From: lkcl <lkcl@web>
Date: Sun, 29 Aug 2021 12:19:02 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: DRAFT_SVP64_0_1~281
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=c89723afeeb1970b48ab7414f2886d6bb547d302;p=libreriscv.git

---

diff --git a/openpower/sv/svp64/appendix.mdwn b/openpower/sv/svp64/appendix.mdwn
index 6e2470a0d..f84257b0e 100644
--- a/openpower/sv/svp64/appendix.mdwn
+++ b/openpower/sv/svp64/appendix.mdwn
@@ -455,9 +455,33 @@ will rely.
 Operations that actually produce or alter CR Field as a result
 do not also in turn have an Rc=1 mode.  However it makes no
 sense to try to test the 4 bits of a CR Field for being equal
-or not equal to zero.
+or not equal to zero. Moreover, the result is already in the
+form that is desired: it is a CR field.
 
-TODO
+There are two primary different types of CR operations:
+
+* Those which have a 3-bit operand field (referring to a CR Field)
+* Those which have a 5-bit operand (referring to a bit within the
+   whole 32-bit CR)
+
+Examining these two as has already been done it is observed that
+the difference may be considered to be that the 5-bit variant
+provides additional information about which CR Field bit
+(EQ, GE, LT, SO) is to be operated on by the instruction.
+
+Thus, logically, we may set the following rule:
+
+* When a 5-bit CR Result field is used in an instruction, the
+  `inv, VLi and RC1` variant of Data-Dependent Fail-First
+  must be used. i.e. the bit of the CR field to be tested is
+  the one that has just been modified by the operation.
+* When a 3-bit CR Result field is used the `inv CRbit` variant
+  must be used in order to select which CR Field bit shall
+  be tested (EQ, LE, GE, SO).
+
+Examples of the former type:
+
+* crand, cror, crnor. These all are 5-bit (BA, BB, BC).
 
 # pred-result mode