diff --git a/cvm/Makefile b/cvm/Makefile
index 6b29eaa..880fddf 100644
--- a/cvm/Makefile
+++ b/cvm/Makefile
@@ -1,4 +1,4 @@
-TARGETS = forth
+TARGETS = forth stage
 OBJS = vm.o
 BLKPACK = ../tools/blkpack
 BLKUNPACK = ../tools/blkunpack
@@ -18,17 +18,19 @@ stage: stage.c $(OBJS) blkfs
 blkfs: $(BLKPACK)
 	$(BLKPACK) ../blk > $@
 
-forth: forth.c $(OBJS) blkfs
+forth: forth.c $(OBJS)
 	$(CC) forth.c $(OBJS) -lncurses -o $@
 
-vm.o: vm.c
+stage: stage.c $(OBJS)
+	$(CC) stage.c $(OBJS) -lncurses -o $@
+
+vm.o: vm.c blkfs
 	$(CC) -DFBIN_PATH=\"`pwd`/forth.bin\" -DBLKFS_PATH=\"`pwd`/blkfs\" -c -o vm.o vm.c
 
 
 .PHONY: updatebootstrap
 updatebootstrap: stage xcomp.fs pack
 	./stage < xcomp.fs > new.bin
-	mv new.bin forth.bin
 
 .PHONY: pack
 pack:
diff --git a/cvm/vm.c b/cvm/vm.c
index 68167e9..d159efe 100644
--- a/cvm/vm.c
+++ b/cvm/vm.c
@@ -67,12 +67,20 @@ static void sw(word addr, word val) {
     vm.mem[addr] = val;
     vm.mem[addr+1] = val >> 8;
 }
-static word pop() { return vm.mem[vm.SP++] | vm.mem[vm.SP++] << 8; }
+static word pop() {
+    if (vm.uflw) return 0;
+    if (vm.SP >= SP_ADDR) { vm.uflw = true; }
+    return vm.mem[vm.SP++] | vm.mem[vm.SP++] << 8;
+}
 static void push(word x) {
     vm.SP -= 2; sw(vm.SP, x);
     if (vm.SP < vm.minSP) { vm.minSP = vm.SP; }
 }
-static word popRS() { word x = gw(vm.RS); vm.RS -= 2; return x; }
+static word popRS() {
+    if (vm.uflw) return 0;
+    if (vm.RS <= RS_ADDR) { vm.uflw = true; }
+    word x = gw(vm.RS); vm.RS -= 2; return x;
+}
 static void pushRS(word val) {
     vm.RS += 2; sw(vm.RS, val);
     if (vm.RS > vm.maxRS) { vm.maxRS = vm.RS; }
@@ -351,6 +359,7 @@ VM* VM_init() {
     vm.IP = gw(0x04) + 1; // BOOT
     sw(SYSVARS+0x02, gw(0x08)); // CURRENT
     sw(SYSVARS+0x04, gw(0x08)); // HERE
+    vm.uflw = false;
     vm.running = true;
     return &vm;
 }
@@ -369,6 +378,10 @@ bool VM_steps(int n) {
         word wordref = gw(vm.IP);
         vm.IP += 2;
         execute(wordref);
+        if (vm.uflw) {
+            vm.uflw = false;
+            execute(gw(0x06)); /* uflw */
+        }
         n--;
     }
     return vm.running;
diff --git a/cvm/vm.h b/cvm/vm.h
index 5d80abc..b0c7c62 100644
--- a/cvm/vm.h
+++ b/cvm/vm.h
@@ -36,6 +36,7 @@ typedef struct {
     word maxRS;
     word minSP;
     bool running;
+    bool uflw;
 } VM;
 
 VM* VM_init();