Missing instruction fetching

[sameo]

The Core crate decodes and executes instructions, but does not fetch instructions from memory.

In order to fetch instructions, a core needs to know where the code is actually loaded in memory.
This could be overwritten by the still missing executable format, but I propose that the default code section starts at 0x100 in memory. Unless overwritten, $RIP should be initially set to that value, and this is where the first instruction will live (a.k.a. as the reset vector).

The fetch-decode-execute loop should then be as simple as:

1. Read 32 bits from $RIP, that's the next instruction to decode and execute
2. Increase RIP by 4 (next instruction, 32 bits further)
3. Decode the instruction
4. Execute the instruction
5. goto step 1

Note that the instruction executions step (step 4) could modify RIP as well (branches and jumps).