Compaq 21264 - Manual (Page 135)

Alpha 21264/EV67 Hardware Reference Manual Cache and External Interfaces 4–47
Bcache Port
In addition to programming the clock CSRs, the data-sample/drive Cbox CSRs, at the
pads, must be set appropriately. Table 4–41 lists these CSRs and provides their pro-
grammed value.
4.8.3 Bcache Transactions
The Cbox uses the programmed clock values to start data read, tag read, data write, and
tag write transactions on the rising edge of a Bcache clock. The Cbox can also be con-
figured to introduce a programmable number of bubbles when changing between write
and read commands. The following three sections describe these Bcache transactions.
4.8.3.1 Bcache Data Read and Tag Read Transactions
The 21264/EV67 always reads four pieces of data (64 bytes) from the Bcache during a
data read transaction, and always interrogates the tag array on the first cycle. Once
started, data read transactions are never cancelled. Assuming that the appropriate values
4.0X-DD 0F0F 0 01
5.0X-DD 7C1F 0 40
6.0X-DD F03F 0 10
7.0X-DD C07F 0 04
8.0X-DD 00FF 0 01
1
These are hexadecimal values.
Table 4–41 Data-Sample/Drive Cbox CSRs
CBOX CSR Description
BC_DDM_FALL_EN[0] Enables the update of the 21264/EV67’s Bcache outputs referenced to the
falling edge of the Bcache forwarded clock. Dual-data RAMs assert this
CSR.
BC_TAG_DDM_FALL_EN[0] Enables the update of the 21264/EV67’s Bcache tag outputs referenced to the
falling edge of the Bcache forwarded clock. Alway deasserted.
BC_DDM_RISE_EN[0] Enables the update of the 21264/EV67’s Bcache outputs referenced to the ris-
ing edge of the Bcache forwarded clock. Always asserted.
BC_TAG_DDM_RISE_EN[0] Enables the update of the 21264/EV67’s Bcache tag outputs referenced to the
rising edge of the Bcache forwarded clock. Always asserted.
BC_DDMF_ENABLE[0] Enables the rising edge of the Bcache forwarded clock. Always asserted.
BC_DDMR_ENABLE[0] Enables the falling edge of the Bcache forwarded clock. Always asserted.
BC_FRM_CLK[0] Forces the 21264/EV67 to only start Bcache transactions on the rising edge
of Bcache clocks that also coincide with the rising edge of GCLK. Must be
asserted for all dual-data parts and single-data parts at 2.5X and 3.5X.
BC_CLKFWD_ENABLE[0] Enables clock forward enable. Always asserted.
Table 4–40 Program Values to Set the Cache Clock Period (Dual-Data Rate) (Continued)
Bcache
Transfer BC_CLK_LD_VECTOR
1
BC_BPHASE_LD_VECTOR
1
BC_FDBK_EN
1