Ken Thompson sent e-mail in to explain that there can be labels mid-line, such as

sys write; 1:0; 4

so I modified the assembler logic to support this, and added some code to write_test.s
to verify that it works.

tools/as7

@@ -20,11 +20,18 @@ my @Mline;     # Source lines associated with mem locations
 my $origline;  # The current input line of code
 
 my $stage = 1; # Pass one or pass two
+my $debug = 0; # Run in debug mode
 
 ### Main program ###
 
+# Optional debug argument
+if ( ( @ARGV > 0 ) && ( $ARGV[0] eq "-d" ) ) {
+    $debug = 1;
+    shift(@ARGV);
+}
+
 # Check the arguments
-die("Usage: $0 file1.s [file2.s ...]\n") if ( @ARGV < 1 );
+die("Usage: $0 [-d] file1.s [file2.s ...]\n") if ( @ARGV < 1 );
 
 # Define the syscalls as variables so that
 # I don't have to hand-code the logic below.
@@ -68,6 +75,7 @@ foreach my $file (@ARGV) {
 # Now do it all again, pass two
 $Var{'.'} = 0;
 $stage = 2;
+print("Now in stage 2\n") if ($debug);
 foreach my $file (@ARGV) {
     parse_file($file);
 }
@@ -75,8 +83,7 @@ foreach my $file (@ARGV) {
 # Now print out the contents of memory
 for my $i ( 0 .. $#Mem ) {
     if ( defined( $Mem[$i] ) ) {
-    printf("%06o: %06o\t%s\n",
+        printf( "%06o: %06o\t%s\n", $i, $Mem[$i], $Mline[$i] || "" );
-	$i, $Mem[$i], $Mline[$i] || "");
     }
 }
 
@@ -97,33 +104,33 @@ sub parse_file {
         $line =~ s{\s*\".*}{};
         #print("=>$line<=\n");
 
-        # Split into a section with possible labels and a
+        # Split the line into commands that are ; separated
-        # section with definitely no labels. The ? makes the
+        # and parse each one
-        # first pattern less greedy. Labels and statements.
+        foreach my $cmd ( split( /;\s*/, $line ) ) {
-        $line =~ m{(.*?)([^:]*$)};
-        my $labelsect = $1;
-        my $stmntsect = $2;
 
-        #print(">$labelsect< >$stmntsect<\n");
+            # Split into a section with possible labels and a
+            # statement section with definitely no labels. The ?
+            # makes the first pattern less greedy.
+            #print("cmd is >$cmd<\n");
+            $cmd =~ m{(.*?)([^:]*$)};
+            my $labelsect = $1;
+            my $statement = $2;
+
+            #print(">$labelsect< >$statement<\n");
 
             # Split $labelsect into labels using the : character
             my @labellist = split( /:\s*/, $labelsect );
 
-        # Split $stmntsect into statements using the ; character
-        # Trim any whitespace first
-        $stmntsect =~ s{^\s+}{};
-        my @stmntlist = split( /;\s*/, $stmntsect );
-
             # First pass: parse the labels
             if ( $stage == 1 ) {
-            foreach my $l (@labellist) { parse_label($l); }
+                foreach my $l (@labellist) {
+                    parse_label($l);
+                }
             }
 
             # Parse the statements on both passes
-        foreach my $s (@stmntlist) {
+            parse_statement($statement);
-            parse_statement($s);
         }
-        #print("\n");
     }
     close($IN);
 }
@@ -153,6 +160,12 @@ sub parse_statement {
     my $location  = $Var{'.'};
     #printf( "Location: 0%o\n", $location );
 
+    # Empty statement, nothing to do
+    return if ( $statement =~ m{^\s*$} );
+
+    # Lose any leading whitespace
+    $statement =~ s{^\s*}{};
+
     # It's an assignment statement: lhs = rhs
     if ( $statement =~ m{(\S+)\s*=\s*(\S+)} ) {
         my $lhs = $1;
@@ -163,6 +176,7 @@ sub parse_statement {
         my $result = parse_expression($rhs);
         die("expression $rhs has no value in assignment\n")
           if ( !defined($result) );
+
         #printf( "Setting variable %s to 0%o\n", $lhs, $result );
         $Var{$lhs} = $result;
         return;
@@ -380,10 +394,10 @@ sub parse_expression {
             die("Unable to subtract $word1 on pass two\n")
               if ( !defined($diff) );
         }
+
         #print("Did a subtraction and got $diff\n");
         return ($diff);
     }
-
     die("I have no idea what $expression is in pass two\n") if ( $stage == 2 );
 }
 
@@ -395,6 +409,7 @@ sub add {
     my $val2 = parse_expression($b);
     #print("Adding $val1 + $val2\n");
     return (undef) if ( !defined($val1) || !defined($val2) );
+
     #print( "  with a good value of ", $val1 + $val2, "\n" );
     return ( $val1 + $val2 );
 }
@@ -407,6 +422,7 @@ sub subtract {
     my $val2 = parse_expression($b);
     #print("Subtracting $val1 - $val2\n");
     return (undef) if ( !defined($val1) || !defined($val2) );
+
     #print( "  with a good value of ", $val1 - $val2, "\n" );
     return ( $val1 - $val2 );
 }

tools/write_test.s

@@ -11,7 +11,13 @@ main:
    lac d1
    sys write; hello; 7
 
-   " open file fred
+   " Test if the assembler can dac into a mid-line label
+   lac helloptr
+   dac 1f
+   lac d1
+   sys write; 1:0; 7
+
+   " Try to open file fred
    sys open; fred; 0; 0
 
    " read 5 words into the buffer from stdin: type in 10 or more characters!
@@ -42,6 +48,7 @@ out: 0
 
 " Hello, world\n, two ASCII chars per word
 hello: 0110145; 0154154; 0157054; 040; 0167157; 0162154; 0144012
+helloptr: hello
 
 " fred as a string, NUL terminated
 fred: 0146162; 0145144; 0