From 9011e4f55012d79ab645b7fa383af7ce519aa99d Mon Sep 17 00:00:00 2001
From: lkcl <lkcl@web>
Date: Thu, 15 Sep 2022 14:42:34 +0100
Subject: [PATCH]

---
 openpower/sv/rfc/ls001.mdwn | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/openpower/sv/rfc/ls001.mdwn b/openpower/sv/rfc/ls001.mdwn
index b68822561..c2397d899 100644
--- a/openpower/sv/rfc/ls001.mdwn
+++ b/openpower/sv/rfc/ls001.mdwn
@@ -702,6 +702,43 @@ Vectorisation of EXT001 is prohibited.
 |--------|---|---|-------|--------|---------|
 | PO (9)?| 1 | 1 | nnnn  | PO2    | SVP64:{EXT000-063} |
 
+# Potential Opcode allocation solution (2)
+
+One of the risks of the bit 6/7 scheme above is that there is no
+room to share PO9 (EXT009) with other potential uses.  A workaround for
+that is as follows:
+
+* EXT009, like EXT001 of Public v3.1, is **defined** as a 64-bit
+  encoding
+* bits 32-33 if 0b11 are indefinitely allocated to SVP64 Format
+  *regardless of what that is*
+* other combinations of bits 32-33 are `RESERVED` for other purposes
+
+
+There are unfortunately some inviolate requirements that directly place
+pressure on the EXT000-EXT063 (32-bit) opcode space to such a degree that
+it risks jeapordising the Power ISA. These requirements are:
+
+* all of the scalar operations must be Vectoriseable
+* all of the scalar operations intended for Vectorisation
+  must be in a 32-bit encoding (not prefixed-prefixed to 96-bit)
+* bringing Scalar Power ISA up-to-date from the past 12 years
+  needs 75% of two Major opcodes all on its own
+
+There exists a potential scheme which meets (exceeds) the above criteria,
+providing plenty of room for both Scalar (and Vectorised) operations,
+*and* provides SVP64-Single with room to grow.  It
+is based loosely around Public v3.1 EXT001 Encoding.[^ext001]
+
+| 0-5 | 6 | 7 | 8-31  | Description               |
+|-----|---|---|-------|---------------------------|
+| PO  | 0 | 0 | 0000  | new-suffix `RESERVED1`               |
+| PO  | 0 | 0 | !zero | new-suffix, scalar (SVP64Single), or `RESERVED3` |
+| PO  | 1 | 0 | 0000  | new scalar-only word, or `RESERVED2`               |
+| PO  | 1 | 0 | !zero | old-suffix, scalar (SVP64Single), or `RESERVED4` |
+| PO  | 0 | 1 | nnnn  | new-suffix, vector (SVP64)       |
+| PO  | 1 | 1 | nnnn  | old-suffix, vector (SVP64)       |
+
 # Example Legal Encodings and RESERVED spaces
 
 This section illustrates what is legal encoding, what is not, and
-- 
2.30.2