RAMDAC
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Random Access Memory Digital-to-Analog Converter is a combination of three fast DACs with a small SRAM used in graphics display adapters to store the color palette and to generate the analog signals (usually a voltage amplitude) to drive a colour monitor. The logical colour number from the display memory is fed into the address inputs of the SRAM to select a palette entry to appear on the output of the SRAM. This entry is composed of three separate values corresponding to the three components (red, green, and blue) of the desired physical colour. Each component value is fed to a separate DAC, whose analog output goes to the monitor, and ultimately to one of its three electron guns (or equivalent in non-CRT displays).
DAC word lengths range usually from 6 to 10 bits. The SRAM's wordlength is three times the DAC's word length. The SRAM acts as a color lookup table (CLUT). It usually has 256 entries (and thus an 8-bit address). If the DAC's word length is also 8 bits, we have a 256 x 24-bit SRAM which allows a selection of 256 out of 16777216 possible colours for the display. The contents of the SRAM can be changed while the display is not active (during display blanking times).
The SRAM can usually be bypassed and the DACs can be fed directly by display data, for true color modes. In fact this has become very much the normal mode of operation of a RAMDAC since the mid-1990s, so the programmable palette is mostly retained only as a legacy feature to ensure compatibility with old software. In many newer graphics cards, the RAMDAC can be clocked much faster in true color modes, when the SRAM is not used.
History
The term "RAMDAC" did not enter into common PC-terminology until IBM introduced the IBM VGA display adapter in 1987. The IBM VGA adapter used the Inmos 171 RAMDAC. The Inmos VGA RAMDAC was a separate chip, featured a 256-color (8-bit CLUT) display from a palette of 262,144 possible values, and supported pixel-rates up to ~30MPixel/sec.
As clone manufacturers copied IBM VGA hardware, they also copied the Inmos VGA RAMDAC. Advances in semiconductor manufacturing and PC processing-power allowed RAMDACs to add "direct-color" operation, which is a mode of operation that allows the SVGA-controller to pass a pixel's color-value directly to the DAC-inputs, thereby bypassing the RAM lookup-table. Another innovation was Edsun's CEGDAC, which featured hardware-assisted anti-aliasing for line/vector draw-operations.
By the early 1990s, the PC chip-industry had advanced to the point where RAMDACs were integrated into the SVGA display controller chip, thus reducing the # discrete chips and the cost of SVGA video cards. Consequently, the market for standalone VGA RAMDACs disappeared. Today, RAMDACs are still manufactured and sold for niche applications, but in obviously limited quantity.
In modern PCs, the VGA RAMDAC(s) are integrated into the display controller chip, which itself may be mounted on an add-in-board or integrated into the motherboard core-logic chispet. The original purpose of the RAMDAC, to provide a CLUT-based display-mode, is rarely used, having been supplanted by true-color display-modes. As LCD and other digital flat panel technology becomes increasingly mainstream, the "DAC" portion of the RAMDAC will likely become obsolete.
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.