"Why would PMT and virtual memomory be an advantage on an STB"
PMT lets you have one task decode video stream at same time as another reads from DVD, something RISC OS is not very good at. In the case of STB then the more things it needs to do at the same time the better PMT is. Virtual Memory is probably less important in an STB unless it has a harddrive. In AIX a version of UNIX you can do I/O by paging which is more efficient than disk I/O, not sure if Linux has this though.