From: lkcl <lkcl@web>
Date: Mon, 19 Jun 2023 10:52:17 +0000 (+0100)
Subject: (no commit message)
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=747c7e792dc1352dec62c23b7eb520d0d497a9a5;p=libreriscv.git

---

diff --git a/openpower/sv/po9_encoding/discussion.mdwn b/openpower/sv/po9_encoding/discussion.mdwn
index e92ae8137..78f256513 100644
--- a/openpower/sv/po9_encoding/discussion.mdwn
+++ b/openpower/sv/po9_encoding/discussion.mdwn
@@ -102,31 +102,7 @@ Length detection:
     else                     return 64
 ```
 
-# alternative 32-64 encoding (3) **current**
-
-this idea gets even simpler. the 55-bit area is eliminated and may only be
-reintroduced by sacrificing parts of EXT200-263.
-
-```
-|0-5| 6-29   |30 31|32-37   | 38-663 | Description      |
-|---|--------|-----|--------|--------|------------------|
-|PO9| xxxx   | x x | 010001 |  ////  | RESERVED         |
-|PO9| xxxx   | x x | 000001 |  ////  | RESERVED         |
-|PO9| !ZERO  | 1 1 |  !PO9  |     xxxx        | SSingle:EXT232-263 |
-|PO9|  0000  | 1 1 |  !PO9  |     xxxx        | Scalar EXT232-263  |
-|PO9|  SVRM  | 1 0 |  !PO9  |     xxxx        | SVP64:EXT232-263   |
-|PO9|  0000  | 0 1 | Defined Word-instruction | 32-bit EXT300-363  |
-|PO9| !ZERO  | 0 1 | Defined Word-instruction | SSingle:EXT000-063 |
-|PO9|  SVRM  | 0 0 | Defined Word-instruction | SVP64:EXT000-063   |
-```
-* bit 31 is "SSingle"
-* bit 30 is EXT200-263 if set otherwise EXT000-063
-
-This was one of the original early PO9 Encodings.
-
-# alternative **no 32 encoding* (4) **current**
-
-
+# alternative **no 32 encoding** (3) **current**
 
 aim of this idea is to attempt simplification of area identification
 and length.  the 55-bit area is eliminated and may only be reintroduced
@@ -300,3 +276,135 @@ test.  Only EXT000-063 is not by a "process of elimination" (if ... return True)
     return True
 ```
 
+# alternative 32-64 encoding (4) **current**
+
+this idea gets even simpler. the 55-bit area is eliminated and may only be
+reintroduced by sacrificing parts of EXT200-263.
+
+```
+|0-5| 6-29   |30 31|32-37   | 38-663 | Description      |
+|---|--------|-----|--------|--------|------------------|
+|PO9| xxxx   | x x | 010001 |  ////  | RESERVED         |
+|PO9| xxxx   | x x | 000001 |  ////  | RESERVED         |
+|PO9| !ZERO  | 1 1 |  !PO9  |     xxxx        | SSingle:EXT232-263 |
+|PO9|  0000  | 1 1 |  !PO9  |     xxxx        | Scalar EXT232-263  |
+|PO9|  SVRM  | 1 0 |  !PO9  |     xxxx        | SVP64:EXT232-263   |
+|PO9|  0000  | 0 1 | Defined Word-instruction | 32-bit EXT300-363  |
+|PO9| !ZERO  | 0 1 | Defined Word-instruction | SSingle:EXT000-063 |
+|PO9|  SVRM  | 0 0 | Defined Word-instruction | SVP64:EXT000-063   |
+```
+* bit 31 is "SSingle"
+* bit 30 is EXT200-263 if set otherwise EXT000-063
+
+This was one of the original early PO9 Encodings.
+
+**Identification**:
+
+The following identification-criteria may be easily verified by taking
+a copy of the table above and striking out each line that fails each
+test.  Only EXT000-063 is not by a "process of elimination" (if ... return True)
+
+**Length detection**: (critically-important to be short)
+
+```
+    if PO1                   return 64
+    elif not PO9             return 32
+    elif Word[30:31] = 0b00  return 32
+    else                     return 64
+```
+
+**EXT232-263:**
+
+```
+    if not PO9                return False
+    # eliminate EXT900
+    if Word[30:31] == 0b00    return False
+    # eliminate EXT0xx
+    if Word[31] == 0b1        return False
+    # eliminate EXT900 and reserved areas
+    if Word[33:37] != 0b10001 return False
+    # 
+    return True
+```
+
+
+
+**EXT000-063**: (includes Prefixed)
+
+```
+    if PO1                     return False # or other reserved
+    if not PO9                 return True  # prefixed needs more
+    # eliminate EXT900
+    if Word[30:31] =  0b00     return False
+    # eliminate Prefixed-EXT900, RESERVED and EXT200
+    if Word[31]    =  0b0      return False
+    # eliminate 32-bit Unvec in 64b area
+    if Word[30:31] =  0b01 and
+       Word[6:29 ] =  0x000000 return False
+    return True
+```
+
+**SVP64**:
+
+```
+    # easy-elimination, first
+    if not PO9                 return False
+    if Word[30]    =  0b0      return False
+    # eliminate anything not SVP64:EXT900
+    if Word[30:31] =  0b10 and
+       Word[33:37] = 0b10001 and
+       Word[62:63] != 0b01    return False
+    # eliminate anything not SVP64:EXT200-231
+    if Word[31:32] =  0b00    return False
+    # all remaining combinations are SVP64:EXTnnn
+    return True
+```
+
+**SVP64Single**:
+
+```
+    # easy-elimination, first
+    if not PO9                 return False
+    # eliminate 32-bit EXT900
+    if Word[30:31] =  0b00     return False
+    # eliminate SVP64:EXT000-063
+    if Word[30:31] =  0b11     return False
+    # eliminate anything "Identity"
+    if  Word[6:29 ] =  0x000000 return False
+    # eliminate SVP64:EXT200-231
+    if Word[30:32] =  0b101     return False
+    # eliminate anything not SSingle:EXT900
+    if Word[30:31] =  0b10 and
+       Word[33:37] = 0b10001 and
+       Word[62:63] != 0b00      return False
+    # everything left is SVP64Single
+    return True
+```
+
+**RESERVED**: (no need to pass for further detailed decode)
+
+```
+    # easy-elimination, first
+    if not PO9                 return False
+    # eliminate EXT900
+    if Word[30:31] =  0b00     return False
+    # eliminate SVP64:EXT000-031
+    if Word[30:31] =  0b11     return False
+    # eliminate SSingle:EXT000-063
+    if Word[6:29 ] != 0x000000 and
+       Word[30:31]  =  0b01    return False
+    # eliminate EXT200-231
+    if Word[30:31] =  0b10 and
+       Word[33:37] = 0b10001   return False
+    # eliminate SSingle:EXT900
+    if Word[30:31] =  0b10 and
+       Word[6:29 ] != 0x000000 and
+       Word[33:37] = 0b10001 and
+       Word[62:63] = 0b00      return False
+    # eliminate SVP64:EXT900
+    if Word[30:31] =  0b10 and
+       Word[33:37] = 0b10001 and
+       Word[62:63] = 0b01      return False
+    # all else needs further detailed decode
+    return True
+```