userland: move getchar from cpp-cheat

README.adoc

@@ -3785,6 +3785,25 @@ Simulated exit code not 0! Exit code is 1
 
 which we parse in link:run[] and then exit with the correct result ourselves...
 
+==== gem5 syscall emulation mode program stdin
+
+gem5 shows its own stdout to terminal, and does not allow you to type stdin to programs.
+
+Instead, you must pass stdin non-interactively with the through a file with the `--se.py --input` option, e.g.:
+
+....
+printf a > f
+./run --emulator gem5 --userland userland/c/getchar.c --static -- --input f
+....
+
+leads to gem5 output:
+
+....
+enter a character: you entered: a
+....
+
+Source: link:userland/c/getchar.c[]
+
 ==== User mode vs full system benchmark
 
 Let's see if user mode runs considerably faster than full system or not.