From 7243b094b7a1885b898b12464276c07b95c01bf2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Thu, 13 Jan 2022 15:46:29 +0100
Subject: [PATCH] dcs: decrqss: add note about Ps in the reply

---
 dcs.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/dcs.c b/dcs.c
index b0d111b7..e99e2d7b 100644
--- a/dcs.c
+++ b/dcs.c
@@ -219,6 +219,17 @@ decrqss(struct terminal *term)
     const uint8_t *query = term->vt.dcs.data;
     const size_t n = term->vt.dcs.idx;
 
+    /*
+     * A note on the Ps parameter in the reply: many DEC manual
+     * instances (e.g. https://vt100.net/docs/vt510-rm/DECRPSS) claim
+     * that 0 means “request is valid”, and 1 means “request is
+     * invalid”.
+     *
+     * However, this appears to be a typo; actual hardware inverts the
+     * response (as does XTerm and mlterm):
+     * https://github.com/hackerb9/vt340test/issues/13
+     */
+
     if (memcmp(query, "r", n) == 0) {
         /* DECSTBM - Set Top and Bottom Margins */
         char reply[64];