(no commit message)

diff --git a/openpower/sv/overview/discussion.mdwn b/openpower/sv/overview/discussion.mdwn
index 49d7e958112f7deb2e596843abce0f9a578b9e0e..58486f7e197889bb69bf2d938cc392a23c8a7dc3 100644 (file)
--- a/openpower/sv/overview/discussion.mdwn
+++ b/openpower/sv/overview/discussion.mdwn
@@ -2,3 +2,37 @@
 
 * <http://lists.libre-soc.org/pipermail/libre-soc-dev/2020-December/001742.html>
 * <http://lists.libre-soc.org/pipermail/libre-soc-dev/2020-December/001748.html>
+
+# BE Regfile confusion
+
+In the overview page is this:
+
+       | byte0 | byte1 | byte2 | byte3 | byte4 | byte5 | byte6 | byte7 |
+       | ----- | ----- | ----- | ----- | ----- | ----- | ----- | ----- |
+    r0 | b[0]  | b[1]  | b[2]  | b[3]  | b[4]  | b[5]  | b[6]  | b[7]  |
+    r1 | b[8]  | b[9]  | b[10] | b[11] | b[12] | b[13] | b[14] | b[15] |
+
+Starting an elwidth=16 loop from r0 and extending for
+7 elements would begin at r0 and extend partly over r1.  Note that
+b0 indicates the low byte (lowest 8 bits) of each 16-bit word, and
+b1 represents the top byte:
+
+       | byte0 | byte1 | byte2 | byte3 | byte4 | byte5 | byte6 | byte7 |
+       | ----- | ----- | ----- | ----- | ----- | ----- | ----- | ----- |
+    r0 | s[0].b0  b1   | s[1].b0  b1   | s[2].b0  b1   |  s[3].b0  b1  |
+    r1 | s[4].b0  b1   | s[5].b0  b1   | s[6].b0  b1   |  unmodified   |
+
+If data is loaded at elwidth=16 as above, then accessed as elwidth=8,
+then in order to get at it in an arithmetically-expected order of
+lowest byte first followed by highest byte, would require this ordering:
+
+       | byte0 | byte1 | byte2 | byte3 | byte4 | byte5 | byte6 | byte7 |
+       | ----- | ----- | ----- | ----- | ----- | ----- | ----- | ----- |
+    r0 | b[1]  | b[0]  | b[3]  | b[2]  | b[5]  | b[4]  | b[7]  | b[6]  |
+
+Whilst this is technically possible to do (with 2D REMAP) it is not
+reasonable to "punish" BE developers by forcing them into the
+situation of having to perform such element-level reordering, nor
+is it reasonable for the complexity to bleed over into LE Mode
+in order to "compensate" by making BE easier but LE more difficult.
+