AGP interface overcomes PCI bandwidth limitations

AGP interface overcomes PCI bandwidth limitations

Fast floating-point performance allows CPUs to draw graphical 3-D meshes and add depth complexity to graphically generated scenes. To add realism, PC-based graphics systems render 3-D images by adding textures, alpha-blended transparencies, and texture-mapping. To speed this performance, Intel Corp. (Santa Clara, CA) has developed the accelerated graphics port (AGP), a dedicated high-speed port that can move large blocks of 3-D texture data between a PC`s graphics controller and system memory.

Positioned between the PC`s chipset and graphics controller (see figure), the AGP increases the bandwidth available to a graphics accelerator. As a 32-bit-wide channel, the AGP runs at 66 MHz and provides a bandwidth of 266 Mbytes/s, as opposed to the PCI-specified 133-Mbytes/s bandwidth. The AGP also supports two optional faster modes, known as AGP x2 and AGP x4, with throughputs of 533-Mbyte/s and 1-Gbyte/s, respectively.

According to Intel, the AGP interface uses the 66-MHz PCI specification as a baseline and provides enhancements to the PCI specification to optimize AGP for 3-D graphics applications. These enhancements, which include pipelined-memory read-and-write operations and demultiplexing of address and data on the bus, allow almost 100% bus efficiency and AC timing for 133-MHz data-transfer rates. The results achieve real data throughput in excess of 500 Mbyte/s.

Physically, logically, and electrically independent of the PCI bus, the AGP is designed specifically for graphics controllers; it is not intended to replace PCI. Says Jeff Wilson, vice president of sales and marketing at Bitflow (Woburn, MA), "The AGP is a one-only slot that is intended to support high-end display cards. In theory, its peak data rate is 2x PCI, and (by its single-slot nature) the I/O bandwidth is not shared among multiple peripherals." Using the AGP slot for other purposes renders it unusable for the VGA display, so the VGA display controller must plug into an available PCI or ISA slot. "Finally," says Wilson, "the AGP is PC-specific, which makes it a nonstarter for other platforms that provide PCI I/O capability, such as the Compaq/DEC Alpha or the Apple Power Macintosh."

Despite these limitations, graphics-controller and frame-grabber vendors are endorsing the AGP standard. The Number Nine Visual Technology (NNVT; Lexington, MA) 128-bit SR9 accelerators, for example, are both compatible with Intel AGP 4x-based motherboards and the new DVI specification (see "Digital interfaces promise plug-and-play monitors, flat-panels," see p. 9). According to NNVT, forthcoming systems based on the board are expected to transfer graphical data across the AGP at rates to 1 Gbyte/s, which doubles those of current AGP 2x-based systems. The PCI versions of the SR9 are also available for users with today`s PCI-based systems.

Integral Technologies (Indianapolis, IN) has also endorsed the AGP bus. However, rather than produce a graphics display-controller card, Integral has developed a frame-grabber/display controller combination, the FlashPoint 3D. Supplied in both AGP and PCI versions, the board can capture and display full-frame color and RS-170 video in real time to VGA display memory. Supplied with multiple video inputs and triggers and composite and S-video outputs, the board`s 128-bit memory interface accommodates display resolutions to 1600 x 1200 pixels at 85 Hz.

Supporting nondestructive overlay of text and graphics on live video, the FlashPoint 3D Plus supports applications that require crosshairs, selector boxes, or other objects to be displayed on live video while the underlying video is captured or displayed. Independent control of graphics and video color allow simultaneous display and capture of up to 768 x 576 video on a 1280 x 1024-pixel display.

In operation, FlashPoint 3D Plus can multiplex up to six composite, three S-Video, and four RS-170 video inputs in both NTSC and PAL formats. By supporting the AGP interface, motherboard, graphics-controller, and frame-grabber vendors have overcome the current 132-Mbyte/s limitation of PCI-based systems. In the future, the introduction of smart digital video cameras is expected to allow frame-grabber vendors, in particular, to reduce the cost of such boards and provide a direct digital video input to computer motherboards.