402-00005-00
Theory of Operation
2–10
Rev 02; February 8, 2002
In 8-bit mode and Decimate by 2, the BMC transfers every other byte (pixel) read from the image memory buffer.
The byte select bit (BYTESEL) selects the even or odd bytes for transfer, as shown in Figure 2–7. The transfer count
must be divided by 2; the transferred output data is half the area of the original image data in the image buffer
memory.
7 ... 0
Image Buffer Memory
pixel6
pixel4
pixel2
pixel0
Bus Master Controller Output
Decimate by 2, Select Even
pixel7
pixel5
pixel3
pixel1
Bus Master Controller Output
Decimate by 2, Select Odd
lsb
msb
lsb
msb
pixel6
7 ... 0
pixel5
7 ... 0
pixel4
7 ... 0
pixel3
7 ... 0
pixel2
7 ... 0
pixel1
7 ... 0
pixel0
Figure 2–8. Bus Master Decimate, 8-bit Mode
In 12-bit mode, Decimate by 2 performs a shift and pack operation. This is very useful for quickly displaying 8-bit
data from un-normalized 12-bit data, without using host computer resources for shift, zoom, decimate, or pack op-
erations to get an 8-bit display from 12-bit data.
In 12-bit mode each 12-bit pixel occupies two bytes. If the data is not normalized to 16 bits by the input LUT, the low
byte contains pixel bits 7 through 0, and the high byte contains pixel bits 11 through 8 with the upper 4 bits filled with
“don’t care” values, as shown in Figure 2–9.
In 12-bit Decimate by 2 mode, the bus master controller reads a word of image buffer data, right-shifts four places,
and packs only the new low byte for output to the PCI-bus. Each byte output contains the upper 8 bits (11–4) of the
12-bit pixel, with no “don’t care” bits.
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