Random Access Memory Digital-to-Analog Converter


Random Access Memory Digital-to-Analog Converter

(hardware)(RAMDAC) A combination of three fast DACs with asmall SRAM used in graphics display adapters to store thecolour palette and to generate the analog signals to drive acolour monitor. The logical colour number from the displaymemory is fed into the address inputs of the SRAM to select apalette entry to appear on the output of the SRAM. This entryis composed of three separate values corresponding to thethree components (red, green, and blue) of the desiredphysical colour. Each component value is fed to a separateDAC, whose analog output goes to the monitor, and ultimatelyto one of its three electron guns (or equivalent innon-CRT displays).

DAC word lengths range usually from 6 to 10 bits. The SRAM'swordlength is three times the DAC's word length. The SRAMacts as a colour lookup table. It usually has 256 entries(and thus an 8-bit address). If the DAC's word length is also8 bits, we have a 256 x 24-bit SRAM which allows a selectionof 256 out of 16777216 possible colours for the display. Thecontents of the SRAM can be changed while the display is notactive (during display blanking times). The SRAM canusually be bypassed and the DACs can be fed directly bydisplay data (for true colour modes).