Wrote new ISR for AP entry

src/x86/isr.c

@@ -24,6 +24,15 @@ void isr_double_fault(void* frame, unsigned int error)
     asm("hlt");
 }
 
+__attribute__ ((interrupt, naked))
+void isr_ap_start(void* frame)
+{
+    asm(".code16");
+    //...
+    asm(".code32");
+    // do something useful
+}
+
 __attribute__ ((interrupt))
 void isr_syscall(void* frame)
 {