diff --git a/tools/write_test.s b/tools/write_test.s
new file mode 100644
index 0000000..e9b4de9
--- /dev/null
+++ b/tools/write_test.s
@@ -0,0 +1,50 @@
+" Test program for several system calls: open, read, write, close, exit
+main:
+   " Test the lac, dac instructions
+   lac in
+   dac out
+
+   " The assembler doesn't know sys write, so iot 5 for now
+   lac d1
+   iot 5; hello; 7
+
+   " open file fred
+   iot 3; fred; 0; 0
+
+   " read 5 words into the buffer from stdin: type in 10 or more characters!
+   lac d0
+   iot 4; buf; 5
+
+   " Stop and dump memory, so you can see five words at location 0400
+   " Comment out the hlt instruction to test close and exit
+   hlt
+
+   " close stdin
+   lac d0
+   iot 9
+
+   " exit
+   iot 14
+
+   " We should not get to the halt instruction
+   hlt
+
+
+" Some memory locations for lac and dac
+. = 0100
+in: 023
+
+. = 0200
+out: 0
+
+" Hello, world\n, two ASCII chars per word
+d0: 0
+d1: 1
+hello: 0110145; 0154154; 0157054; 040; 0167157; 0162154; 0144012
+
+" fred as a string, NUL terminated
+fred: 0146162; 0145144; 0
+
+" Input buffer for read
+. = 0400
+buf: 0