Random Access Memory Digital-to-Analog Converter definition
<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)
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