Example: load instruction
Layer (d) could be replaced by a page fault to other machines on a LAN that are sharing the memory (function: sharing), or
Layer (c) by access to a distributed cache over a multiprocessor’s network (function: sharing).
Layer (b) could be replaced by access to a PCI I/O bus (function: device access) which at layer (c) is bridged to an ISA bus (function: compatibility).