[otbn] Add a code snippet that shows off the loop instructions
This might be helpful for readers, but is mostly intended as a sanity
test for the simulator.
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/sw/otbn/code-snippets/loop.S b/sw/otbn/code-snippets/loop.S
new file mode 100644
index 0000000..2f77b7f
--- /dev/null
+++ b/sw/otbn/code-snippets/loop.S
@@ -0,0 +1,33 @@
+/* Copyright lowRISC contributors. */
+/* Licensed under the Apache License, Version 2.0, see LICENSE for details. */
+/* SPDX-License-Identifier: Apache-2.0 */
+/*
+ An example of how hardware loops work in OTBN
+*/
+
+ /* The basic loop instruction (running 2 instructions 3 times) */
+ li x2, 3
+ li x3, 0
+
+ loop x2, 2
+ addi x3, x3, 2
+ addi x3, x3, -1
+
+ /* At this point, we've incremented x3 by 2-1 = 1 on each of three
+ loop iterations, so x3 should equal 3. */
+
+ loopi 5, 1
+ addi x3, x3, -3
+
+ /* Now we've run a loop that decrements x3 by 3 on each of
+ five loop iterations, so it should now equal 3-15 = -12. */
+
+ loop x2, 3
+ loopi 4, 1
+ addi x3, x3, 2
+ nop
+
+ /* The nested loop runs 3 * 4 times, incrementing by 2 each
+ iteration. So x3 should now equal -12 + 2*12 = 12. */
+
+ ecall
