Random Access Memory Digital-to-Analog Converter

hardware (RAMDAC) A combination of three fast DACs with a

small SRAM used in graphics display adapters to store the

colour 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 colour 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 colour modes).

(1996-03-24)