Compaq EN Series Technical Reference Manual page 62

AGP 2X Transfers
In 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.
T1
CLK
AD
AD_STBx
GNT-
TRDY-
ST0..2
00x
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. In Figure 4-7, 4-byte transfer D1A
is latched by the falling edge of AD_STBx while D1B is latched by the falling edge of
AD_STBx-.
T1
CLK
AD
AD_STBx
AD_STBx-
ST0..2
xxx
AGP 4X Data Transfer (Peak Transfer Rate: 1064 MB/s)
T2 T3 T4
D1A D1B D2A D2B
xxx xxx
T2
D1A D1B D2A
00x
Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition - September 1998
Technical Reference Guide
T5 T6
D3A D3B D4A D4B
xxx xxx
xxx
T3
D2B D3A D3B D4A D4B
xxx
T7
T4
xxx
4-13