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Chapter 4 System Support
Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition - December 2000
4-12
AGP 2X Transfers
During AGP 2X transfers, clocking is basically the same as in 1X transfers except that the 66-MHz
CLK signal is used to qualify only the control signals. The data bytes are latched by an additional
strobe (AD_STBx) signal so that an 8-byte transfer occurs in one CLK cycle (Figure 4-6). The first
four bytes (DnA) are latched by the receiving agent on the falling edge of AD_STBx and the
second four bytes (DnB) are latched on the rising edge of AD_STBx.
Figure 4-6. AGP 2X Data Transfer (Peak Transfer Rate: 532 MB/s)
AGP 4X Transfers
The AGP 4X transfer rate allows sixteen bytes of data to be transferred in one clock cycle. As in
2X transfers the 66-MHz CLK signal is used only for qualifying control signals while strobe
signals are used to latch each 4-byte transfer on the AD lines. As shown in Figure 4-7, 4-byte block
DnA is latched by the falling edge of AD_STBx while DnB is latched by the falling edge of
AD_STBx-.
Figure 4-7. AGP 4X Data Transfer (Peak Transfer Rate: 1064 MB/s)
T1 T2 T3 T4 T5 T6 T7
TRDY-
GNT-
CLK
AD_STBx
AD
D1A D2A D3A
D1B D2B D3B D4A D4B
ST0..2
00x
xxx
xxx
xxx
xxx
xxx
T1 T2 T3
T4
AD_STBx
AD
D1A D2A D3AD1B D2B D3B D4A D4B
CLK
AD_STBx-
00xxxx xxx xxx
ST0..2