page_title: RAMDAC
section_title: History
hierarchical_section_title: RAMDAC / History
caption_attribution_description: English: INMOS G171 RAMDAC
context_page_description: A random-access memory digital-to-analog converter is a combination of three fast digital-to-analog converters with a small static random-access memory used in computer graphics display controllers to store the color palette and to generate the analog signals to drive a color monitor. The logical color number from the display memory is fed into the address inputs of the SRAM to select a palette entry to appear on the data output of the SRAM. This entry is composed of three separate values corresponding to the three components of the desired physical color. 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.
  As the use of DVI, HDMI, DisplayPort and other digital interface technology becomes increasingly mainstream, the DAC portion of the RAMDAC will likely become obsolete. The video data is transferred digitally via transition-minimized differential signaling or low-voltage differential signaling and no digital-to-analog conversion takes place until the actual display pixels are actuated.
  DAC word lengths range usually from 6 to 10 bits. The SRAM's word length is three times the DAC's word length.
context_section_description: 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 G171 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 approximately 30 Mpix/s.
  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 spatial 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 display controller chip, thus reducing the number of discrete chips and the cost of video cards. Consequently, the market for standalone RAMDACs disappeared.  Today, RAMDACs are still manufactured and sold for niche applications, but in obviously limited quantity.
  In modern PCs, the 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 chipset.  The original purpose of the RAMDAC, to provide a CLUT-based display-mode, is rarely used, having been supplanted by True Color display modes. However, many CAD and video editing applications use hardware overlay, combined with the programmable palette, to ensure the user interface does not disrupt the rendering of editing window.
caption: IMS G171 RAMDAC on the VGA board
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