diff --git a/CurtaSim.xcodeproj/project.pbxproj b/CurtaSim.xcodeproj/project.pbxproj
index 58edd5c..ea3012a 100644
--- a/CurtaSim.xcodeproj/project.pbxproj
+++ b/CurtaSim.xcodeproj/project.pbxproj
@@ -64,6 +64,7 @@
 		9D24618F20D8B30C00227E1D /* curta.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; path = curta.c; sourceTree = "<group>"; };
 		9D24619020D8B30C00227E1D /* joystick.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = joystick.h; sourceTree = "<group>"; };
 		9D24619120D8B30C00227E1D /* curtaUI.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; path = curtaUI.c; sourceTree = "<group>"; };
+		9D45ABB620E1E564008C3F16 /* errorFilter.sh */ = {isa = PBXFileReference; lastKnownFileType = text.script.sh; name = errorFilter.sh; path = make/errorFilter.sh; sourceTree = "<group>"; };
 		9D65A1BF20DCA472004268B7 /* LICENSE */ = {isa = PBXFileReference; lastKnownFileType = text; path = LICENSE; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
@@ -122,6 +123,7 @@
 				9D24617020D8B2AE00227E1D /* createDiskImage */,
 				9D24617220D8B2AE00227E1D /* DevApple.vii */,
 				9D24617420D8B2AE00227E1D /* dos33_template.dsk */,
+				9D45ABB620E1E564008C3F16 /* errorFilter.sh */,
 				9D24617620D8B2AE00227E1D /* head.mk */,
 				9D24617820D8B2AE00227E1D /* prodos_template.dsk */,
 				9D24617A20D8B2AE00227E1D /* tail.mk */,
diff --git a/CurtaSim.xcodeproj/project.xcworkspace/xcuserdata/jrand.xcuserdatad/UserInterfaceState.xcuserstate b/CurtaSim.xcodeproj/project.xcworkspace/xcuserdata/jrand.xcuserdatad/UserInterfaceState.xcuserstate
index acc0024..8f66c77 100644
Binary files a/CurtaSim.xcodeproj/project.xcworkspace/xcuserdata/jrand.xcuserdatad/UserInterfaceState.xcuserstate and b/CurtaSim.xcodeproj/project.xcworkspace/xcuserdata/jrand.xcuserdatad/UserInterfaceState.xcuserstate differ
diff --git a/CurtaSim/make/errorFilter.sh b/CurtaSim/make/errorFilter.sh
new file mode 100755
index 0000000..30259c1
--- /dev/null
+++ b/CurtaSim/make/errorFilter.sh
@@ -0,0 +1,59 @@
+#!/bin/sh
+
+PWD=`pwd`
+
+$* 2> /tmp/errors.$$
+RESULT=$?
+
+if [ $RESULT -ne 0 ]
+then
+    exec /usr/bin/perl -x $0 $RESULT /tmp/errors.$$ "$PWD"
+fi
+exit 0
+
+#!/usr/bin/perl
+
+use strict;
+
+my $result = $ARGV[0];
+my $filename = $ARGV[1];
+my $pwd = $ARGV[2];
+
+open(FILE, "<$filename") || die "$0: Unable to open file, $!";
+
+my $unresolvedRefError = undef;
+
+while (<FILE>) {
+    chomp;
+
+    if (defined $unresolvedRefError) {
+        if (m/^  ([^(]+)\(([0-9]+)\)/) {
+            my $file = $1;
+            my $lineno = $2;
+
+            if (! -f "$pwd/$file") {
+                $file =~ s/\.s$/.c/;
+            }
+
+            $_ = "$pwd/$file:$lineno:0: Error: $unresolvedRefError";
+        } else {
+            $unresolvedRefError = undef;
+        }
+    }
+
+    if (m/^(Unresolved external .* referenced) in:/) {
+        $unresolvedRefError = $1;
+        $_ = "";
+    } elsif (m/^([^(]+)\(([0-9]+)\):(.*)$/) {
+        my $file = $1;
+        my $lineno = $2;
+        my $error = $3;
+
+        $_ = "$pwd/$file:$lineno:0:$error";
+    }
+    print STDERR "$_\n";
+}
+
+unlink($filename);
+
+exit($result);
diff --git a/CurtaSim/make/tail.mk b/CurtaSim/make/tail.mk
index 6463582..4d72c06 100644
--- a/CurtaSim/make/tail.mk
+++ b/CurtaSim/make/tail.mk
@@ -95,7 +95,7 @@ cleanMacCruft:
 	rm -rf pkg
 
 $(PGM): $(OBJS)
-	$(CL65) $(MACHCONFIG) --mapfile $(MAPFILE) $(LDFLAGS) -o "$(PGM)" $(OBJS)
+	make/errorFilter.sh $(CL65) $(MACHCONFIG) --mapfile $(MAPFILE) $(LDFLAGS) -o "$(PGM)" $(OBJS)
 
 $(DISKIMAGE): $(PGM)
 	make/createDiskImage $(AC) $(MACHINE) "$(DISKIMAGE)" "$(PGM)" "$(START_ADDR)" $(COPYDIRS)
@@ -104,12 +104,12 @@ execute: $(DISKIMAGE)
 	osascript make/V2Make.scpt "$(CWD)" "$(PGM)" "$(CWD)/make/DevApple.vii" "$(EXECCMD)"
 
 %.o:	%.c
-	$(CL65) $(MACHCONFIG) $(CFLAGS) --create-dep -c -o $@ $<
+	make/errorFilter.sh $(CL65) $(MACHCONFIG) $(CFLAGS) --create-dep -c -o $@ $<
 	sed -i .bak 's/\.s:/.o:/' $(@:.o=.u)
 	rm -f $(@:.o=.u).bak
 
 %.o:	%.s
-	$(CL65) $(MACHCONFIG) --cpu $(CPU) $(ASMFLAGS) -l -c -o $@ $<
+	make/errorFilter.sh $(CL65) $(MACHCONFIG) --cpu $(CPU) $(ASMFLAGS) -l -c -o $@ $<
 
 $(OBJS): Makefile