From: lkcl <lkcl@web>
Date: Thu, 5 May 2022 20:04:48 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~2420
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=92735709357c9c5f848a7103e033fce720810db3;p=libreriscv.git

---

diff --git a/openpower/sv/SimpleV_rationale.mdwn b/openpower/sv/SimpleV_rationale.mdwn
index 4708e73e5..f6f717b71 100644
--- a/openpower/sv/SimpleV_rationale.mdwn
+++ b/openpower/sv/SimpleV_rationale.mdwn
@@ -276,3 +276,18 @@ of the problem-space:
   DCT, FFT, Parallel Prefix-Sum and other common transformations
   that require significant programming effort in other ISAs.
 
+**What is missing from Power Scalar ISA that a Vector ISA needs?**
+
+Remarkably, very little.
+
+* The traditional `iota` instruction may be
+  synthesised with an overlapping add, that stacks up incrementally
+  and sequentially.  Although it requires two instructions, one to
+  start the sum-chain, the technique has the advantage of allowing
+  increments by arbitrary amounts, and is not limited to addition,
+  either.
+* Big-integer addition (arbitrary-precision arithmetic) is an
+  emergent characteristic the carry-in, carry-out capability of
+  Power ISA `adde` instruction. `sv.adde` naturally emerges from the
+  sequential chaining of these scalar instructions.
+*