Intel 815 manual Bits 65=00, Bits 65=01, Bits 65=1x

Intel® 815 Chipset: Graphics Controller PRM, Rev 1.0

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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.

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