Intel 460GX 7.3Bandwidth, Table 7-3.Burst Write Combining Modes, 7.2.7.11Fast Back-to-BackTransactions

AGP Subsystem

Table 7-3. Burst Write Combining Modes

Write Command	Transfer	Data Length	Combining Supported
Used	Mode
Memory Write	1X	< 4 DW	Can not be combined with the next access.
		>= 4 DW	Can combine next access if it is sequential, regardless
			of next access size.
Memory Write	2X	< 8 DW	Can not be combined with the next access.
		>= 8 DW	Can combine next access if it is sequential, regardless
			of next access size.
Memory Write	4X	< 16 DW	Can not be combined with the next access.
		>= 16 DW	Can combine next access if it is sequential, regardless
			of next access size.
Memory Write	All	must be Line	Can only combine sequential lines.
Invalidate

Table 7-4. Burst Write Combining Examples with 3 Writes in 1X Transfer Mode

1st write	2nd write	3rd write	Transferred as:
x DW	y DW	z DW	if y < 4
(x<4)			x DW followed by y DW followed by z DW
			else
			x DW followed by (y+z) DW
x DW	y DW	z DW	if y < 4
(x>=4)			(x+y) DW followed by z DW
			else
			(x+y+z) DW

7.2.7.11Fast Back-to-Back Transactions

The GXB as a PCI target will accept fast back-to-back cycles from a PCI master accessing different agents during back-to-back sequence. As an initiator the GXB does not generate a fast back-to- back cycle.

7.3Bandwidth

The bandwidth of AGP 4X mode is 1 GB peak. The sustained bandwidth will be less than this. The sustainable bandwidth obtainable with the GX is dependent on several things including the size of the requests from the graphics card. Table 7-5shows an estimate of obtainable bandwidth. There is a fairly broad range for each of the sizes. The bandwidth will depend on the mix of traffic. Traffic consisting of all reads by the graphics card or all writes by the processor to the card will result in higher bandwidths than a 50/50 mix of reads by the graphics card and writes by the processor to the card. This table will change as the implementation is completed and should be viewed as a guideline for the graphics card designer for relative performance trade-offs.

Intel® 460GX Chipset Software Developer’s Manual 7-13