AGP FAQ's

AGP FREQUENTLY ASKED QUESTIONS

1. What is AGP?
2. How does AGP make high-end 3D graphics affordable for the general public?
3. Is AGP designed to be the replacement for the PCI bus?
4. What operating systems are able to support AGP?
5. Is AGP another name for UMA?
6. What different features does AGP have over PCI?
7. What are the benefits of AGP?
8. What's the architecture of AGP?
9. What's Direct Memory Execute (DIME)?
10. How does AGP performance compare with a traditional VGA display card?

1. What is AGP?

AGP stands for Accelerated Graphics Port. It is an interface specification specifically
written to provide for high performance graphics, especially 3D graphics, at affordable
prices. It would put a home computer on par with a 3D workstation.

2. How does AGP make high-end 3D graphics affordable for the general public?

Higher bandwidth is achieved by utilizing a dedicated pipeline to access main memory. This
allows a faster data transfer rate. Cost is reduced by moving graphic data to main memory,
thus allowing AGP interfaces to use system memory for texturing, z-buffering, and alpha
blending. This removes the burden of expensive on board graphic memory.

3. Is AGP designed to be the replacement for the PCI bus?

No. AGP is designed solely for use with graphic controllers. It's not intended to replace the
role of the PCI interface as the general I/O interface bus. AGP is physically, logically, and
electrically independent of the PCI bus. The interface uses a new bus design which is not
compatible with the existing bus connector.

4. What operating systems are able to support AGP?

So far, Microsoft has announced to support the AGP bus in future releases of both Win95
(OSR2.1, VGARTD.VXD from Intel) and Win NT 4.0 (with Service Pack 3) operating
systems. Over one hundred companies, including major graphic component manufacturers
and software companies, have joined the AGP Implementor Forum world wide. Other
major OS players which have not yet announced AGP support should soon announce AGP
support.

5. Is AGP another name for UMA?

No. The primary purpose of AGP is to support high-end 3D graphics on a PC by utilizing
system memory for the 3D effects such as textures, alpha buffers, and z-buffers, while still
having dedicated on-board frame buffer memory on the card itself. UMA architecture is
aiming to move all the on-board frame buffer memory to the system memory in order to
reduce cost caused by expensive on-board dedicated buffer memory. AGP allows for
dynamic memory allocation which means the OS will be able to reclaim the system memory
used for 3D effects by the AGP architecture, but UMA allocates a portion of the system
memory at system boot-up and can not be reclaimed by the OS.

6. What different features does AGP have over PCI?

A comparison table below lists the differences:

AGP	PCI
Pipelined requests	Non-pipelined
Address/data de-multiplexed	Address/data multiplexed
Peak at 533MB/s in 32 bits	Peak at 133MB in 32 bits
Single target, single master	Multi-target, multi-master
Memory read/write only, no other I/O operations	Link to entire system
High/low priority queues	No priority queues

7. What are the benefits of AGP?

1.) Direct texturing from main memory:
-Two memory pipes are provided to graphic engines for concurrency.
-Richer textures with no frame buffer growth.
-Graphics & CPU get a continuous view of graphic data structures from "GART"
(Graphics Address Re-mapping Table) Hardware.
2.) De-multiplexed address and data.
-Enables pipelining and concurrency.
3.) 533 Mbytes /s peak bandwidth. -Delivers high performance data control.
4.) Peak bandwidth can be 4 times the PCI bus bandwidth, and higher sustained rates
via Sideband and pipelining.
5.) Direct Memory Execute Textures.
6.) Reduced Contention with the CPU and I/O devices for bus and memory access. The
PCI bus serves disk controllers, LAN chips, and possibly video capture. AGP
operates concurrently with and independent from most PCI operations. Furthermore,
the CPU can accesses system RAM while with the AGP graphic chip reads RAM,
because of out-of-order queuing hardware support in the chip set. Therefore, in spite
of the heavy access from the graphic chip, there should be no audio breakup or other
CPU degradation.
7.) A separate port for the graphics ship to access memory which allows for concurrent
texture reads from AGP memory while read/writing from local memory. Efficient
utilization of the bandwidths allows the graphic chip to obtain 1.3 GB/s peak by using

both ports simultaneously, versus 0.8 GB/s from the local RAM.
8.) Allowing the CPU to write directly to AGP shared system memory.

8. What's the architecture of AGP?

The figure below shows a simplified diagram of an AGP system, with peak bus bandwidths
and memory usage. The bus on the right hand side of the chipset, between the memory
controller and system memory is typically 64 bits wide, and must be shared by the CPU,
graphic card, and PCI bus. Because of the large amount of RAM support for the system
memory, its clock rate is usually slower than the graphic local memory bus. The initial chip
sets will support 528 MB/s peak on that system bus, but future versions may increase it.
The 800 MB/s peak between local memory and the graphic chip comes from a 64-but bus
at 100MHz as defined in a typical high performance graphics card.

9. What's Direct Memory Execute (DIME)?

The most powerful feature of AGP is the use of DIME. With DIME implemented,
AGP graphic chips have the capability to access main memory directly (called DIME, Direct
Memory Execute) for the complex operation of texture mapping. Without DIME, most PCI
generated graphic controllers can only do texture-mapping from local graphic memory
attached directly to the graphic chip. This would require extra copying from system memory
to CPU then through the chipset to local memory via the PCI bus for every data transfer.
Some initial AGP graphics chips available on the market today may only implement PCI-like
bus mastering, or maybe none at all. AGP bus will provide faster transfer rates, but they will
have to transfer the entire texture map to their local memory via AGP, then re-access the
texture from that local RAM to map individually.

10. How does AGP performance compare with a traditional VGA display card?

AGP cards should be tested like any other graphic card. However, for a more accurate
AGP test measurements you should do a test specifying a large amount of textures (> 8 MB)
and since 3D is the area that really differentiates an AGP display card with a traditional VGA
display card a more suitable test program is the one that does a lot of 3D scenario
measurements. The current version of Winbench 3D has a scene with large textures for a
more realistic test environment.