Compaq Deskpro EP 6233 Technical Reference Manual page 62

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 or 2X) 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 north bridge supports both SBA and AD addressing methods and all three data transfer
rates, 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 bus allows 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 the higher transfer
rates.
AGP 1X Transfers
In AGP 1X transfers 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
have their traditional PCI meaning. The ST0..3 signals are used for priority encoding, with "000"
for low priority and "001" indicating high priority.
T1
CLK
AD
GNT-
TRDY-
ST0..2
Figure 4–5. AGP 1X Data Transfer (Peak Transfer Rate: 266 MB/s)
4-12 Compaq Deskpro EP Series of Personal Computers
T2 T3
D1A D1B
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First Edition - April 1998
T4 T5 T6
D2A D2B
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T7
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