Compaq 21264, EV67 specifications Dual-data mode, bcfrm is twice the ratio, Ratio is even or odd

Bcache Port

priate programming of the Bcache clock period and delay parameters to satisfy SSRAM setup/hold requirements of the Bcache address latch, a Bcache write transaction pro- ceeds through the Cbox as follows:

1.The Cbox transmits the index and write control signals during an Int_Adr_BcClk edge.

2.The data is placed on Bcache data, tag, and tag status pins on the appropriate

Int_Data_BcClk edge from 0 to 7 Bcache bit-times later, based on the Cbox CSR BC_LATE_WRITE_NUM[2:0]. The BC_LATE_WRITE_NUM[2:0] supports the late-write SSRAM, which optimize Bcache data bus bandwidth by minimizing bubbles between read and write transactions. For example, single-data late-write SSRAMs would need this CSR programmed to a value of one, and dual-data late- write SSRAMs would need this CSR programmed to a value of two.

3.The difference between the data delivery (Int_Data_BcClk) and forwarded clocks out provides the setup for the data at the Bcache data flip-flop.

4.For Bcache writes, the 21264/EV67 drivers are enabled on the GCLK BPHASE preceding the start of a write transfer, and disabled on the succeeding GCLK BPHASE at the end of a write transfer. Thus, the write data is enveloped by the 21264/EV67 drivers to guarantee that every data transfer has the same data-valid window.

4.8.3.3Bubbles on the Bcache Data Bus

When changing between read and write transactions on the bidirectional bus, it is often necessary to introduce NOP cycles (bubbles) to allow the bus to settle and to drain the Bcache read pipeline. The Cbox provides two CSRS, BC_RD_WR_BUBBLES[5:0] and BC_WR_RD_BUBBLES[3:0], to help control the bubbles between read and write transactions.

The optimum parameters for these CSRs are determined by formulas that include the following terms:

Term	Description
bcfrm	Bcache frame clock.
	• In dual-data mode, bcfrm is twice the ratio.
	• In single-data mode, the value for bcfrm is determined by whether
	the ratio is even or odd:
	– When the ratio is even, bcfrm is equal to the ratio.
	– When the ratio is odd, bcfrm is twice the ratio.
	For example, in single-data mode:
	Ratio	Bcfrm
	2	2
	2.5	5
GCLK	The processor clock.

Alpha 21264/EV67 Hardware Reference Manual Cache and External Interfaces 4–49