Memory Primer- Giordan on Graphics | 3 | WebReference

Memory Primer- Giordan on Graphics | 3

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Types of Memory

While the term memory almost always refers to DRAM based system memory, other types of RAM also exist, and are usually set aside to perform specific tasks. By paying attention to these special requirements, you can dramatically impact your system performance.


VRAM stands for Video Random Access Memory. If you run your monitor from on-board video, you are using VRAM to control the information sent to your screen. If you use a separate video card, plugged into a PCI slot, then you do not have to worry about VRAM.

The first thing that VRAM does is to look at the total number of calculations it needs to do to support your monitor. Once it has figured this out, it then tells you how complex each of those calculations can be. One unique thing about VRAM is that it can be read from and written to at the same time. This allows the video processor to write data to the screen while it is reading the next screen image.

Consider for a moment the amount of data required just to put an image on your monitor. Most monitors display roughly 72 pixels per linear inch of space. This means that a square inch of monitor space contains 5184 pixels. A standard 13 in diagonal measure monitor contains 84 square inches of space. Multiply 5184 pixels per square inch by 84 inches, and you get 435,456 pixels on the entire monitor. (If you have a 19 inch display, you can almost double that amount). VRAM must calculate a specific value for each pixel. This shows the sheer number of calculations that VRAM must support for an average display.

The next thing VRAM determines is exactly how complex each pixel calculation will be. The complexity of the calculation determines how many different shades of color can be shown for each pixel, which is referred to as bit depth. 24 bit color calculates 8 bits of color per pixel. Considering that each pixel contains a red, green and blue pixel component, we must multiply each by 8 bits, which equals 24 bits of color information. The end result is 16.7 million different color variations for each of the 435,456 pixels on my 13 inch monitor.

You should buy only enough VRAM to support the display you are using. You do not gain any speed benefits by installing more VRAM than is required to get 24 bits of color information. To view 24 bits of color information, a two page display must be running 4MB of VRAM. 2MB of VRAM is all that is necessary on a full page, or 17 inch display, while 1MB will support a 13 inch monitor at 24bit.

Cache Memory

Cache memory is a special section of memory that uses a fast static RAM chips,(SRAM) , and serves as a shortcut to allow the processor access to certain types of data at very high speeds. Data being written to the processor from standard RAM can take as much as 180 nanoseconds counting the request, access and retrieval. Cache memory allows frequently requested data to go into the processor in as little as 45 nanoseconds. Using Cache Memory dramatically improves your computer's overall system performance.

Cache memory exists as a buffer between the processor and the standard DRAM modules. The most recent instructions and bits of data are stored in cache memory. When the processor looks for information, it first looks to the Cache segments, then to main memory.

The processor looks to cache memory first, then to RAM. Your system actually has two types of cache memory, called level one and level two cache. Level one cache, also called internal cache, is located inside the processor chip itself, and is obviously not modifiable.

Level One cache typically ranges from 1KB to 32KB in size, depending on the age and speed of the processor. For example, the Power Macintosh 601 chip has a 32KB cache inside the processor itself

Level two cache is located outside of the processor, either on the system board, or on a modular cache card. The general rule of thumb says that for smaller amounts of RAM (up to 24MB), you should use 32KB of cache for every 1024KB of DRAM installed. This equals 256KB per 8MB, or 768KB per 24MB. This usually gets rounded up to 1024KB, at which point it remains constant.

How Much Speed Will A Cache Card Give Me?

Cache cards can effect a number of areas in your system. The most obvious one is the processor area, given the basic way that cache memory works. In addition, if you are using onboard video, you will find that screen redraw and QuickDraw functions can increase due to an increase in cache memory.

Using 256KB of cache, you can expect to see approximately a 30% increase in the performance of processor and video functions, as they pertain to imaging and graphics tasks. This number jumps to almost 50% with 1024KB of cache installed. Going beyond 1024KB seems to yield diminishing returns, especially compared with the increases seen with the first 1024Kb of cache installed.

One point to keep in mind is that if you are going to use a cache, be sure you have enough of it. This is because a smaller cache segment will be able to store fewer instructions in reserve. When the processor looks first to the cache, and then to the main memory, you can suffer something of a performance hit if the processor repeatedly finds nothing in the cache area. It is looking in two places instead of one, which can result in as much as a 25% decrease in performance. Keeping a sizable cache will yield the performance gains listed above. For this reason, my recommendation is that if you are going to use a cache, bump it up to 1024KB, and optimize things right out of the box.


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Produced by Daniel Giordan

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URL: 14/
Created: May 17, 1999
Revised: May 17, 1999