29 September 1997 – Subject To Change
Clocks, Cache, and External Interface
4–11
Physical Address Considerations
system environment as to which INT8s are accessed. Write merging is permitted.
Write accesses are INT32 requests with a mask indicating which INT4s are actually
modified.
The 21164PC never writes more than 32 bytes at a time in noncached space.
The 21164PC does not broadcast accesses to the CBU IPR region if they map to a
CBU IPR. Accesses in this region, that are not to a defined CBU IPR, produce
UNDEFINED results. The system should not probe this region.
Table 4–4 shows the 21164PC physical memory regions.
4.3.2 Data Wrapping
The 21164PC requires that wrapped read operations be performed on INT16 bound-
aries. READ and FLUSH commands are all wrapped on INT16 boundaries as
described here. The valid wrap orders for 64-byte blocks are selected by
addr_h<5:4>. They are:
0, 1, 2, 3
1, 0, 3, 2
2, 3, 0, 1
3, 2, 1, 0
Similarly, when the system interface supplies a command that returns data from the
21164PC caches, the values that the system drives on addr_h<5:4> determine the
order in which data is supplied by the 21164PC.
BCACHE VICTIM commands provide the data with the same wrap order as the read
miss that produced them.
Table 4–4 Physical Memory Regions
Region
Address Range
Description
Memory-like
00 0000 0000 –
01 FFFF FFFF
16
Write invalidate cached, load, and store merging
allowed.
Noncacheable
80 0000 0000 –
FF FFEF FFFF
16
Not cached, load merging limited.
IPR region
FF FFF0 0000 –
FF FFFF FFFF
16
Accesses do not appear on the interface unless an
undefined location is accessed (which produces
UNDEFINED results).