diff --git a/contrib/gdb_cmds.gdb b/contrib/gdb_cmds.gdb
new file mode 100644
index 0000000..8f0ba8c
--- /dev/null
+++ b/contrib/gdb_cmds.gdb
@@ -0,0 +1,42 @@
+# misc helpers for gdb debugging
+
+# first run lc3, then
+#
+# 	echo 0 | doas tee /proc/sys/kernel/yama/ptrace_scope
+#
+# to disable security measures that prevent debugging,
+# then
+#
+# 	rust-gdb -p $(pgrep lc3)
+#
+# then in the gdb shell
+#
+# 	source contrib/gdb_cmds.gdb
+#
+# you can then use the commands defined here
+
+
+define vmb
+	# set a breakpoint at VM addr $0
+	break lc3::vm::instruction::execute_instruction if vm.registers.pc == $arg0 + 1
+	set $vmb_break = $bpnum
+end
+
+define vmj
+	# jump execution to VM addr $0
+	vmb $arg0
+	c
+	d $vmb_break
+	p vm.registers.pc-1
+end
+
+define vms
+	# step in vm with breakpoint $0
+	# before this break at the second line in lc3::vm::instruction::execute_instruction
+	# pass the number of this breakpoint in as $0
+	enable $arg0
+	c
+	p opcode
+	p vm.registers.pc-1
+	disable $arg0
+end
diff --git a/src/vm/instruction.rs b/src/vm/instruction.rs
index 3594e8c..65f67e6 100644
--- a/src/vm/instruction.rs
+++ b/src/vm/instruction.rs
@@ -276,7 +276,7 @@ fn op_not(vm: &mut VM, instr: u16) {
     // NOTE
     // rustc is very friendly and tells you off if you use ~ as bitwise not
     let res = !vm.registers.get_reg(sr);
-    vm.registers.set_reg_with_cond(sr, res);
+    vm.registers.set_reg_with_cond(dr, res);
 }
 
 ////////////////