When you use Oracle VM Server for SPARC, you can assign most of the memory in a SPARC T8 server to logical domains. However, a small portion of the server's memory is preassigned to software components, the hypervisor, and certain I/O devices. Also, if DIMM sparing is active, a portion of memory is reserved to enable the server to operate normally after a DIMM has failed.
To determine which portions of memory are unavailable for logical domains, type:
# ldm ls-devices -a mem
In the output, look for rows that include _sys_ in the Bound column. Each of those portions of memory is not available for logical domains.