Accelerated Graphics Port review

Just when you thought the processor wars had reached a brief hiatus, Intel is attempting to consolidate on the critical success of the Pentium II with a new performance-boosting chipset. The 440LX is the much-awaited successor to the 440FX, and contains a host of improved features, the most important of which is the Accelerated Graphics Port or AGP.

Intel believes that the PCI peak bandwidth of 132Mb, which originally seemed boundless, is now looking a little meagre when faced with the enormous data throughput requirements of 3D. To avoid the bottleneck, Intel developed AGP as a separate connector that operates off the processor bus. The AGP chipset acts as the intermediary between the processor and Level 2 cache contained in the Pentium II's Single Edge Contact Cartridge, the system memory, the graphics card and the PCI bus. This is called Quad Port acceleration.

The AGP connector looks uncannily like a PCI bus. In fact, it's much more sophisticated, with a few extra wires to enable advanced features. The connection itself has a double row of contacts rather than the single row used by PCI. Therefore, the card must be more accurately seated, as I discovered when I had to re-install the card to get it running.

AGP operates at the speed of the processor bus, which is now known as the frontside bus, so it has a clock rate of 66MHz. This is double the PCI clock speed, and means that the peak base throughput is 264Mbytes/sec. Initial graphics adaptors ported from PCI will get immediate benefit from this improved bandwidth. However, AGP has quite a few more tricks up its sleeve.

The first is that data can be sent during both the up and down clock cycle, doubling clock rate to 133MHz and peak transfer to 528Mbytes/sec. This is known as X2, and is available only for cards that have been specifically designed to support it. To improve the length of time that AGP can maintain this peak transfer, the bus supports pipelining, which is another improvement over PCI. A pipelining X2 graphics card will be able to sustain throughput at 80 per cent of the peak. As if that wasn't enough, AGP also supports queuing of up to 32 commands over the extra wires mentioned above. To assist, there are eight extra lines for a process called Sideband Addressing (SBA), where these commands are sent while data is being received. This allows the bus to sustain peak performance for 95 per cent of the time, according to Intel.

AGP is primarily intended to boost 3D performance, so there are other improvements that are specifically aimed at this function. With its increased access speed to system memory over the PCI bus, AGP can use system memory as if it's actually on the graphics card. This is called Direct Memory Execute (DIME). A device called a Graphics Aperture Remapping Table (GART) handles the RAM addresses so they can be distributed in small chunks throughout system memory rather than hijacking one large section, and presents them to a DIME-enabled graphics card as if they're part of on-board memory. The main use for DIME is to allow much larger textures to be used because the graphics card can have a much larger memory space in which to load the bitmaps used.

Other improvements with the LX are more like housekeeping, bringing the Pentium II chipset up to the feature set of 430TX. SDRAM is now supported, which will further assist the MMX multimedia performance improvements. In terms of disk I/O, this is backed by Ultra DMA IDE channels, for up to 33Mbytes/sec burst throughput from hard disks. The final piece of the puzzle is the inclusion of the Advanced Configuration and Power Interface (ACPI). This allows quick power down and up, remote startup over a LAN for remote network management, plus temperature and fan speed sensors. The chipset also has better integration with the capabilities of the Pentium II, such as support for dynamic execution and processor pipelining.