From: lkcl <lkcl@web>
Date: Fri, 14 Jun 2019 14:16:23 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: convert-csv-opcode-to-binary~4627
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=9fefa47074127fe24da40abaf979853dbc246a0b;p=libreriscv.git

---

diff --git a/isa_conflict_resolution/isamux_isans.mdwn b/isa_conflict_resolution/isamux_isans.mdwn
index b638d8cb1..b694119a1 100644
--- a/isa_conflict_resolution/isamux_isans.mdwn
+++ b/isa_conflict_resolution/isamux_isans.mdwn
@@ -159,6 +159,17 @@ trap_exit(x_cause):
 </pre>
 </code>
 
+# Is this like MISA? <a name="misa"></a>
+
+No.
+
+* MISA's space is entirely taken up (and running out).
+* There is no allocation (provision) for custom extensions.
+* MISA switches on and off entire extensions: ISAMUX/NS may be used to switch multiple opcodes (present and future), to alternate meanings.
+* MISA is WARL and is inaccessible from everything but M-Mode (not even readable). 
+
+MISA is therefore wholly unsuited to U-Mode usage; ISANS is specifically permitted to be called by userspace to switch (with no stalling) between namespaces, repeatedly and in quick succession.
+
 # What happens if this scheme is not adopted? Why is it better than leaving things well alone? <a name="lassezfaire"></a>
 
 At the first sign of an emergency non-backwards compatible and unavoidable