From: lkcl <lkcl@web>
Date: Sat, 20 Aug 2022 19:17:50 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~816
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=5ffdc881f4af51007cd6f1599c84e639bd1b3a33;p=libreriscv.git

---

diff --git a/openpower/sv/ldst.mdwn b/openpower/sv/ldst.mdwn
index aaeba9009..b45cef665 100644
--- a/openpower/sv/ldst.mdwn
+++ b/openpower/sv/ldst.mdwn
@@ -554,5 +554,16 @@ because the generic abstracted concept of "Remapping", when applied to
 LD/ST, will give that same capability, with far more flexibility.
 
 Also LD/ST with immediate has a Pack/Unpack option similar to VSX
-'vpack' and 'vunpack'.
+'vpack' and 'vunpack', as well as the VSX Pixel instructions. Enabling
+this mode on SubVectors is straightforward and does not involve
+the setup cost of REMAP. Unlike REMAP, Pack/Unpack on LD/ST does not have
+Saturation (or Fail-first) at the same time.
+
+*Programmer's note: a decision on what is best if combining Saturation
+with Pack/Unpack is required will depend on resources.  REMAP will
+require less registers but is more costly to set up. On the other
+hand LDST Pack/Unpack followed by Saturated MV or arithmetic requires
+intermediary registers at full width prior to reduced saturated width.
+A balanced decision is therefore needed*.
+