TERM OF THE DAY - Video Random Access Memory

What does it mean?

Video RAM, or VRAM, is a dual-ported variant of dynamic RAM (DRAM), which was once commonly used to store the framebuffer in graphics adapters.

Video random access memory (VRAM or video RAM) is a high-speed array of dynamic random access memory (DRAM) used to store the image and video data that a computer displays. VRAM is an integrated circuit that serves as a buffer between the CPU and video card. VRAM was originally designed as a high-resolution graphics adapter. The higher the video memory, the higher the capability of the system to handle more complex graphics at a faster pace.
VRAM is also known as a frame buffer or simply video memory.

VRAM has two sets of data output pins, and thus two ports that can be used simultaneously. The first port, the DRAM port, is accessed by the host computer in a manner very similar to traditional DRAM. The second port, the video port, is typically read-only and is dedicated to providing a high throughput, serialized data channel for the graphics chipset.

Typical DRAM arrays normally access a full row of bits (i.e. a word line) at up to 1,024 bits at one time, but only use one or a few of these for actual data, the remainder being discarded. Since DRAM cells are destructively read, each row accessed must be sensed, and re-written. Thus, 1,024 sense amplifiers are typically used. VRAM operates by not discarding the excess bits which must be accessed, but making full use of them in a simple way. If each horizontal scan line of a display is mapped to a full word, then upon reading one word and latching all 1,024 bits into a separate row buffer, these bits can subsequently be serially streamed to the display circuitry. This will leave access to the DRAM array free to be accessed (read or write) for many cycles, until the row buffer is almost depleted. A complete DRAM read cycle is only required to fill the row buffer, leaving most DRAM cycles available for normal accesses.