Compaq 21264 Hardware Reference Manual page 90

Introduction to the External Interfaces
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The Bcache interface includes a 128-bit bidirectional data bus, a 20-bit unidirec-
tional address bus, and several control signals.
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The system interface includes a 64-bit bidirectional data bus, two 15-bit
unidirectional address buses, and several control signals.
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Figure 4–1 shows a simplified view of the external interface. The function and purpose
of each signal is described in Chapter 3.
Cache and External Interfaces
4–2
The BcDataOutClk_x[3:0] clocks are free-running and are derived from the
internal GCLK. The period of BcDataOutClk_x[3:0] is a programmable mul-
tiple of GCLK.
The Bcache turns the BcDataOutClk_x[3:0] clocks around and returns them
to the 21264/EV67 as BcDataInClk_H[7:0]. Likewise, BcTagOutClk_x
returns as BcTagInClk_H.
The Bcache interface supports a 64-byte block size.
The SysAddOutClk_L clock is free-running and is derived from the internal
GCLK. The period of SysAddOutClk_L is a programmable multiple of
GCLK.
The SysAddInClk_L clock is a turned-around copy of SysAddOutClk_L.
Alpha 21264/EV67 Hardware Reference Manual