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4–12
Clocks, Cache, and External Interface
29 September 1997 – Subject To Change
Bcache Structure
4.3.3 Noncached Read Operations
Read operations to physical addresses that have addr_h<39> asserted are not cached
in the Dcache or Bcache. They are merged like other read operations in the miss
address file (MAF). To prevent several read operations to noncached memory from
being merged into a single 32-byte bus request, software must insert memory barrier
(MB) instructions or set MAF_MODE IPR bit (IO_NMERGE). The MAF merges as
many Dstream read operations together as it can and sends the request to the BIU.
Rather than merging two 32-byte requests into a single 64-byte request, the BIU
requests a READ MISS from the system. Signals int4_valid_h<3:0> indicate which
of the four quadwords are being requested by software. The system should return the
fill data to the 21164PC as usual. The 21164PC does not write the Dcache or Bcache
with the fill data. The requested data is written in the register file or Icache.
Note:
A special case using int4_valid_h<3:0> occurs during an Icache fill. In
this case the entire returned block is valid although int4_valid_h<3:0>
indicates zero.
4.3.4 Noncached Write Operations
Write operations to physical addresses that have addr_h<39> asserted are not writ-
ten to any of the caches. These write operations are merged in the write buffer before
being sent to the system. If software does not want write operations to merge, it must
insert MB or WMB instructions between them.
When the write buffer decides to write data to noncached memory, the BIU requests
a WRITE BLOCK. During each data cycle, int4_valid_h<3:0> indicates which
INT4s within the INT16 are valid.
4.4 Bcache Structure
The 21164PC supports a .5, 1, 2, and 4MB Bcache. The size is under program con-
trol and is specified by CBOX_CONFIG<13:12> (BC_SIZE<1:0>). The Bcache
block size is 64-byte blocks.
Industry-standard, burst-mode synchronous static RAMs (SSRAMs) may be con-
nected to the 21164PC without many extra components, although fanout buffers may
be required for the index lines. The SSRAMs are directly controlled by the 21164PC,
and the Bcache data lines are connected to the 21164PC data bus.