Popular DRAM Advances and Technologies


Many technological improvements have been made to DRAM in order to create a faster type of memory. The following section describes some of these improvements.

Fast Page Mode (FPM) DRAM

FPM DRAM is faster than DRAM, but is relatively slow compared to other enhancements. With FPM, the memory controller knows ahead of time to look in the pages of addressed memory after the CPU’s read or write requests. This reduces the amount of time the memory controller has to wait to take instructions from the CPU and read from or write to memory. FPM DRAM is not suitable for motherboard bus speeds greater than 60MHz.

Extended Data Output (EDO) DRAM

EDO DRAM was the first memory introduced with the ability to hold several pieces of information at a time without having to be refreshed. In other words, if the CPU needs to access the same information several times, the information can wait in EDO memory until the CPU is through accessing it. The information does not have to be continuously reloaded or reregistered into memory. EDO was intended to run with Pentium systems rated between 60MHz and 75MHz.

EDO is faster than FPM memory. EDO memory was advertised to increase system performance by 60%. True benchmarks of EDO showed a 10% to 15% increase in performance.

Burst EDO (BEDO) DRAM

Burst EDO memory is a form of EDO DRAM that can process multiple (up to four) memory addresses at a time in small bursts. Burst EDO did not have great success because it could not retain its synchronization with the processor for long periods.

Synchronous Dynamic Random Access Memory

SDRAM is similar to DRAM. What distinguishes SDRAM is that it uses an internal clock to synchronize input and output operations with the CPU. The synchronization between the memory and CPU results in enhanced performance. SDRAM uses burst mode (automatic retrieval of data before it is requested) for read and write operations. SDRAM sends data in high-speed bursts by utilizing burst mode. It is important to note that SDRAM speed is not measured in nanoseconds. It is measured in megahertz.

Double Data Rate SDRAM (DDR SDRAM)

DDR SDRAM is a type of synchronous dynamic random access memory that dramatically increases memory throughput by allowing data to be transferred on both the rising and falling edges of the system clock as opposed to just the rising edge. What does this mean in English? Data throughput is (approximately) doubled on a memory chip that implements DDR SDRAM technology. It is important to note that the DDR SDRAM is very effective for laptop computer systems because it draws less power. It is also sometimes referred to as SDRAM II. DDR SDRAM can be easily purchased in 128MB, 256MB, and 512MB increments. It is very important to note that DDR SDRAM has a bus clock speed of 100MHz and a transfer of data rate equal to 200MHz. It comes packaged on a 184-pin DIMM.

RDRAM (RAMBUS) DRAM

Rambus RDRAM is proprietary memory from Rambus, Inc. RDRAM improves on memory latency by transferring data in and out of memory at about 600MHz. RDRAM can achieve this speed by synchronizing directly with the memory bus instead of the motherboard bus. Rambus memory uses a narrow bus width and comes on proprietary memory modules called RIMMs. Rambus RDRAM is being used in conjunction with the newer Pentium 4s offered by Intel.

The following Advanced Horizons, Inc. Web site offers excellent insight regarding various RAM specifications. Pay special attention to the “Evolution of Memory” table located at this site: http://www.ahinc.com/hhmemory.htm.

Video Memory

The need for high-speed graphics acceleration, higher resolution, and faster video refresh rates has spawned a growing need for better engineered video memory. For the A+ core test, you need to be familiar with the following three types of video memory.

Video Random Access Memory (VRAM)

A computer screen is made up of many tiny dots called pixels. The bit depth is the number of bits assigned through VRAM to each pixel. The more VRAM that can be assigned to each pixel, the greater the bit depth will be. This results in better resolution and color scale. The larger the monitor, the more pixels there are to fill, and thus there is a greater need for video memory.

VRAM is memory specifically designed for video. VRAM acts as a buffer between the CPU and the monitor. It is designed with two access paths (or dual ports), which provide separate passages to the same area of memory or memory address. This means that two devices can access VRAM at the same time. With this design, the video adapter chip, known as RAMDAC (RAM Digital-Analog Converter), can convert the digital signals to analog, to be displayed on the screen; and at the same time, the video controller (processor) can bring more data into VRAM. VRAM does not need to be refreshed as often as DRAM.

Windows Random Access Memory (WRAM)

Do not be confused—WRAM does not mean Microsoft Windows™ memory. WRAM is similar to VRAM, with the exception that WRAM makes better use of the dual ports available through VRAM. WRAM can take advantage of more memory address storage space, resulting in better color depth and video resolution (1600 1200). WRAM is faster than VRAM.

Synchronous Graphics RAM (SGRAM)

SGRAM is a form of DRAM that uses a single port. SGRAM uses its own program instructions, called masked write and block write commands, to provide better throughput for graphic-intensive applications. SGRAM is synchronized with the CPU clock speed and can support up to 100MHz. If you ever receive a nonmaskable interrupt error, you are probably experiencing defective RAM or SGRAM.

Cached Memory

As mentioned in Chapter 16, cached memory is memory the processor uses for very fast access to information. It is very important to remember that Level 1 cache is considered primary cache and is internal or built into the processor itself. Level 2 cache is secondary cache that is external to the processor. Level 3 cache adds 1MB (512K) on to the CPU chip. With memory caching, a memory cache controller anticipates (about 90% correctly) what the processor is going to require from memory. This method eliminates the processor’s constant need to access the slower DRAM.

Virtual Memory (Swap File)

Today’s popular operating systems are typically installed with a predetermined amount of hard drive space set aside to act as a memory buffer area for main memory (RAM). This area of hard drive space is referred to as virtual memory, swap file, or page file. (The different names refer to the same area of hard drive space.) Data is temporarily moved, or swapped between memory and the hard drive. Moving data out of main memory and placing it into a swap file frees up valuable space in main memory for other purposes. The swap file size can vary depending on the amount of free space available on the hard drive. You can manually configure the swap file size or let the operating system take care of its configuration.

If you are running an application that uses up all the current RAM, the extra memory needed to run the application can be provided automatically from virtual memory. This memory area is managed differently than main memory. As you may recall, data is stored in memory addresses within RAM. These addresses are known as real memory addresses. With virtual memory, the operating system logically divides the set-aside hard drive space into memory pages that contain virtual memory addresses. These virtual memory pages can typically hold more memory addresses than RAM can. The process by which these virtual memory addresses are converted into real memory is called memory mapping. In order for virtual memory to be utilized, the operating system must be able to run in protected mode. Disk Operating System (DOS) was only able to run in real mode. Microsoft Windows 3x introduced 386 enhanced mode, which paved the way for virtual memory utilization.




The A+ Certification & PC Repair Handbook
The A+ Certification & PC Repair Handbook (Charles River Media Networking/Security)
ISBN: 1584503726
EAN: 2147483647
Year: 2003
Pages: 390

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net