Memory Primer- Giordan on Graphics | 3
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
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.
Produced by Daniel Giordan
All Rights Reserved. Legal Notices.
URL: https://www.webreference.com/graphics/column 14/
Created: May 17, 1999
Revised: May 17, 1999
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