Service manual

System Overview 1-71
The directory module is associated with the local memory contained within a QBB. In
systems with more than one QBB, a directory module is required in each.
The directory functions as the focal point for memory coherency. It is used to store the
processor ID of the current owner and node masks or presence bits of the nodes that have
acquired shared copies of a cache block belonging to memory in the local QBB. The
directory module is accessed for every local memory reference issued by a local processor,
remote processor, or I/O device to determine if the memory contains a valid copy of the
requested block or if the block is dirty elsewhere in a remote processor’s cache. If the
requested block on the local QBB is not the most current, the directory module forwards
the request to the CPU in the system that has it. The directory module also sends an
invalidate command to nodes that may have a shared copy of the requested block when
the requester of the block intends to modify it. See Appendix B for a description of cache
coherency.
As shown in Figure 1–31, the directory module contains two ASICs, the directory address
controller (DMA) and directory data controller (DMD) as well as eight custom DIMM
modules. The DIMMs have two independent arrays, one with 20 data bits and the second
with 21 address and control bits.
The directory module detects single-bit, double-bit, and some greater-than-double-bit
errors and performs error correction for single-bit errors. The DMD ASIC provides a
data path, error detection/correction, directory response, and directory state updates
based on the incoming command and the current state. The DMA ASIC provides address
and control for the SDRAM, diagnostic/self-test functions, and interfaces to the incoming
commands from the QSA and to the directory’s microprocessor.
DIMM types used on directory modules must be large enough to hold data for each block
of data in all memories on a QBB. Table 1–11 shows the directory DIMM type and part
number associated with a given array size.
Table 1– 11 Directory Array DIMM Types
Dir DIMM Type Memory Array Size Dir DIMM Part Number
0 256 MB 54-25019-Ax
1 1 GB 54-25023-Ax
2 2 GB 54-25023-Bx
3 2 GB 54-25023-Cx
4 4 GB 54-25023-Dx