From: lkcl <lkcl@web>
Date: Sat, 13 Aug 2022 10:18:09 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~872
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=a8b88534da1b5028fcf8c794b8606758b1bbacc4;p=libreriscv.git

---

diff --git a/openpower/sv/normal.mdwn b/openpower/sv/normal.mdwn
index 9f7134f9b..cfbe04b1e 100644
--- a/openpower/sv/normal.mdwn
+++ b/openpower/sv/normal.mdwn
@@ -34,6 +34,9 @@ and FP.
 is as if the 
 *destination* predicate bit was zero even before starting the operation. 
 When Rc=1 the CR element however is still stored in the CR regfile, even if the test failed.  See appendix for details.
+* **Pack/Unpack** mode, only available when SUBVL is vec2/3/4, performs
+basic structure packing on sub-elements. Bits 4-5 (normally elwidth) are
+taken up as Pack/Unpack bits.
 
 Note that ffirst and reduce modes are not anticipated to be high-performance in some implementations.  ffirst due to interactions with VL, and reduce due to it requiring additional operations to produce a result.  normal, saturate and pred-result are however inter-element independent and may easily be parallelised to give high performance, regardless of the value of VL.
 
@@ -45,7 +48,8 @@ The Mode table for Arithmetic and Logical operations
 | 00  |   0 |  dz  sz | normal mode                      |
 | 00  |   1 | 0  RG   | scalar reduce mode (mapreduce), SUBVL=1 |
 | 00  |   1 | 1  /    | parallel reduce mode (mapreduce), SUBVL=1 |
-| 00  |   1 | SVM RG  | subvector reduce mode, SUBVL>1   |
+| 00  |   1 | SVM 0   | subvector reduce mode, SUBVL>1   |
+| 00  |   1 | SVM 1   | Pack/Unpack mode, SUBVL>1   |
 | 01  | inv | CR-bit  | Rc=1: ffirst CR sel              |
 | 01  | inv | VLi RC1 |  Rc=0: ffirst z/nonz |
 | 10  |   N | dz   sz |  sat mode: N=0/1 u/s |
@@ -139,8 +143,10 @@ executed in sequential Program Order, element 0 being the first.
 Thus the new VL comprises a contiguous vector of results, 
 all of which pass the testing criteria (equal to zero, less than zero).
 
-The CR-based data-driven fail-on-first is new and not found in ARM
-SVE or RVV. It is extremely useful for reducing instruction count,
+The CR-based data-driven fail-on-first is "new" and not found in ARM
+SVE or RVV. At the same time it is "old" because it is almost
+identical to a generalised form of Z80's `CPIR` instruction.
+It is extremely useful for reducing instruction count,
 however requires speculative execution involving modifications of VL
 to get high performance implementations.  An additional mode (RC1=1)
 effectively turns what would otherwise be an arithmetic operation