HP Deskpro EX Technical Reference Manual page 63

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, or 4X) 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 GMCH 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 82815 GMCH supports three transfer rates: 1X, 2X,
and 4X. 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.
T1
CLK
AD
GNT-
TRDY-
ST0..2
Figure 4-5. AGP 1X Data Transfer (Peak Transfer Rate: 266 MB/s)
T2 T3
D1A D1B
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Compaq Deskpro EX Series of Personal Computers
First Edition - August 2000
Technical Reference Guide
T4 T5 T6
D2A D2B
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T7
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