Accelerated Graphics Port (AGP) is a high-speed point-to-point channel designed to connect video cards and 3D accelerators to a computer's motherboard. Developed by Intel in 1997, AGP revolutionized graphics performance by offering significantly faster data transfer rates compared to its predecessor, the PCI bus.
Understanding AGP
AGP operates on a 32-bit wide bus at 66 MHz, providing a base bandwidth of 266 MB/s. It also supports faster modes, offering up to 533 MB/s and 1.07 GB/s. This dedicated graphics port simplifies data transfer by eliminating many of the inefficiencies found in the PCI standard.
Key features of AGP include:
- Direct path to system memory
- Faster transfer of video information
- Ability to store 3D textures in main memory
- Sideband addressing for improved efficiency
History and Evolution
Intel launched AGP in August 1997, initially supporting Slot 1 P6 Pentium II and Socket 7 Intel P5 Pentium processors. The first Socket 7 chipsets to support AGP were:
- SiS 5591/5592
- VIA Apollo VP3
- ALI Aladdin V
AGP quickly gained popularity, with early boards often using graphics processors built on PCI architecture and bridged to AGP. Over time, AGP evolved through several versions:
- AGP 1.0 (1997): 1x and 2x signaling, 266-533 MB/s
- AGP 2.0 (1998): 4x signaling, 1,066 MB/s
- AGP 3.0 (2000): 8x signaling, 2,133 MB/s
Features and Benefits
AGP offers several advantages over PCI:
- Higher bandwidth and faster performance
- Dedicated communication between GPU and CPU
- Improved clock speed
- Direct access to system memory
- Efficient texture map handling
- Forward and backward compatibility (within voltage limits)
Compatibility and Power Supply
AGP compatibility depends on motherboard support rather than the graphics card itself. Cards are generally forward and backward compatible within their voltage specifications (1.5V or 3.3V). Universal slots can support both voltage types.
Power supply through an AGP 3.0 slot can reach up to 48.25 watts, though most cards draw less than 40 watts in practice.
Working Process
AGP utilizes several techniques to improve performance:
- Dedicated Port: Provides a direct path between CPU and GPU
- Pipelining: Allows parallel processing of data requests
- Sideband Addressing: Separates addressing from data packets for improved efficiency
RAM Benefits
AGP improves texture map storage by allowing the operating system to dynamically assign RAM for video card use. This non-local video memory, or AGP memory, enhances processing capability by not being limited to the video card's onboard RAM.
The Graphics Address Remapping Table (GART) further optimizes memory usage by re-addressing borrowed system memory, making it appear as if texture maps are stored in the graphics card's frame buffer.
Connector Pinout
The AGP connector features 132 pins across two sides (A and B), with 66 contacts on each side. The pinout includes various signals for power, data transfer, and control functions.
Types of AGP
Besides the main AGP versions (1.0, 2.0, and 3.0), there are several variants and extensions:
- AGP Pro: Extended specification for workstations
- Ultra-AGP and Ultra-AGP II: Unofficial standards used by SiS
- AGP Express, AGI, AGX, XGP, and AGR: PCI-based variations with limited AGP compatibility
Is AGP Better Than PCI?
AGP offers several advantages over PCI:
- 4x higher peak bandwidth
- Direct texture map execution from system memory
- Dedicated connection for graphics subsystem
- More efficient data requests and transfers
- Lower latency due to simplified addressing
These improvements make AGP significantly better for graphics-intensive applications, especially 3D rendering and gaming.
Conclusion
Accelerated Graphics Port represented a significant leap forward in graphics performance when it was introduced. While it has since been superseded by PCI Express, understanding AGP's role in computer history provides valuable insight into the evolution of graphics technology.