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Chapter 4 System Support
4.3.1.1 Data Request
Requesting data is accomplished in one of two ways; either multiplexed addressing (using the AD
lines for addressing/data) or demultiplexed (“sideband”) addressing (using the SBA lines for
addressing only and the AD lines for data only). Even though there are only eight SBA lines (as
opposed to the 32 AD lines) sideband addressing maximizes efficiency and throughput by
allowing the AD lines to be exclusively used for data transfers. Sideband addressing occurs at the
same rate (1X, 2X, 4X, or 8X) as data transfers. The differences in rates will be discussed in the
next section describing data transfers. Note also that sideband addressing is limited to 48 bits
(address bits 48-63 are assumed zero). The IGP component supports both SBA and AD
addressing, but the method and rate is selected by the AGP graphics adapter.
4.3.1.2 Data Transfers
Data transfers use the AD lines and occur as the result of data requests described previously. Each
transaction resulting from a request involves at least eight bytes, requiring the 32 AD lines to
handle at least two transfers per request. The AGP v.2.0 specification (used on D315 models)
supports three transfer rates: 1X, 2X, and 4X. The AGP v3.0 specification (used on d325 models)
supports a fourth transfer rate, 8X. Regardless of the rate used, the speed of the bus clock is
constant at 66 MHz. The following subsections describe how the use of additional strobe signals
makes possible higher transfer rates.
AGP 1X Transfers
During a AGP 1X transfer the 66-MHz CLK signal is used to qualify the control and data signals.
Each 4-byte data transfer is synchronous with one CLK cycle so it takes two CLK cycles for a
minimum 8-byte transfer (Figure 4-5 shows two 8-byte transfers). The GNT- and TRDY- signals
retain their traditional PCI functions. The ST0..3 signals are used for priority encoding, with
“000” for low priority and “001” indicating high priority. The signal level for AGP 1X transfers
may be 3.3 or 1.5 VDC.
xxx
xxx
xxx
xxx
xxx
00x
ST0..2
D1B D2B D2A D1A
AD
CLK
GNT
-
TRDY
-
T6 T7 T5 T4 T3 T2 T1
Figure 4-5. AGP 1X Data Transfer (Peak Transfer Rate: 266 MB/s)
Compaq D315 and hp d325 Personal Computers
Featuring the AMD Athlon XP Processor
Second Edition – April 2003
4-10