Bandwidth; Burst Write Combining Modes; Burst Write Combining Examples With 3 Writes In 1X Transfer Mode - Intel 460GX Software Developer’s Manual

Table 7-3. Burst Write Combining Modes
Write Command
Used
Memory Write
Memory Write
Memory Write
Memory Write
Invalidate
Table 7-4. Burst Write Combining Examples with 3 Writes in 1X Transfer Mode
1st write
x DW
(x<4)
x DW
(x>=4)
7.2.7.11 Fast 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.
7.3

Bandwidth

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.
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
Transfer
Data Length
Mode
1X < 4 DW
>= 4 DW
2X < 8 DW
>= 8 DW
4X < 16 DW
>= 16 DW
All must be Line
2nd write 3rd write
y DW z DW
y DW z DW
Table 7-5
Combining Supported
Can not be combined with the next access.
Can combine next access if it is sequential, regardless
of next access size.
Can not be combined with the next access.
Can combine next access if it is sequential, regardless
of next access size.
Can not be combined with the next access.
Can combine next access if it is sequential, regardless
of next access size.
Can only combine sequential lines.
Transferred as:
if y < 4
x DW followed by y DW followed by z DW
else
x DW followed by (y+z) DW
if y < 4
(x+y) DW followed by z DW
else
(x+y+z) DW
shows an estimate of obtainable bandwidth. There is
AGP Subsystem
7-13