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272 AMD Geode™ LX Processors Data Book
Graphics Processor Register Definitions
33234H
6.4.2.20 Command Write (GP_CMD_WRITE)
This register points to the next location to be written with command buffer data from the processor. After the processor
writes out a complete command buffer starting at this address, it should write to this register to update the value to point to
the next location to be written. This write is what queues the GP that there is command buffer data that needs to be fetched
and activates the command buffer logic within GP. If the Wrap bit is set in a command buffer control WORD, this register
should be written with the same value as that found in GP_CMD_TOP (GP Memory Offset 50h) after the CPU has com-
pleted loading the command buffer in memory.
6.4.2.21 Offset (GP_CH3_OFFSET)
The GP_CH3_OFFSET register is used during a BLT to give a starting location for the BLT data in the channel 3 region of
memory. The register consists of two fields to compose the address, the OFFSET and Nibble Select. The OFFSET field is
a pointer, which when added to the channel 3 base address, gives the memory location of the byte containing the first pixel
of the BLT. As in the destination and source offsets, this value must be aligned correctly for BLT direction and pixel depth.
When host source data is used, the two LSBs of OFFSET must still be initialized with the byte location of the first source
pixel in the host source data stream. Nibble Select is used when the source is 4-bpp, to give an offset within the specified
byte to the nibble representing the starting pixel. Both the OFFSET LSBs and Nibble Select are used to index into the first
DWORD of every new line of source data.
For a rotation of 90° counterclockwise, the offset should point to the top rightmost byte of the source bitmap. For a rotation
of 90° clockwise, the offset should point to the bottom leftmost byte of the source bitmap. For a rotation of 180°, the offset
should point to the opposite corner from that pointed to by the destination offset (e.g., If GP_BLT_MODE (GP Memory Off-
set 40h) indicates a left to right, top to bottom fill, then the destination offset should point to the upper left corner and the
channel 3 offset should point to the bottom right most byte of the source bitmap).
GP Memory Offset 5Ch
Typ e R /W
Reset Value 00000000h
GP_CMD_WRITE Register Map
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
RSVD CMD_WRITE
GP_CMD_WRITE Bit Descriptions
Bit Name Description
31:24 RSVD Reserved. Read returns 0.
23:0 CMD_WRITE Command Write. Pointer to where the next command buffer will be written in the com-
mand buffer region.
GP Memory Offset 60h
Typ e R /W
Reset Value 00000000h
GP_CH3_OFFSET Register Map
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
YLSBS XLSBS N
RSVD
OFFSET