How do processors fetch instructions that are longer than the word length? From what I have read, instructions under x86 can be as long as 15 bytes. Since x86 is a 32-bit architecture (64-bit for x86-64 obviously) how does it handle these long instructions? How are they stored in memory, and how are they fetched?
In general there is not relation between the processor register size (32-bit for x86, 64-bit for x86_64) and the instruction size. Instructions are fetched by a dedicated hardware unit and the current instruction is stored the instruction register, but this register does not need to (and usually does not) have the same size as the data registers. On x86, this register must be able to hold 15 bytes. Instructions are fetched like everything else by issuing read commands to the memory. In general to speed up things, the CPU has an instruction cache so it will not just fetch one instruction at a time but rather an entire cache line at once.