diff --git a/src/cmd/ed1.s b/src/cmd/ed1.s
index 3210d09..1a05695 100644
--- a/src/cmd/ed1.s
+++ b/src/cmd/ed1.s
@@ -70,28 +70,28 @@ chkwrp:
    jmp error
 
 comand:
-   lac char
+   lac char		" Get the character entered
    sad o141
-   jmp ca
+   jmp ca		" a command, append lines of text
    sad o143
-   jmp cc
+   jmp cc		" c command, change lines of text
    sad o144
-   jmp cd
+   jmp cd		" d command, delete lines of text
    sad o160
-   jmp cp
+   jmp cp		" p command, print lines of text
    sad o161
-   jmp cq
+   jmp cq		" q command, quit the editor
    sad o162
-   jmp cr
+   jmp cr		" r command, read in a file
    sad o163
-   jmp cs
+   jmp cs		" s command, substitute text
    sad o167
-   jmp cw
+   jmp cw		" w command, write out the file
    sad o12
-   jmp cnl
+   jmp cnl		" newline
    sad o75
-   jmp ceq
-   jmp error
+   jmp ceq		" = command
+   jmp error		" unrecognised, give an error
 ca:
    jms newline
    jms setfl
@@ -334,10 +334,10 @@ setfl: 0
    jmp i setfl
 
 newline: 0
-   jms getsc; tal
+   jms getsc; tal	" Get a character into tal
    sad o12
-   jmp i newline
-   jmp error
+   jmp i newline	" Return if a newline
+   jmp error		" else an error
 
 addres: 0
    dzm minflg "..) [stray scan mark?]
@@ -768,4 +768,4 @@ error:
    sys write; 1f; 1
    jmp advanc
 1:
-   077012
\ No newline at end of file
+   077012
diff --git a/tools/a7out b/tools/a7out
index eeb81c7..ceab567 100755
--- a/tools/a7out
+++ b/tools/a7out
@@ -13,8 +13,10 @@ use Data::Dumper;
 ### Global variables ###
 my $debug      = 0;    # Debug flag
 my $singlestep = 0;    # Are we running in single-step mode?
+my $coverage   = 0;    # Print out code coverage
 my %Breakpoint;        # Hash of defined breakpoints
 my @Mem;               # 8K 18-bit words of main memory
+my @CC;		       # Code coverage: what addrs executed instructions
 my @FD;                # Array of open filehandles
 my @ISBINARY;          # Array of filehandle flags: ASCII or binary files?
 
@@ -49,10 +51,16 @@ while ( defined( $ARGV[0] ) && ( $ARGV[0] =~ m{^-} ) ) {
         shift(@ARGV);
         $Breakpoint{ oct( shift(@ARGV) ) } = 1;
     }
+
+    # -c: print out code coverage
+    if ( $ARGV[0] eq "-c" ) {
+        shift(@ARGV);
+        $coverage= 1;
+    }
 }
 
 # Check the arguments
-die("Usage: $0 [-d] [-b breakpoint] a.outfile [arg1 arg2 ...]\n")
+die("Usage: $0 [-c] [-d] [-b breakpoint] a.outfile [arg1 arg2 ...]\n")
   if ( @ARGV < 1 );
 
 # Load the a.out file into memory
@@ -63,8 +71,16 @@ set_arguments();
 #dump_memory(0, MAXADDR, 0);
 #exit(0);
 simulate();
+print_coverage() if ($coverage);
 exit(0);
 
+# Print out the code coverage
+sub print_coverage {
+  foreach my $addr (0 .. MAXADDR ) {
+    printf("%06o: %d\n", $addr, $CC[$addr]) if ($CC[$addr]);
+  }
+}
+
 ### Load the a.out file into memory
 sub load_code {
     my $filename = shift;
@@ -212,7 +228,8 @@ sub simulate {
     while (1) {
 
         # Get the instruction pointed to by PC and decode it
-        my $instruction = $Mem[$PC];
+	# Also do code coverage
+        my $instruction = $Mem[$PC]; $CC[$PC]++;
         my $opcode      = ( $instruction >> 12 ) & 074;
         my $indirect    = ( $instruction >> 13 ) & 1;
         my $addr        = $instruction & MAXADDR;
@@ -694,6 +711,7 @@ sub cal {
 # Exit system call
 sub sys_exit {
     dprintf( "exit system call, pid %06o\n", $$ );
+    print_coverage() if ($coverage);
     exit(0);
 }