System Port

The command precedes data by at least one SYSCLK period. Table 4–25shows the number of SYSCLK cycles between SysAddOut and SysData for all system clock ratios (clock forwarded bit times) and system framing clock multiples.

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

3

2

2

2

2

1

1

1

1

2

2

2

1

1

1

1

1

1

1

1

4

1

1

1

1

1

1

1

1

1

1

 

 

 

 

 

 

 

 

 

 

 

Figure 4–5show 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

SysAddOut_L[14:0] Probe Response

SysData_L[63:0]

D0

D1

D2

D3

D4

D5

D6

SYSCLK

SysAddOutClk_L

INT_FWD_CLK

GCLK

FM05822B.AI4

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.

4–32Cache and External Interfaces

Alpha 21264/EV67 Hardware Reference Manual

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Compaq EV67, 21264 specifications Sysclk Cycles Between SysAddOut and SysData, 32Cache and External Interfaces