M69000 Asiliant Technologies, M69000 Datasheet - Page 163

• Manufacturer Part Number: M69000
• Manufacturer: Asiliant Technologies

Specifications of M69000

• Lead Free Status / Rohs Status: Supplier Unconfirmed

13-2
Palette Registers
Accessing Color Data Locations Within the Palette
A complex sub-indexing scheme using separate read and write access indices and a data port is used to
access both the standard and alternate palette locations within the palette where color data is stored. The
Palette Read Index Register is used to select the palette location to be read from via the the Palette Data
Register, while the Palette Write Index Register is used to select the palette location to be written to. This
arrangement allows the same data port to be used for reading from and writing to two different palette
locations.
To read a palette location, the index of the desired palette location must first be written to the Palette Read
Index Register. Then all three bytes of data in that palette location may be read, one at a time, via the
Palette Data Register. The first byte read from the Palette Data Register retrieves the 8-bit value specifying
the intensity of the red color component while the second and third byte reads are for the green and blue
color components, respectively. After completing the third read operation, the Palette Read Index Register
is automatically incremented so that the data of the next palette location becomes accessible for being read.
This allows the contents of all 256 palette locations to be read by specifying only the index of the 0th location
in the Palette Read Index Register, and then simply performing 768 successive reads from the Palette Data
Register.
Writing palette locations entails a very similar procedure. The index of the desired palette location must first
be written to the Palette Write Index Register. Then all three bytes of data to specify a given color may be
written, one at a time, to the selected palette location via the Palette Data Register. The first byte written to
the Palette Data Register specifies the intensity of the red color component, while the second and third byte
writes are for the green and blue color components, respectively. One important detail is that all three of
these bytes must be written before the hardware will actually update these three values in the selected
palette location. When all three bytes have been written, the Palette Write Index Register is automatically
incremented so that the next palette location becomes accessible for being written. This allows the contents
of all 256 palette locations to be written by specifying only the index of the 0th palette location in the Palette
Write Index Register, and then simply performing 768 successive writes to the Palette Data Register.
In addition to the standard set of 256 palette locations, there is also an alternate set of 8 palette locations
used to specify the colors used to draw cursors 1 and 2, and these are also accessed using the very same
sub-indexing scheme. Bit 0 of the Pixel Pipeline Configuration 0 Register (XR80) determines whether the
standard 256 palette locations or the alternate 8 palette locations are to be accessed.
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69000 Databook
Subject to Change Without Notice
Revision 1.3 8/31/98