Fetch-Decode-Execute Loop

Putting it together:

The loop is straightforward; the per-instruction logic is the real work. Get one instruction right; the rest are templates.

Edge cases:

• Misaligned pc (set jal to odd address): exception.
• Memory bounds: catch Python IndexError; raise as exception.
• Division by zero (M extension): defined behavior (returns -1 for signed div).

System instructions:

• ECALL: software interrupt. In our toy, dispatch syscall by a7.
• EBREAK: debugger trap. Pause for inspection.
• FENCE: memory ordering. No-op for single-core.