Some comments

e577a62f · Frans Kaashoek · bd71a450 · e577a62f
--- a/main.c
+++ b/main.c
@@ -82,7 +82,13 @@ bootothers(void)
      continue;

    // Tell bootother.S what stack to use, the address of mpboot and pgdir;
-    stack = boot_alloc();    // We need a stack below 4Mbyte with bootpgdir
+    // We cannot use kpgdir yet, because the AP processor is running in low 
+    // memory, so we use bootpgdir for the APs too.  kalloc can return addresses
+    // above 4Mbyte (the machine may have much more physical memory than 4Mbyte), which 
+    // aren't mapped by bootpgdir, so we must allocate a stack using boot_alloc();
+    // This introduces the constraint that xv6 cannot invoke until after these last boot_alloc 
+    // invocations.
+    stack = boot_alloc();
    *(void**)(code-4) = stack + KSTACKSIZE;
    *(void**)(code-8) = mpboot;
    *(int**)(code-12) = (void *) v2p(bootpgdir);