Instruction manual

Position Field Function
31 BAD_TCP Bad tag control parity.
35:32 BC_PA_DIS 4-bit field. Set. Only physical addresses with A<33:32> = 00
can be cached.
36 BAD_DP Bad data parity.
Note
The 300 model has a faster read/write access to its Bcache than other
models, because the 300 model drives only half as many Bcache RAMs.
2.6 Bcache Tag Space
The Bcache is the system module-level secondary cache. From physical address
0.8000.0000 to address 0.FFFF.FFFF, there are 4096 successive copies of the 16
384 Bcache tags accessible for writes only. These tags are spaced 32 bytes apart.
Thus the tag for block 0 of the Bcache first appears at address 0.8000.0000; the
tag for block 1 of the Bcache first appears at address 0.8000.8020.
Here is a sample Bcache tag for the 300/400/600/700 models. On the 500/800/900
models the ‘‘block to write to’’ fields also covers bits 19 and 20 fora2MBcache.
05 04 0019 182931 3033
MR−0064−93RAGS
010 NEW TAG VALUE BLOCK TO WRITE TO UNUSED
When a store operation references this address space, the data are ignored. The
tag portion of the address (<29:19>) is used as data for writing the cache tag
specified by <18:5>. <30,4:0> are ignored.
When the tag is written, the control bits associated with the block are set to
indicate that the block is dirty. You can write incorrect parity for a tag during
a tag space write operation, by storing to an offset of 8 bytes from the normal
tag space address. Thus a store to 0.8000.0008 will write incorrect tag parity for
block 0 of the Bcache.
2–12 Memory and I/O Addressing