From a5ed3bf90454fe7dbdde72138eb5a12627ba8c36 Mon Sep 17 00:00:00 2001
From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Sat, 16 Sep 2023 13:11:58 +0100
Subject: [PATCH] Revert unauthorized change to the specification without
 proper consultation or review

Revert "fix scalar EXTRA2 in EXTRA2/3 decoding algorithms"

This reverts commit 24576f370d5b0b0282b821062c66e1ff39ab8019.
---
 openpower/sv/svp64.mdwn          | 16 +++++++---------
 openpower/sv/svp64/appendix.mdwn | 29 ++++++++++++-----------------
 2 files changed, 19 insertions(+), 26 deletions(-)

diff --git a/openpower/sv/svp64.mdwn b/openpower/sv/svp64.mdwn
index 8bc7f784b..c623c123a 100644
--- a/openpower/sv/svp64.mdwn
+++ b/openpower/sv/svp64.mdwn
@@ -1194,15 +1194,13 @@ A pseudocode algorithm explains the relationship, for INT/FP (see
 
 ```
     if extra3_mode:
-        if EXTRA3[0]: # vector
-            return (RA << 2) | EXTRA3[1:2]
-        else:         # scalar
-            return (EXTRA3[1:2] << 5) | RA
-    else:  # EXTRA2 mode
-        if EXTRA2[0]: # vector
-            return (RA << 2) | (EXTRA2[1] << 1)
-        else:
-            return (EXTRA2[1] << 5) | RA
+        spec = EXTRA3
+    else:
+        spec = EXTRA2 << 1 # same as EXTRA3, shifted
+    if spec[0]: # vector
+         return (RA << 2) | spec[1:2]
+    else:         # scalar
+         return (spec[1:2] << 5) | RA
 ```
 
 Future versions may extend to 256 by shifting Vector numbering up.
diff --git a/openpower/sv/svp64/appendix.mdwn b/openpower/sv/svp64/appendix.mdwn
index 60c8962c7..73426d430 100644
--- a/openpower/sv/svp64/appendix.mdwn
+++ b/openpower/sv/svp64/appendix.mdwn
@@ -615,22 +615,17 @@ applies, **not** the `CR_bit` portion (bits 3-4):
 
 ```
     if extra3_mode:
-        is_vec = EXTRA3[0]
-        extra = EXTRA3[1:2]
+        spec = EXTRA3
     else:
-        is_vec = EXTRA2[0]
-        if is_vec:
-            extra = EXTRA2[1] << 1
-        else:
-            extra = EXTRA2[1]
-    if is_vec:
-        # vector constructs "BA[0:2] extra 00 BA[3:4]"
-        return ((BA >> 2) << 6) | # hi 3 bits shifted up
-               (extra << 4) |     # to make room for these
-               (BA & 0b11)        # CR_bit on the end
+        spec = EXTRA2<<1 | 0b0
+    if spec[0]:
+       # vector constructs "BA[0:2] spec[1:2] 00 BA[3:4]"
+       return ((BA >> 2)<<6) | # hi 3 bits shifted up
+              (spec[1:2]<<4) | # to make room for these
+              (BA & 0b11)      # CR_bit on the end
     else:
-        # scalar constructs "00 extra BA[0:4]"
-        return (extra << 5) | BA
+       # scalar constructs "00 spec[1:2] BA[0:4]"
+       return (spec[1:2] << 5) | BA
 ```
 
 Thus, for example, to access a given bit for a CR in SV mode, the v3.0B
@@ -638,12 +633,12 @@ algorithm to determine CR\_reg is modified to as follows:
 
 ```
     CR_index = (BA>>2)      # top 3 bits 
-    if is_vec:
+    if spec[0]:
         # vector mode, 0-124 increments of 4
-        CR_index = (CR_index << 4) | (extra << 2)
+        CR_index = (CR_index<<4) | (spec[1:2] << 2)
     else:
         # scalar mode, 0-32 increments of 1
-        CR_index = (extra << 3) | CR_index
+        CR_index = (spec[1:2]<<3) | CR_index
     # same as for v3.0/v3.1 from this point onwards
     bit_index = (BA & 0b11) # low 2 bits
     CR_reg = CR{CR_index}     # get the CR
-- 
2.30.2