent for display, they are first read by the processor as data from some form of main storage RAM and then written to video RAM. From video RAM (the frame buffer), the data is converted by a RAM digital-to-analog converter (RAMDAC) into analog signals that are sent to the display presentation mechanism such as a cathode ray tube (CRT). Usually, video RAM comes in a 1 to 4 megabyte package and is located on the video or graphics card in the computer. Mostly video RAM is dual-ported. For your information, RAMDAC (random access memory digital-to-analog converter) is a microchip that converts digital image data into the analog data needed by a computer display. Then there is Synchronised Graphics RAM (SGRAM), it is clock synchronised RAM that is used for video memory. It is relatively low-cost.

Another form of VRAM is called Window RAM (WRAM). However, this has no relation with Microsoft Windows. It is high-performance video RAM that is dual-ported and has about 25 per cent more bandwidth than VRAM but costs less. It has features that make it more efficient to read data. Multibank Dynamic RAM (MDRAM) is another high-performance RAM, developed by MoSys, that divides memory into multiple 32 KB parts or "banks" that can be accessed individually.

The last among the VRAMs is Rambus Dynamic RAM (RDRAM). It is a video RAM designed by Rambut that includes a proprietary bus that speeds up the data flow between video RAM and the frame buffer. Itís optimised for video streaming.

Branded PCs come with the special kind of RAM best suited for the hardware that has been used. Therefore, the buyer does not generally need to pay much attention to it. However, if you plan to buy a computer from the grey market (assembled PCs) you must check whether the right kind of RAM has been installed. It is generally best to read the manuals of the motherboard carefully before deciding on what kind of RAM needs to be installed. Only then you can expect the best from your PC.

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