Hello,
first of all which device are you going to use?
Second after you have made your choice look for application notes about memory interface and/or boot methods: usually there are a lot of documents with example source code available from manufacturer.
Parallel interface: I suppose you are referring to parallel memories interface (e.g. 8-bit, 16-bit ...) where accesses to the memory chip are done by means of a address and data bus, while serial SPI memories uses less connection due to the communications method (simpler but slower).
HPI stands for Host Port Interface: the HPI port is a particular interface that makes the internal memory of the host processor accessible from the outside. In this way an external master processor can directly load the program into the host device and successively send the start execution command to the host.
Regards
Mowgli