Move underscore section back up. Remove mentions of version from naming sections

Mention underscore as a separor of multi-letter extensions more
consistently.

Move the special case requiring an underscore for "P" extension to
the Version Numbering section.

Author: topperc

src/naming.adoc

@@ -45,13 +45,18 @@ Some ISA extensions depend on the presence of other extensions, e.g.,
 may be implicit in the ISA name: for example, RV32IF is equivalent to
 RV32IFZicsr, and RV32ID is equivalent to RV32IFD and RV32IFDZicsr.
 
+=== Underscores
+
+Underscores "_" may be used to separate ISA extensions to improve
+readability and to provide disambiguation, e.g., "RV32I2_M2_A2".
+
 === Additional Standard Unprivileged Extension Names
 
 Standard unprivileged extensions can also be named by using a single "Z" followed by an
-alphanumeric name. The name must end with an alphabetical character or an 
-optional version number. The second letter from the end cannot be numeric if the
+alphanumeric name. The name must end with an alphabetical character.
+The second letter from the end cannot be numeric if the
 last letter is "p". For example, "Zifencei" names the instruction-fetch fence extension
-described in <<zifencei>>; "Zifencei2" and "Zifencei2p0" name version 2.0 are the same.
+described in <<zifencei>>.
 
 The first letter following the "Z" conventionally indicates the most
 closely related alphabetical extension category, IMAFDQLCBKJTPVH. For the
@@ -68,20 +73,20 @@ separated from other multi-letter extensions by an underscore, e.g.,
 === Supervisor-level Instruction-Set Extension Names
 
 Standard extensions that extend the supervisor-level virtual-memory
-architecture are prefixed with the letters "Sv", followed by an alphabetical
-name and an optional version number, or by a numeric name with no version number.
+architecture are prefixed with the letters "Sv", followed by an alphanumeric
+name. Other standard extensions that extend the supervisor-level architecture are
+prefixed with thel letters "Ss", followed by an alphanumeric name. The name
+must end with an alphabetical character. The second letter from the end cannot
+be numeric if the last letter is "p". These extensions are further defined in
+Volume II.
 
-Other standard extensions that extend the supervisor-level architecture are 
-named by using "Ss" as a prefix, followed by an alphanumeric name. The name 
-must end with an alphabetical character or an optional 
-version number. The second letter from the end cannot be numeric character
-if the last letter is "p". Supervisor-level extensions must be separated 
-from other multi-letter extensions by an underscore. These extensions are further 
-defined in Volume II.
+The extensions "sv32", "sv39", "sv48", and "sv59" were defined before the rule
+against extension names ending in numbers was established.
 
 Standard supervisor-level extensions should be listed after standard
-unprivileged extensions. If multiple supervisor-level extensions are
-listed, they should be ordered alphabetically.
+unprivileged extensions, and like other multi-letter extensions, must be
+separated from other multi-letter extensions by an underscore. If multiple
+supervisor-level extensions are listed, they should be ordered alphabetically.
 
 === Hypervisor-level Instruction-Set Extension Names
 
@@ -102,16 +107,17 @@ Standard machine-level instruction-set extensions are prefixed with the
 letters "Sm".
 
 Standard machine-level extensions should be listed after standard
-lesser-privileged extensions. If multiple machine-level extensions are
-listed, they should be ordered alphabetically.
+lesser-privileged extensions, and like other multi-letter extensions, must be
+separated from other multi-letter extensions by an underscore. If multiple
+machine-level extensions are listed, they should be ordered alphabetically.
 
 === Non-Standard Extension Names
 
 Non-standard extensions are named by using a single "X" followed by the alphanumeric 
-name. The name must end with an alphabetic character or an optional version number. The 
+name. The name must end with an alphabetic character. The 
 second letter from the end cannot be numeric if the last letter is 
 "p". For example, "Xhwacha" names the Hwacha vector-fetch ISA 
-extension; "Xhwacha2" and "Xhwacha2p0" name version 2.0 of same.
+extension;
 
 Non-standard extensions must be listed after all standard extensions, and,
 like other multi-letter extensions, must be separated from other multi-letter
@@ -120,15 +126,25 @@ For example, an ISA with non-standard extensions Argle and
 Bargle may be named "RV64IZifencei_Xargle_Xbargle".
 
 If multiple non-standard extensions are listed, they should be ordered
-alphabetically.
+alphabetically. Like other multi-letter extensions, they should should be
+separated from other multi-leter extensions by an underscore.
 
 === Version Numbers
 
 Recognizing that instruction sets may expand or alter over time, we
 encode extension version numbers following the extension name. Version
 numbers are divided into major and minor version numbers, separated by a
 "p". If the minor version is "0", then "p0" can be omitted from
-the version string. Changes in major version numbers imply a loss of
+the version string. To avoid ambiguity, no extension name may end with a number
+or a "p" preceded by a number.
+
+Because the "P" extension for Packed SIMD can be confused for the
+decimal point in a version number, it must be preceded by an underscore
+if it follows another extension with a version number. For example, "rv32i2p2"
+means version 2.2 of RV32I, whereas "rv32i2_p2" means version 2.0 of RV32I with
+version 2.0 of the P extension.
+
+Changes in major version numbers imply a loss of
 backwards compatibility, whereas changes in only the minor version
 number must be backwards-compatible. For example, the original 64-bit
 standard ISA defined in release 1.0 of this manual can be written in
@@ -140,17 +156,6 @@ Hence, we define the default version of a standard extension to be the
 version present at that time, e.g., "RV32I" is equivalent to
 "RV32I2".
 
-=== Underscores
-
-Underscores "_" may be used to separate ISA extensions to improve
-readability and to provide disambiguation, e.g., "RV32I2_M2_A2".
-
-Because the "P" extension for Packed SIMD can be confused for the
-decimal point in a version number, it must be preceded by an underscore
-if it follows a number. For example, "rv32i2p2" means version 2.2 of
-RV32I, whereas "rv32i2_p2" means version 2.0 of RV32I with version 2.0
-of the P extension.
-
 === Subset Naming Convention
 
 <<isanametable>> summarizes the standardized extension