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, 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 MCH 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 82865 GMCH supports four transfer rates: 1X, 2X,
4X, and 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
ransfer rates.
t
GP 1X Transfers
A
During a AGP 1X transfer the 66-MHz CLK signal is used to qualify the control and data signa
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- sign
retain their traditional PCI functions. The ST0..3 signals are used for priority encoding, with
“000” for low priority an
ls.
als
d “001” indicating high priority. The signal level for AGP 1X transfers
ay be 3.3 or 1.5 VDC.
Figure 4-5.
AGP 1X Data Transfer (Peak Transfer Rate: 266 MB/s)
m
T1 T2 T3 T4 T5 T6 T7
TRDY
-
GNT
-
CLK
AD
D1A
D2A
D2B
D1B
ST0..2
00x
xxx
xxx
xxx
xxx
xxx
hp compaq d330 and d530 Series of Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition – June 2003
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