AMD Geode™ LX Processors Data Book 239
Graphics Processor 33234H
6.3.1 Command Buffer
The AMD Geode LX processor supports a command buffer
interface in addition to the normal two-deep pipelined regis-
ter interface. It is advised that software use either the com-
mand buffer interface or the register interface. It is possible
to use both, however, all pending operations should be
allowed to complete before making the switch. The com-
mand buffer interface is controlled through four registers
that specify the starting address of the command buffer,
the ending address of the command buffer, the current
write pointer and the current read pointer. The base
address (top 12 bits) of the command buffer is specified in
the GLD_MSR_CONFIG (MSR A0002001h). During initial-
ization, a block of memory is allocated to be the command
buffer space. This block must be entirely contained within a
non-cacheable 16 MB region of physical memory. The
Geode LX processor will not issue coherent transactions
for the command buffer or any other memory operations.
The starting address should be written to GP_CMD_TOP
and the ending address should be written to
GP_CMD_BOT (GP Memory Offset 50h and 54h respec-
tively). The starting address should also be written to
GP_CMD_READ (GP Memory Offset 58h). Writing to
GP_CMD_READ automatically updates GP_CMD_WRITE
(GP Memory Offset 5Ch). From this point, software can ini-
tiate an action in the processor by writing a command
buffer structure into memory at the write address
(GP_CMD_WRITE), then updating the write address to
point to the next available space in the command buffer,
either the next contiguous DWORD address, or the buffer
starting address (GP_CMD_TOP) if the wrap bit is set in
the command buffer control word. Command buffers are
allowed to wrap around the end of the command buffer
space (i.e., whenever the end of the space is reached, the
hardware will continue fetching at the beginning of the
space creating a circular buffer). However, software may
force a wrap before the end of the buffer space is reached
by setting the wrap bit in the control word, which causes
the hardware to reset its read pointer to the beginning of
the buffer space when the current command buffer is com-
plete.
Do not attempt to perform a BLT that expects host source
data for both the old source channel and channel 3 unless
one of the channels is receiving its host source data
through the command buffer, and the other is receiving it
directly from the processor. If this rule is violated, the GP
and/or the entire system may hang.
The structure of a BLT command buffer is as follows:
Table 6-8. BLT Command Buffer Structure
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
W 0 0 S RSVD Write Enables
GP_RASTER_MODE Data
GP_DST_OFFSET Data
GP_SRC_OFFSET Data
GP_STRIDE Data
GP_WID_HEIGHT Data
GP_SRC_COLOR_FG Data
GP_SRC_COLOR_BG Data
GP_PAT_COLOR_0 Data
GP_PAT_COLOR_1 Data
GP_ PAT_ DATA_ 0 Da ta
GP_ PAT_ DATA_ 1 Da ta
GP_CH3_OFFSET Data
GP_CH3_MODE_STR Data
GP_CH3_WIDHI Data
GP_BASE_OFFSET Data
GP_BLT_MODE Data
DTYPE RSVD DCOUNT
Optional Data Word 0
Optional Data Word 1
...
Optional Data Word n