Computer Memory

From NP Technology

There are several different technologies available today when it comes to memory.

ROM

This is read-only memory, memory that can only be read, but cannot be written to. ROM is used in situations where the data must be held permanently. This is due to the fact that it is non-volatile memory. This means the data is "hard-wired" into the ROM chip. You can store the chip forever and the data will always be there. Besides, the data is very secure. The BIOS is stored on ROM because the user cannot disrupt the information.

There are different types of ROM, too:

• Programmable ROM(PROM). This is basically a blank ROM chip that can be written to, but only once. It is much like a CD-R drive that burns the data into the CD. Some companies use special machinery to write PROMs for special purposes.
• Erasable Programmable ROM (EPROM). This is just like PROM, except that you can erase the ROM by shining a special ultra-violet light into a sensor atop the ROM chip for a certain amount of time. Doing this wipes the data out, allowing it to be rewritten.
• Electrically Erasable Programmable ROM (EEPROM). Also called flash BIOS. This ROM can be rewritten through the use of a special software program. Flash BIOS operates this way, allowing users to upgrade their BIOS.

  ROM is slower than RAM, which is why some try to shadow it to increase speed.

RAM

Random Access Memory (RAM) is what most of us think of when we hear the word memory associated with computers. It is volatile memory, meaning all data is lost when power is turned off. The RAM is used for temporary storage of program data, allowing performance to be optimum.

Like ROM, there are different types of RAM:

• Static RAM (SRAM). This RAM will maintain it's data as long as power is provided to the memory chips. It does not need to be re-written periodically. In fact, the only time the data on the memory is refreshed or changed is when an actual write command is executed. SRAM is very fast, but is much more expensive than DRAM. SRAM is often used as cache memory due to its speed.

  There are a few types of SRAM:

• Async SRAM. An older type of SRAM used in many PC's for L2 cache. It is asynchronous, meaning that it works independently of the system clock. This means that the CPU found itself waiting for info from the L2 cache.
• Sync SRAM. This type of SRAM is synchronous, meaning it is synchronized with the system clock. While this speeds it up, it makes it rather expensive at the same time.
• Pipeline Burst SRAM. Commonly used. SRAM requests are pipelined, meaning larger packets of data re sent to the memory at once, and acted on very quickly. This breed of SRAM can operate at bus speeds higher than 66MHz, so is often used.

• Dynamic RAM (DRAM). DRAM, unlike SRAM, must be continually re-written in order for it to maintain its data. This is done by placing the memory on a refresh circuit that re-writes the data several hundred time per second. DRAM is used for most system memory because it is cheap and small.

  There are several types of DRAM, complicating the memory scene even more:

• Fast Page Mode DRAM (FPM DRAM). FPM DRAM is only slightly faster than regular DRAM. Before there was EDO RAM, FPM RAM was the main type used in PC's. It is pretty slow stuff, with an access time of 120 ns. It was eventually tweaked to 60 ns, but FPM was still too slow to work on the 66MHz system bus. For this reason, FPM RAM was replaced by EDO RAM. FPM RAM is not much used today due to its slow speed, but is almost universally supported.
• Extended Data Out DRAM (EDO DRAM). EDO memory incorporates yet another tweak in the method of access. It allows one access to begin while another is being completed. While this might sound ingenious, the performance increase over FPM DRAM is only around 30%. EDO DRAM must be properly supported by the chipset. EDO RAM comes on a SIMM. EDO RAM cannot operate on a bus speed faster than 66MHz, so, with the increasing use of higher bus speeds, EDO RAM has taken the path of FPM RAM.
• Burst EDO DRAM (BEDO DRAM). Original EDO RAM was too slow for the newer systems coming out at the time. Therefore, a new method of memory access had to be developed to speed up the memory. Bursting was the method devised. This means that larger blocks of data were sent to the memory at a time, and each "block" of data not only carried the memory address of the immediate page, but info on the next several pages. Therefore, the next few accesses would not experience any delays due to the preceding memory requests. This technology increases EDO RAM speed up to around 10 ns, but it did not give it the ability to operate stably at bus speeds over 66MHz. BEDO RAM was an effort to make EDO RAM compete with SDRAM.
• Synchronous DRAM (SDRAM). SDRAM is really the new standard for PC memory. Its speed is synchronous, meaning that it is directly dependent on the clock speed of the entire system. Standard SDRAM can handle higher bus speeds. In theory, it can operate at up to 100MHz, although it has been found that higher quality DIMMs must be used for stable operation at such speeds. Hence PC100 SDRAM. Although SDRAM is faster, the speed difference isn't noticed by many users due to the fact that the system cache masks it. Also, many users are working on a relatively slow 66MHz bus speed, which doesn't use the SDRAM to is full capacity. Using 100MHz chipsets, like the BX and other more modern chipsets, you can easily run your PC100 SDRAM at full speed. With some newer chipsets by Via and others, we now have PC-133 as well.
• RAMBus DRAM (RDRAM). Developed by Rambus, Inc. and endorsed by Intel as the chosen successor to SDRAM. RDRAM narrows the memory bus to 16-bit and runs at up to 800 MHz. Since this narrow bus takes up less space on the board, systems can get more speed by running multiple channels in parallel. Despite the speed, RDRAM has had a tough time taking off in the market because of compatibility and timing issues. Heat is also an issue, but RDRAM has heatsinks to dissipate this. Cost is a major issue with RDRAM, with manufacturers needing to make major facility changes to make it and the product cost to consumers being too high for people to swallow.
• DDR-SDRAM. This type of memory is the natural evolution from SDRAM and most manufacturers prefer this to Rambus because not much needs to be changes to make it. Also, memory makers are free to manufacture it because it is an open standard, whereas they would have to pay license fees to Rambus, Inc. in order make RDRAM. DDR stands for Double Data Rate. PC-100 and PC-200 DDR-SDRAM both use the 100 MHz bus speed, but DDR shuffles data over the bus over both the rise and fall of the clock cycle, effectively doubling the speed. Of course, chipset support is necessary, but Via, ALi, and Micron have already decided they will support DDR-SDRAM in their chipsets rather than RDRAM.