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4–32 Cache and External Interfaces
Alpha 21264/EV67 Hardware Reference Manual
System Port
The command precedes data by at least one SYSCLK period. Table 4–25 shows the
number of SYSCLK cycles between SysAddOut and SysData for all system clock
ratios (clock forwarded bit times) and system framing clock multiples.
Figure 4–5 show a simple example of a fast transfer. The data rate ratio is 1.5X with a
4:1 SYSCLK to INT_FWD_CLK ratio.
Figure 4–5 Fast Transfer Timing Example
In fast data mode, movement of data into the 21264/EV67 requires turning around the
SysData bus that is being actively driven by the 21264/EV67. Given a SysDc fill com-
mand (ReadDataError, ReadData, ReadDataShared, ReadDataShared/Dirty, ReadData-
Dirty), the 21264/EV67 responds as follows:
1. Three GCLK cycles after perceiving the SysDc fill command, the 21264/EV67
turns off its drivers, interrupting any ongoing fast data write transactions.
2. The 21264/EV67 drivers stay off until the last piece of fill data is received, or a new
SysDc write command overrides the current SysDc fill command. It is the responsi-
bility of the external system to schedule SysDc fill or write commands so that there
is no conflict on the SysData bus.
3. The 21264/EV67 samples fill data in the GCLK clock domain, 10 +
SYSDC_DELAY GCLK cycles after perceiving the SysDc fill command. The
Cbox CSR SYSDC_DELAY[4:0] provides GCLK granularity for precisely placing
fills into the processor pipeline discussed in Section 2.2.
Table 4–25 SYSCLK Cycles Between SysAddOut and SysData
GCLK/INT_FWD_CLK (Data Rate Ratio)
System framing clock ratio 1.5X 2.0X 2.5X 3.0X 3.5X 4.0X 5.0X 6.0X 7.0X 8.0X
1 4 32 2 2 21111
2 2 21 1 1 11111
4 1 11 1 1 11111
FM05822B.AI4
SysData_L[63:0]
SYSCLK
Probe
SysAddOut_L[14:0]
SysAddOutClk_L
D0
Response
D1 D2 D3 D4 D5 D6
INT_FWD_CLK
GCLK