Wednesday, January 31, 2007
June 2004 HelpLine
In aperture or aperture-preferred mode, you can set your exposure based on the aperture that will give you the depth of field that you want and let the camera pick the shutter speed to match the ƒ-stop you've selected.
Q) When I was researching my next big CompactFlash card purchase, I came across a "feature/benefit" on one manufacturer's card called MultiWord DMA. What is that?
A) Let's break apart the term—MultiWord and DMA—and deal with the two components separately. DMA stands for Direct Memory Access, a technology that has been used for years in the computer industry. The concept is a fairly simple one. In the early days of computers, the CPU, or central processing unit, was more like a central control unit that did everything.
As computer manufacturers started making computers faster and faster, they recognized that they had to offload some of the more mundane tasks to separate computer chips. That's where DMA comes in. Instead of the CPU's controlling the writing of each bit of data in memory to the hard drive, a separate chip directly accesses the memory without the intervention of the CPU.
Now on to MultiWord. In computer speak, a "word" is another way of saying a group of bits (of data). When DMA was first developed, it used the typical method of transferring one "word" at a time. Think of each word as a package of information. Each package has to be wrapped, sent from memory to the storage device, unwrapped and acknowledged that it was received.
Unfortunately, a "word" is a very small amount of data. The overhead in sending each "word" makes the process a little inefficient. What if you could put more "words" into each package? That's just what MultiWord does—it allows multiple "words" of data to be sent during each transmission. When you put MultiWord and DMA together, you get a technology that removes the CPU from the task of writing to memory and also speeds up the process.
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