K7VAT
System Board 23
AGP Technical
Background
Speed of data transfers
The PCI Bus runs at 33MHz, the AGP at double that frequency, 66MHz. This is
independent of the mode that is used. AGP cards currently are available in 3 modes, 1X, 2X
and 4X. These are explained in more detail below:
1X mode: The data is clocked only at the rising edge of the clock. Since the databus
is 32 bits (4 bytes) wide the transfer rate is 66 x 4 = 264MB/s.
2X mode: The data is clocked on both the rising and falling edge of the clock,
yielding a transfer rate of 133 x 4 = 532MB/s.
4X mode: The sideband signals allow a transfer rate of 266 x 4 = 1064MB/s. 4X
mode is only supported in the AGP 2.0 Specification.
Note that the transfer rates above are ideal rates, the real transfer rates will be lower due to
main memory latency.
Sideband signals
The 32-bit bus in AGP is used for both address and data. While transactions (reads or writes)
are running, commands (a commands) cannot be issued for queuing until the
transactions finish. Sideband addressing prevents this by separating the commands and
transactions paths. So, the transactions can keep on running without intervening with
commands.
Memory Sharing
Texture data take a large amount of memory in 3D games. If the memory on the AGP card
is insufficient performance will suffer. Consider a graphics card with 4 Mbytes memory.
Under resolution 800x600 with 16-bit color depth & 16-bit Z-buffer, 2.74Mbytes are used
under double buffer configuration and only 1.36Mbytes are left for texture cache. That is
not enough for most games, so textures will constantly be swapped in and out of local
memory. Now with textures located in the shared AGP memory, only one access is required.
That is, from AGP memory to frame buffer instead of a copy from system memory to VGA
local memory, and next, from VGA local memory to frame buffer. The amount of memory
shared is known as the AGP Aperture, the size can be set in the BIOS. Other names for
memory sharing are AGP Execute Mode, DME, and DIME.
