Christian Vogt told us about OS21 in

http://curl.haxx.se/mail/lib-2008-04/0443.html
2008-04-30 21:32:35 +00:00 · 2008-04-30 21:32:35 +00:00 · 12ffcf0b45
--- a/docs/INSTALL
+++ b/docs/INSTALL
@ -790,7 +790,8 @@ PORTS
        - PowerPC Linux
        - PowerPC Mac OS 9
        - PowerPC Mac OS X
-        - SuperH4 Linux 2.6.X
+        - SH4 Linux 2.6.X
+        - SH4 OS21
        - SINIX-Z v5
        - Sparc Linux
        - Sparc Solaris 2.4, 2.5, 2.5.1, 2.6, 7, 8, 9, 10