diff --git a/file-intro/pstack.c b/file-intro/pstack.c
index b8e4f58..8aafb08 100644
--- a/file-intro/pstack.c
+++ b/file-intro/pstack.c
@@ -1,70 +1,74 @@
-// Persistent stack example of how mmap() naturally creates a software abstraction of persistent memory.
-// Author: Terence Kelly <tpklly@acm.org>
-
-// Note: Use 'dd' to make the initial (empty) image, e.g., 
-//   dd if=/dev/zero of=ps.img bs=32 count=1
-// makes a 32-byte empty file (the first integer being the size of the stack,
-// the remaining integers being values)
-
-#include <assert.h>
-#include <fcntl.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <string.h>
-#include <unistd.h>
-#include <sys/mman.h>
-#include <sys/stat.h>
-#include <sys/types.h>
-
-int open_or_die(const char *path, int oflag) {
-    int fd = open(path, oflag);
-    assert(fd > 0);
-    return fd;
-}
-
-int fstat_or_die(int fildes, struct stat *buf) {
-    int rc = fstat(fildes, buf);
-    assert(rc == 0);
-    return rc;
-}
-
-void *mmap_or_die(void *addr, size_t len, int prot, int flags, int fd, off_t offset) {
-    void *p = mmap(addr, len, prot, flags, fd, offset);
-    assert(p != MAP_FAILED);
-    return p;
-}
-
-typedef struct {
-    size_t n;
-    int stack[]; // zero-length per C99
-} pstack_t; 
-
-int main(int argc, char *argv[]) {
-    int fd, rc;
-    struct stat s;
-    size_t file_size;
-    pstack_t *p;
-    
-    fd = open_or_die("ps.img", O_RDWR);
-    rc = fstat_or_die(fd, &s);
-
-    file_size = (size_t) s.st_size;
-    assert(file_size % sizeof(int) == 0);
-
-    p = mmap_or_die(NULL, file_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
-
-    for (int i = 1; i < argc; i++)
-	if (strcmp(argv[i], "pop") == 0) {  
-	    if (p->n > 0)  // stack not empty
-		printf("%d\n", p->stack[--p->n]);
-	    else
-		printf("stack empty\n");
-	} else
-	    if (sizeof(pstack_t) + p->n * sizeof(int) < file_size)  // stack not full
-		p->stack[p->n++] = atoi(argv[i]);
-	    else
-		printf("stack full: %d not added\n", atoi(argv[i]));
-
-    close(fd);
-    return 0;
-}
+// Persistent stack example of how mmap() naturally creates a software abstraction of persistent memory.
+// Author: Terence Kelly
+//         tpkelly @ { acm.org, cs.princeton.edu, eecs.umich.edu }
+
+// Note:  Use 'truncate' to make the initial (empty) backing file,
+// whose size should be a multiple of the system page size:
+//
+//     prompt> getconf PAGESIZE
+//     4096
+//     prompt> truncate -s 4096 ps.img
+//
+// makes a 4096-byte empty file.  The first sizeof(size_t) bytes will
+// be interprted as the number of items on the stack; the remainder
+// of the file will contain the integers contained in the stack.
+// Now you can run the program:
+//
+//     prompt> ./pstack 7 13 47 pop
+//     47
+//     prompt> ./pstack pop pop 99
+//     13
+//     7
+//     prompt> ./pstack pop
+//     99
+//
+// Notice that items push'd in one invocation of the program persist
+// until the next invocation:  The stack is persistent.
+//
+// You can use hexdump to examine the contents of the backing file:
+//
+//     prompt> hexdump ps.img
+
+#include <assert.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+typedef struct {
+    size_t n;
+    int stack[]; // zero-length per C99
+} pstack_t;
+
+int main(int argc, char *argv[]) {
+    int fd, rc;
+    struct stat s;
+    size_t file_size;
+    pstack_t *p;
+
+    fd = open("ps.img", O_RDWR);
+    assert(fd > -1);
+    rc = fstat(fd, &s);
+    assert(rc == 0);
+
+    file_size = (size_t) s.st_size;
+    assert(file_size >= sizeof(pstack_t) && file_size % sizeof(int) == 0);
+
+    p = (pstack_t *) mmap(NULL, file_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
+    assert(p != MAP_FAILED);
+
+    for (int i = 1; i < argc; i++)
+        if (strcmp(argv[i], "pop") == 0) {
+            if (p->n > 0)  // stack not empty
+                printf("%d\n", p->stack[--p->n]);
+        } else {  // pop
+          if (sizeof(pstack_t) + (1 + p->n) * sizeof(int) <= file_size)  // stack not full
+                p->stack[p->n++] = atoi(argv[i]);
+        }
+    (void) close(fd);
+    return 0;
+}