RAMDAC

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 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. 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).

This article is based on material from FOLDOC, used with permission.