Chapter 4: AGP Memory Mapping So called AGP memory is just dynamically-allocated areas of system memory, which the graphics controller can access quickly. The access speed comes from built-in hardware in the 440LX chipset which translates addresses, allowing the graphics controller and its software to see a contiguous space in main memory, when in fact the pages are disjointed. Thus the graphics controller can access large data structures like texture bitmaps (typically 1 KByte to 128 KByte) as a single entity. The built-in chipset hardware is called the GART (Graphics Address Remapping Table), similar in function to the paging hardware in the CPU. The processor "linear" virtual addresses are translated by its paging hardware into physical addresses. These physical addresses are used to access system memory, the local frame buffer, and AGP memory. The CPU accesses to the local frame buffer and AGP memory use the same addresses as the graphics controller does. The operating system therefore sets up the CPU paging hardware to a straight 1:1 non-translation of virtual to physical address. For accesses to AGP memory, the graphics controller and CPU use a contiguous aperture of several megabytes. But the GART translates these to various, possibly disjointed, 4 KByte page addresses in system memory. PCI devices that access to the AGP memory aperture (for example, for live video capture) also go through the GART. Chapter 5 Table of Contents