diff --git a/build/Makefile b/build/Makefile
index c8215c7..9afa5e3 100644
--- a/build/Makefile
+++ b/build/Makefile
@@ -13,6 +13,10 @@ a.rim:
 	$(AS) -f rim  -o a.rim $(SYS)/sop.s $(SYS)/s[1-8].s
 	$(AS) -f list -o a.lst $(SYS)/sop.s $(SYS)/s[1-8].s
 
+coldboot:
+	$(AS) -f rim  -o a.rim $(SYS)/sop.s $(SYS)/s[1-9].s
+	$(AS) -f list -o a.lst $(SYS)/sop.s $(SYS)/s[1-9].s
+
 image.fs:
 	$(MKFS) --debug --format simh proto
 
diff --git a/src/sys/s4.s b/src/sys/s4.s
index 5d37eec..e34ddc6 100644
--- a/src/sys/s4.s
+++ b/src/sys/s4.s
@@ -3,11 +3,11 @@
 
 	" allocate a free disk block for a file (data or indirect)
 alloc: 0
-   -1
-   tad s.nfblks
+   -1			" Decrement the # free block numbers in the cache
+   tad s.nfblks		" kept at s.fblks. Jump to 1f if no free blocks left.
    spa
    jmp 1f
-   dac s.nfblks
+   dac s.nfblks		" Update the count of free block numbers
    tad fblksp
    jms laci
    dac 9f+t
@@ -16,7 +16,7 @@ alloc: 0
    jms dskwr
    dzm .savblk
    lac 9f+t
-   jmp alloc i
+   jmp alloc i		" Return from routine
 1:
    lac s.nxfblk
    sna
@@ -30,7 +30,7 @@ alloc: 0
    dac s.nfblks
    jmp alloc+1
 
-	" free a disk block
+	" free the disk block whose number is in AC
 free: 0
    lmq
    lac s.nfblks
@@ -53,7 +53,7 @@ free: 0
    dzm .savblk
    lac d1
    dac s.nfblks
-   jmp free i
+   jmp free i				" Return from the routine
 t = t+1
 
 	" load AC indirect (without using indirect!)
diff --git a/src/sys/s9.s b/src/sys/s9.s
index 044350a..930a967 100644
--- a/src/sys/s9.s
+++ b/src/sys/s9.s
@@ -5,27 +5,28 @@
 
 " zero i-list
 
-   dzm ii
-   jms copyz; dskbuf; 64
+   dzm ii			" Set ii to zero
+   jms copyz; dskbuf; 64	" Zero the 64-word dskbuf
 1:
-   lac ii
+   lac ii			" Zero blocks 0 to 709
    jms dskio; 07000
-   isz ii
+   isz ii			" Move up to next block number
    -710
    tad ii
    sza
-   jmp 1b
+   jmp 1b			" Loop back until we reach block 710
+				" i.e. we don't write block 710
 
 " free rest of disk
 
 1:
-   lac ii
+   lac ii			" Free blocks 710 up to 6399
    jms free
    isz ii
    -6400
    tad ii
    sza
-   jmp 1b
+   jmp 1b			" Loop back until we reach block 6400
 
 " read in tapes
 
diff --git a/tools/sdump b/tools/sdump
index 5e8c66c..43908f7 100755
--- a/tools/sdump
+++ b/tools/sdump
@@ -6,6 +6,7 @@
 #
 use strict;
 use warnings;
+use Fcntl qw(:flock SEEK_SET);
 
 ### read a word from a file in SimH format
 ### return -1 on EOF
@@ -14,10 +15,10 @@ sub read_word {
     # Convert four bytes into one 18-bit word
     return -1 if ( read( $F, my $four, 4 ) != 4 ); # Not enough bytes read
     my ( $b1, $b2, $b3, $b4 ) = unpack( "CCCC", $four );
-    return ((($b1 & 0xff) << 24 ) |
-            (($b2 & 0xff) << 16 ) |
-            (($b3 & 0xff) << 8) |
-             ($b4 & 0xff));
+    return (($b1 & 0xff)  |
+            (($b2 & 0xff) << 8 ) |
+            (($b3 & 0xff) << 16) |
+            (($b3 & 0xff) << 24));
 }
 
 ### Main program
@@ -27,6 +28,11 @@ open(my $IN, "<", $ARGV[0]) || die("Couldn't open $ARGV[0]: $!\n");
 
 use constant NUMBLOCKS     => 8000;    # Number of blocks on a surface
 use constant WORDSPERBLK   => 64;      # 64 words per block
+use constant BYTESPERWORD  => 4;       # We encode each word into 4 bytes
+
+# Skip the first surface
+seek($IN, NUMBLOCKS*WORDSPERBLK*BYTESPERWORD, SEEK_SET) ||
+				die("Cannot seek: $!\n");
 
 foreach my $blocknum ( 0 .. NUMBLOCKS*2 - 1 ) {
     printf("Block %d (%06o)\n", $blocknum, $blocknum );