RIVA128ZX datasheet(14/85 Pages) STMICROELECTRONICS

14 / 85 page

128-BIT 3D MULTIMEDIA ACCELERATOR

RIVA128ZX

14/85

ACCELERATED GRAPHICS PORT (AGP) INTERFACE

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

do not have special access ordering or coher-

ency problems.

2 Shifting textures balances the bandwidth load

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

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.

3 Texture size is dependent upon application

quality rather than on display resolution, and

therefore subject to the greatest pressure for

growth.

4 Texture data is not persistent; it resides in

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).

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

AGP chipset

RIVA128ZX

System

memory

CPU

I/O

PCI

AGP

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