From 4800f32290bcf8d2cfb0fa6c0b06a18e7f21eeb1 Mon Sep 17 00:00:00 2001 From: lkcl Date: Sat, 24 Sep 2022 21:19:25 +0100 Subject: [PATCH] --- openpower/sv/overview/discussion.mdwn | 32 +++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) diff --git a/openpower/sv/overview/discussion.mdwn b/openpower/sv/overview/discussion.mdwn index 8eebff62c..b4287ba28 100644 --- a/openpower/sv/overview/discussion.mdwn +++ b/openpower/sv/overview/discussion.mdwn @@ -5,6 +5,38 @@ # BE Regfile confusion +Defining how an element-addressable variable-width regfile is to +be accessed is exceptionally difficult to pin down when conventions +are used that start MSB numbering from zero (MSB0) rather than at the +Industry-standard convention, LSB0. The confusion arises from the +possibility of MSB0 numbering being potentially applicable to: + +* bits +* bytes +* register numbers +* element numbers + +Then there is the completely separate issue of whether: + +* Memory should be the sole location where LE/BE byteswapping should + be applicable, Architecturally +* Both the register file *and* Memory are, both, Architecturally, + LE/BE reorderable and thus **direct and explicit** control over + reordering over Memory **and the bytes inside regfile data** + is provided at the ISA Level. + +Simple-V has made the decision that providing programmers with explicit +control over the byte-ordering when data is transferred in and out of +the register files is too great, and has chosen to require that, +**Architecturally**, the register file is to be treated as a Little-Endian +byte-ordered SRAM with elements to be accessed as if in a Little-Endian +Software Environment. This does **not** impact Memory-ordering in any way +and it does **not** impact arithmetic operations. +"Architecturally" is defined by Industry-standard convention to mean +that how the Hardware is implemented is entirely down to the implementor, +but that as far as *software* running on that hardware is concerned, +"Architecturally" everything appears as described. + In the overview page is this: | byte0 | byte1 | byte2 | byte3 | byte4 | byte5 | byte6 | byte7 | -- 2.30.2