From 37939f24c2fbb12a57a628fedd024a4865741e74 Mon Sep 17 00:00:00 2001
From: Frans Kaashoek <kaashoek@mit.edu>
Date: Fri, 26 Aug 2016 08:46:13 -0400
Subject: better comment

---
 vm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/vm.c b/vm.c
index d4982ee..c8b8d35 100644
--- a/vm.c
+++ b/vm.c
@@ -168,7 +168,9 @@ switchuvm(struct proc *p)
   cpu->gdt[SEG_TSS].s = 0;
   cpu->ts.ss0 = SEG_KDATA << 3;
   cpu->ts.esp0 = (uint)proc->kstack + KSTACKSIZE;
-  cpu->ts.iomb = (ushort) 0xFFFF;  // forbid I/O instructions from user space
+  // setting IOPL=0 in eflags *and* iomb beyond the tss segment limit
+  // forbids I/O instructions (e.g., inb and outb) from user space
+  cpu->ts.iomb = (ushort) 0xFFFF;
   ltr(SEG_TSS << 3);
   if(p->pgdir == 0)
     panic("switchuvm: no pgdir");
-- 
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