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This chapter is from the book

Video RAM (VRAM)

VRAM (Video RAM) and WRAM (Windows RAM) have been mostly supplanted by DDR memory chips, but you may find a question about VRAM (pronounced "vee-ram") on the exam. Video RAM was designed to provide two access paths to the same memory address. It's as if VRAM were a café that has two doors: one in the front and one in the back. Information comes in one "entrance" at the same time that other information flows out the other "exit." When the video controller reads the memory for information, it accesses an address with one of the paths. When the CPU writes data to that memory, it accesses the address via the other path. Because of these two access paths, we say that VRAM is dual-ported.

Manipulating graphics is processing-intensive, and so this capability to push data in and out of the chip at the same time helps a moving image appear continuous. In a way, the concept is similar to pipelining, but dual-porting uses one channel for "in" and the other channel for "out." Pipelining uses only one channel, but doesn't have to ask for instructions twice. VRAM chips are about 20% larger than DRAM chips because of extra circuitry requirements. (Modern computers usually have basic graphics processing integrated right onto the motherboard, with the AGP providing for faster video processing.)

VRAM, WRAM, and AGP

The AGP acronym stands for Accelerated Graphics Port. Most computers include this accelerated port, which is an integrated part of the I/O system. An AGP is not the same thing as VRAM or a video accelerator card, nor is it the same thing as today's integrated graphics. Although some video cards still use the main expansion bus, most connect with the AGP.

To say that a computer has "AGP memory" or "comes with AGP" can be confusing at best. At worst, it can demonstrate a faulty knowledge of the distinction between video memory and I/O subsystems. AGP is discussed in the "Accelerated Graphics Port (AGP)" section of Chapter 9, "Peripherals: Output Devices."

WRAM is short for Windows RAM, and has no connection with Microsoft, even though the acronym includes the word "Windows." WRAM, like VRAM, is dual-ported, but uses large block addressing to achieve higher bandwidth. Additional features provided better performance than video RAM at lower manufacturing costs. With the advent of AGP and DDR memory, both VRAM and Windows RAM have faded from the marketplace. That's not to say that add-on graphics accelerator cards have vanished.

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