402-00005-00
Registers
3–70
Rev 02; February 8, 2002
separate, automatic right-shift of 12-bit data. BMSHIFT is a left-shift. In both 8-bit and 12-bit modes, the transfer
count should be half the original image value in bytes.
A 1024 pixel line of 12-bit pixels occupies 2048 bytes or 512 DWORDs in image memory. The transfer count
should be 512 DWORDs if no decimation occurs. The transfer count should be 256 DWORDs if decimate by 2 is
enabled.
3.5.2.5 Pad Mode Enable (PADEN) R/W
The PADEN bit enables pad mode for bus master cycles. Pad mode modifies data bytes in the Bus Master FIFO. The
BMC inserts a byte of 0x80 in every other pixel location as data is passed to the PCI-bus. This mode is useful when
displaying 8-bit monochrome image data on the 16-bit secondary (or YCrCb) surface (leaving the primary surface
for Windows and Overlay).
In 8-bit mode (8-bit image data in the frame memory), the zoom factor (BMZOOM bits) must be programmed to
“zoom by 2” or “zoom by 4” when using pad mode, and the transfer count must be multiplied by 2 or 4, to account for
the extra data added to the image. Zoom by 2 doubles the image data sent to the bus master controller, and padding
replaces the second (replicated) copy of each pixel with 0x80 in the upper byte of each WORD in the FIFO. Zoom by
4 with pad mode produces a padded zoom by 2 (horizontal) image.
In 12-bit mode, the pixel data is left-shifted by 4 and stored in the low byte of each WORD of the FIFO, and 0x80
stored in the upper byte of each WORD. BMSHIFT and Decimate by 2 are not used with PADEN in 12-bit mode.
This is a separate and independent shift. Refer to Chapter 2 for additional discussion and illustrations.
PADEN
Function
0
Disable pad mode
1
Enable pad mode
3.5.2.6 Clip Mode Enable (CLIPEN) R/W
The CLIPEN bit enables clip mode for bus master cycles. Clip mode modifies data bytes in the Bus Master FIFO.
Any pixel value between 0x00 and 0x0F is forced to 0x10 and any value between 0xF0 and 0xFF is forced to 0xEF.
Clipping these pixel value ranges allows for direct image transfer to a VGA display without seeing color dots.
CLIPEN
Function
0
Disable clipping
1
Enable clipping
3.5.2.7 Bus Master Scan Direction (SCANDIR) R/W
The SCANDIR bit defines the direction or order for reading data from the DWORD memory location to the
DWORD FIFO in the Bus Master Controller (BMC).
SCANDIR
Function
0
Normal forward scan
1
Reverse scan
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