I've got a very simple cat program working, which reads from stdin and

sends it to stdout.

tools/simplecat.s

@@ -0,0 +1,29 @@
+" Simple cat program: echo stdin to stdout until no more words to read
+
+main:
+   " Read 5 words into the buffer from stdin
+   lac d0
+   sys read; buf; 5
+   spa			" Skip if result was >= 0
+     jmp error		" Result was -ve, so error result
+   sna			" Skip if result was >0
+     jmp end		" Result was zero, so nothing left to read
+
+   " Save the count of words read in
+   dac 1f
+
+   " Write 5 words from the buffer to stdout
+   lac d1
+   sys write; buf; 1:0
+   jmp main
+
+error:
+end:
+   " exit
+   sys exit
+
+d0: 0
+d1: 1
+
+" Input buffer for read
+buf: 0; 0; 0; 0; 0