RAMDAC
       Random Access Memory Digital to Analog Converter (RAM)
       

Random Access Memory Digital-to-Analog Converter
RAMDAC

    (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)