User's Manual
Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0
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94
Bit Description
6:5 Shift Register Control. In standard VGA modes, pixel data is transferred from the 4 graphics memory
planes to the palette via a set of 4 serial output bits. These 2 bits of this register control the format in
which data in the 4 memory planes is serialized for these transfers to the palette.
Bits [6:5]=00
One bit of data at a time from parallel bytes in each of the 4 memory planes is transferred to the palette
via the 4 serial output bits, with 1 of each of the serial output bits corresponding to a memory plane. This
provides a 4-bit value on each transfer for 1 pixel, making possible a choice of 1 of 16 colors per pixel.
Serial
Out 1st Xfer 2nd Xfer 3rd Xfer 4th Xfer 5th Xfer 6th Xfer 7th Xfer 8th Xfer
Bit 3 plane 3 bit 7 plane 3 bit 6 plane 3 bit 5 plane 3 bit 4 plane 3 bit 3 plane 3 bit 2 plane 3 bit 1 plane 3 bit 0
Bit 2 plane 2 bit 7 plane 2 bit 6 plane 2 bit 5 plane 2 bit 4 plane 2 bit 3 plane 2 bit 2 plane 2 bit 1 plane 2 bit 0
Bit 1 plane 1 bit 7 plane 1 bit 6 plane 1 bit 5 plane 1 bit 4 plane 1 bit 3 plane 1 bit 2 plane 1 bit 1 plane 1 bit 0
Bit 0 plane 0 bit 7 plane 0 bit 6 plane 0 bit 5 plane 0 bit 4 plane 0 bit 3 plane 0 bit 2 plane 0 bit 1 plane 0 bit 0
Bits [6:5]=01
Two bits of data at a time from parallel bytes in each of the 4 memory planes are transferred to the
palette in a pattern that alternates per byte between memory planes 0 and 2, and memory planes 1 and
3. First the even-numbered and odd-numbered bits of a byte in memory plane 0 are transferred via
serial output bits 0 and 1, respectively, while the even-numbered and odd-numbered bits of a byte in
memory plane 2 are transferred via serial output bits 2 and 3. Next, the even-numbered and odd-
numbered bits of a byte in memory plane 1 are transferred via serial output bits 0 and 1, respectively,
while the even-numbered and odd-numbered bits of memory plane 3 are transferred via serial out bits 1
and 3. This provides a pair of 2-bit values (one 2-bit value for each of 2 pixels) on each transfer, making
possible a choice of 1 of 4 colors per pixel.
Serial
Out 1st Xfer 2nd Xfer 3rd Xfer 4th Xfer 5th Xfer 6th Xfer 7th Xfer 8th Xfer
Bit 3 plane 2 bit 7 plane 2 bit 5 plane 2 bit 3 plane 2 bit 1 plane 3 bit 7 plane 3 bit 5 plane 3 bit 3 plane 3 bit 1
Bit 2 plane 2 bit 6 plane 2 bit 4 plane 2 bit 2 plane 2 bit 0 plane 3 bit 6 plane 3 bit 4 plane 3 bit 2 plane 3 bit 0
Bit 1 plane 0 bit 7 plane 0 bit 5 plane 0 bit 3 plane 0 bit 1 plane 1 bit 7 plane 1 bit 5 plane 1 bit 3 plane 1 bit 1
Bit 0 plane 0 bit 6 plane 0 bit 4 plane 0 bit 2 plane 0 bit 0 plane 1 bit 6 plane 1 bit 4 plane 1 bit 2 plane 1 bit 0
This alternating pattern is meant to accommodate the use of the Odd/Even mode of organizing the 4
memory planes, which is used by standard VGA modes 2h and 3h.
Bits [6:5]=1x
Four bits of data at a time from parallel bytes in each of the 4 memory planes are transferred to the
palette in a pattern that iterates per byte through memory planes 0 through 3. First the 4 most significant
bits of a byte in memory plane 0 are transferred via the 4 serial output bits, followed by the 4 least
significant bits of the same byte. Next, the same transfers occur from the parallel byte in memory planes 1,
2 and lastly, 3. Each transfer provides either the upper or lower half of an 8 bit value for the color for each
pixel, making possible a choice of 1 of 256 colors per pixel.
Serial
Out 1st Xfer 2nd Xfer 3rd Xfer 4th Xfer 5th Xfer 6th Xfer 7th Xfer 8th Xfer
Bit 3 plane 0 bit 7 plane 0 bit 3 plane 1 bit 7 plane 1 bit 3 plane 2 bit 7 plane 2 bit 3 plane 3 bit 7 plane 3 bit 3
Bit 2 plane 0 bit 6 plane 0 bit 2 plane 1 bit 6 plane 1 bit 2 plane 2 bit 6 plane 2 bit 2 plane 3 bit 6 plane 3 bit 2
Bit 1 plane 0 bit 5 plane 0 bit 1 plane 1 bit 5 plane 1 bit 1 plane 2 bit 5 plane 2 bit 1 plane 3 bit 5 plane 3 bit 1
Bit 0 plane 0 bit 4 plane 0 bit 0 plane 1 bit 4 plane 1 bit 0 plane 2 bit 4 plane 2 bit 0 plane 3 bit 4 plane 3 bit 0
This pattern is meant to accommodate mode 13h, a standard VGA 256-color graphics mode.