Built device interrupts

docs/interrupts.md

@@ -113,12 +113,12 @@ TSS/IST is wired up: the handler survived a completely broken stack.
 
 ## What's next (not done here)
 
-- **Device interrupts**: program the local APIC and IO-APIC, wire a timer and the
-  keyboard onto vectors ≥ 32, and (unlike exceptions) actually *return* from them
-  with `iretq` — which `isr_common` already does.
+- **The IO-APIC and the keyboard**: the timer (a local-APIC device interrupt) is
+  covered in [device-interrupts.md](device-interrupts.md); external devices like
+  the keyboard also need the IO-APIC to route their lines onto vectors.
 - **SSE state**: the stubs save general registers but not the vector registers, so
-  recoverable interrupts that return to SSE-using code will need that added. Fine
-  for now, since exceptions here don't return.
+  a returning interrupt whose handler uses SSE will need that added. Fine for now,
+  since our handlers don't.
 
-With faults now debuggable, the paging work that comes next — where a wrong
+With faults now debuggable, the paging work that follows — where a wrong
 page-table entry means an instant #PF — is far less painful.