AMD Geode™ LX Processors Data Book 271
Graphics Processor Register Definitions 33234H
6.4.2.18 Command Bottom (GP_CMD_BOT)
This register defines the ending address of the command buffer within the command buffer region. Bits [23:0] of this register
are combined with the CBASE in GLD_MSR_CONFIG (MSR A0002001h) to form the 32 bit address. This register should
only be changed when the GP is not actively executing out of the command buffer, which can be checked by reading the CE
bit in GP_BLT_STATUS (GP Memory Offset 44h[4]) or by verifying that GP_CMD_READ and GP_CMD_WRITE (GP Mem-
ory Offset 58h and 5Ch respectively) have the same value.
6.4.2.19 Command Read (GP_CMD_READ)
This register points to the location from which the GP fetches the next command buffer data. As data is fetched, this register
increments. When this register equals GP_CMD_BOT (GP Memory Offset 54h) and the data has been fetched, it is
reloaded with the value from GP_CMD_TOP (GP Memory Offset 50h). If the current command buffer had the W (wrap) bit
set in the command word, then this register is reset to GP_CMD_TOP after the execution of the current command buffer.
Typically, this register is read only by the software, and is used in combination with GP_CMD_WRITE (GP Memory Offset
5Ch) to determine how much space is available in the command buffer for new commands. However, this register can be
written. A write to this register also affects the GP_CMD_WRITE register such that when creating and initializing a new
command buffer in memory, the read and write pointers can be updated simultaneously to point to the beginning of the
buffer without the GP thinking that the buffer was non-empty and beginning to fetch. This register must not be written while
the GP is actually executing command buffers as this could cause the GP to hang.
GP Memory Offset 54h
Type R/W
Reset Value 00FFFFE0h
GP_CMD_BOT 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_BOT RSVD
GP_CMD_BOT Bit Descriptions
Bit Name Description
31:24 RSVD Reserved. Read returns 0.
23:5 CMD_BOT Command Bottom. Ending address of the command buffer in the command buffer
region.
4:0 RSVD Reserved. Read returns 0.
GP Memory Offset 58h
Type R/W
Reset Value 00000000h
GP_CMD_READ 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_READ
GP_CMD_READ Bit Descriptions
Bit Name Description
31:24 RSVD Reserved. Read returns 0.
23:0 CMD_READ Command Read. Pointer to the tail of the command buffer in the command buffer region.