4 files changed, 16 insertions, 11 deletions
diff --git a/kernel/memlayout.h b/kernel/memlayout.h
index ef5c1e1..c15e398 100644
--- a/kernel/memlayout.h
+++ b/kernel/memlayout.h
@@ -53,4 +53,15 @@
 // map the trampoline page to the highest address,
 // in both user and kernel space.
 #define TRAMPOLINE (MAXVA - PGSIZE)
-#define KSTACK(p) (TRAMPOLINE - (p+1)* 2*PGSIZE)
+#define KSTACK(p) (TRAMPOLINE - ((p)+1)* 2*PGSIZE)
+
+// User memory layout.
+// Address zero first:
+//   text
+//   original data and bss
+//   fixed-size stack
+//   expandable heap
+//   ...
+//   TRAPFRAME (p->tf, used by the trampoline)
+//   TRAMPOLINE (the same page as in the kernel)
+#define TRAPFRAME (TRAMPOLINE - PGSIZE)
diff --git a/kernel/proc.c b/kernel/proc.c
index b9babed..d10c952 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -156,8 +156,8 @@ proc_pagetable(struct proc *p)
   mappages(pagetable, TRAMPOLINE, PGSIZE,
            (uint64)trampout, PTE_R | PTE_X);
 
-  // map the trapframe, for trampoline.S.
-  mappages(pagetable, (TRAMPOLINE - PGSIZE), PGSIZE,
+  // map the trapframe just below TRAMPOLINE, for trampoline.S.
+  mappages(pagetable, TRAPFRAME, PGSIZE,
            (uint64)(p->tf), PTE_R | PTE_W);
 
   return pagetable;
@@ -169,7 +169,7 @@ void
 proc_freepagetable(pagetable_t pagetable, uint64 sz)
 {
   unmappages(pagetable, TRAMPOLINE, PGSIZE, 0);
-  unmappages(pagetable, TRAMPOLINE-PGSIZE, PGSIZE, 0);
+  unmappages(pagetable, TRAPFRAME, PGSIZE, 0);
   if(sz > 0)
     uvmfree(pagetable, sz);
 }
diff --git a/kernel/proc.h b/kernel/proc.h
index 1524c74..8f94d30 100644
--- a/kernel/proc.h
+++ b/kernel/proc.h
@@ -105,9 +105,3 @@ struct proc {
   struct inode *cwd;           // Current directory
   char name[16];               // Process name (debugging)
 };
-
-// Process memory is laid out contiguously, low addresses first:
-//   text
-//   original data and bss
-//   fixed-size stack
-//   expandable heap
diff --git a/kernel/trampoline.S b/kernel/trampoline.S
index 471a29c..1924de2 100644
--- a/kernel/trampoline.S
+++ b/kernel/trampoline.S
@@ -75,7 +75,7 @@ trampin:
         # user page table.
         #
         # sscratch points to where the process's p->tf is
-        # mapped into user space (TRAMPOLINE - 4096).
+        # mapped into user space, at TRAPFRAME.
         #
         
 	# swap a0 and sscratch