Compaq EV67 User Manual
Alpha 21264/EV67 Hardware Reference Manual
Cache and External Interfaces
4–51
Bcache Port
The Relationship Between Write-to-Read — BC_WR_RD_BUBBLES and wr_rd
The following formulas calculate the relationship between the Cbox CSR
BC_WR_RD_BUBBLES and wr_rd:
BC_WR_RD_BUBBLES and wr_rd:
wr_rd = (BC_WR_RD_BUBBLES – 1) * bcfrm
or
BC_WR_RD_BUBBLES = ((wr_rd + bcfrm – 1) / bcfrm) + 1
There is never a need to use a value of 0 or 1 for BC_WR_RD_BUBBLES.
If
wr_rd = 4*ratio
, then value 3 would be the minimum
BC_WR_RD_BUBBLES value when
bcfrm = 2*ratio
, and value 5 would be the
minimum BC_WR_RD_BUBBLES value when
bcfrm = ratio
.
There is a special case for
ratio = 2.0
in single-data mode. In this case, the for-
mula is:
wr_rd = (BC_WR_RD_BUBBLES – 2) * bcfrm
The Relationship Between Read-to-Write — BC_RD_WR_BUBBLES and rd_wr
Use the following formula to calculate the value for the Cbox CSR
BC_RD_WR_BUBBLES that produces the minimum rd_wr restriction:
BC_RD_WR_BUBBLES that produces the minimum rd_wr restriction:
BC_RD_WR_BUBBLES = rd_wr – 6
Note that a value for BC_RD_WR_BUBBLES of zero really means 64 GCLK cycles.
In that case, amend the formula. For example, it is impossible to have
In that case, amend the formula. For example, it is impossible to have
rd_wr = 6
in
the 1.5x dual-data rate mode case.
4.8.4 Pin Descriptions
This section describes the characteristics of the Bcache interface pins.
4.8.4.1 BcAdd_H[23:4]
The BcAdd_H[23:4] pins are high drive outputs that provides the index for the Bcache.
The 21264/EV67 supports Bcache sizes of 1MB, 2MB, 4MB, 8MB, and 16MB. Table
4–42 lists the values to be programmed into Cbox CSRs BC_ENABLE[0] and
BC_SIZE[3:0] to support each size of the Bcache.
The 21264/EV67 supports Bcache sizes of 1MB, 2MB, 4MB, 8MB, and 16MB. Table
4–42 lists the values to be programmed into Cbox CSRs BC_ENABLE[0] and
BC_SIZE[3:0] to support each size of the Bcache.
Table 4–42 Programming the Bcache to Support Each Size of the Bcache
BC_ENABLE[0]
BC_SIZE[3:0]
Bcache Size
1
0000
1MB
1
0001
2MB
1
0011
4MB
1
0111
8MB
1
1111
16MB