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Chapter 4 System Support
4.3.1.1
D
The differences in rates will be discussed in the next
ction describing data transfers. Note also that sideband addressing is limited to 48 bits (address
its 48-63 are assumed zero). The MCH component supports both SBA and AD addressing , but
er the 66-MHz CLK signal is used to qualify the control and data signals.
is synchronous with one CLK cycle so it takes two CLK cycles for a
inimum 8-byte transfer (Figure 4-5 shows two 8-byte transfers). The GNT- and TRDY- signals
ers
Figure 4-5. AGP 1X Data Transfer (Peak Transfer Rate: 266 MB/s)
Data Request
Requesting data is accomplished in one of two ways; either multiplexed addressing (using the A
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.
se
b
the method and rate is sel d by the P grap adapte
4.3.1.2 Data Transfers
ata transfers use the AD lines and occur as the result of data requests described previously. Each
ans resulting from a request involves at least eight bytes, requiring the 32 AD lines to
andle at least two transfers per request. The 82845 MCH supports three transfer rates: 1X, 2X,
nd gardless of the rate used, the speed of the bus clock is constant at 66 MHz. The
llowing subsections describe how the use of additional strobe signals makes possible higher
an es.
ecte AG hics r.
D
tr action
h
a 4X. Re
fo
tr sfer rat
AGP 1X Transfers
During a AGP 1X transf
Each 4-byte data transfer
m
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 transf
may be 3.3 or 1.5 VDC.
TRDY
-
GNT
-
CLK
D1A D2A D2B D1B
00x
xxx
xxx
xxx
xxx
xxx
ST0..2
AD
T1 T2 T3 T4 T5 T6 T7
Compaq Evo and Workstation Personal Computers
Featuring the Intel Pentium 4 Processor
Second Edition – January 2003
4-12