fix writing by using a virtual image instead of a physical thumbdrive, better document the hack in KeIrqDispatch

code/kernel/ke_irq.c

@@ -18,7 +18,12 @@ __declspec(dllexport) void KeIrqRegister(uint8_t irq, void (*handler)(void)) {
 }
 
 void KeIrqDispatch(uint8_t irq) {
-    // eoi first, handler second because it doesn't work otherwise.
+    // KeIrqDispatch sends EOI after the handler returns, but the handler switched context before returning.
+    // The EOI call is still sitting on the idle thread's stack, waiting for that thread to be resumed.
+    // The LAPIC sees ISR[0x20] still set. Its rule is: don't deliver another interrupt at the same or lower priority until EOI is written.
+    // So the timer never fires again. The user process runs completely unpreempted until it calls SYS_EXIT_PROCESS, which calls KeYield, which cooperatively switches back
+    // and only then does the idle thread resume, KeIrqDispatch finally sends its EOI, and the timer starts firing again.
+    // To fix this we send EOI before calling the handler, not after.
     gHal->Intc.SendEoi();
     if (irq < MAX_IRQS && gIrqHandlers[irq])
         gIrqHandlers[irq]();