STG3005A2S STMicroelectronics, STG3005A2S Datasheet - Page 14

• Manufacturer Part Number: STG3005A2S
• Description: 128-BIT 3D MULTIMEDIA ACCELERATOR
• Manufacturer: STMicroelectronics

RIVA128ZX
4
The Accelerated Graphics Port (AGP) is a high performance, component level interconnect targeted at 3D
graphical display applications and based on performance enhancements to the PCI local bus.
Figure 1. System block diagram showing relationship between AGP and PCI buses
Background to AGP
Although 3D graphics acceleration is becoming a
standard feature of multimedia PC platforms, 3D
rendering generally has a voracious appetite for
memory bandwidth. Consequently there is upward
pressure on the PC’s memory requirement leading
to higher bill of material costs. These trends will in-
crease, requiring high speed access to larger
amounts of memory. The primary motivation for
AGP therefore was to contain these costs whilst
enabling performance improvements.
By providing significant bandwidth improvement
between the graphics accelerator and system
memory, some of the 3D rendering data structures
can be shifted into main memory, thus relieving
the pressure to increase the cost of the local
graphics memory.
Texture data are the first structures targeted for
shifting to system memory for four reasons:
1 Textures are generally read only, and therefore
2 Shifting textures balances the bandwidth load
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do not have special access ordering or coher-
ency problems.
between system memory and local graphics
memory, since a well cached host processor
has much lower memory bandwidth require-
ments than a 3D rendering engine. Texture ac-
cess comprises perhaps the largest single com-
ponent of rendering memory bandwidth (com-
pared with rendering, display and Z buffers), so
avoiding loading or caching textures in graphics
ACCELERATED GRAPHICS PORT (AGP) INTERFACE
RIVA128ZX
AGP
I/O
PCI
AGP chipset
I/O
CPU
128-BIT 3D MULTIMEDIA ACCELERATOR
3 Texture size is dependent upon application
4 Texture data is not persistent; it resides in
Other data structures can be moved to main mem-
ory but the biggest gain results from moving tex-
ture data.
Relationship of AGP to PCI
AGP is a superset of the 66MHz PCI Specification
(Revision 2.1) with performance enhancements
optimized for high performance 3D graphics appli-
cations.
The PCI Specification is unmodified by AGP and
‘reserved’ PCI fields, encodings and pins, etc. are
not used.
AGP does not replace the need for the PCI bus in
the system and the two are physically, logically,
and electrically independent. As shown in Figure 1
I/O
local memory saves not only this component of
local memory bandwidth, but also the band-
width necessary to load the texture store in the
first place. Furthermore, this data must pass
through main memory anyway as it is loaded
from a mass store device.
quality rather than on display resolution, and
therefore subject to the greatest pressure for
growth.
memory only for the duration of the application,
so any system memory spent on texture stor-
age can be returned to the free memory heap
when the application finishes (unlike display
buffers which remain in use).
memory
System