From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Wed, 5 Dec 2018 13:38:29 +0000 (+0000)
Subject: update microarch
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=f25b45b026d9541b98f8fc75643787e967d855f6;p=crowdsupply.git

update microarch
---

diff --git a/updates/003_2018dec04_microarchitecture.mdwn b/updates/003_2018dec04_microarchitecture.mdwn
index 88093b9..b1735c5 100644
--- a/updates/003_2018dec04_microarchitecture.mdwn
+++ b/updates/003_2018dec04_microarchitecture.mdwn
@@ -58,6 +58,8 @@ instruction queue and the ALUs is efficiently leveraged.
 
 Simple!
 
+# Tomasulo Algorithm and Reorder Buffers
+
 There are many other benefits to a multi-issue microarchitecture, and
 these are being discussed
 [here](http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2018-December/000198.html)
@@ -125,5 +127,65 @@ towards Reorder Buffers and Tomasulo as a good, clean fit.  In part that
 is down to more research having been done on that particular algorithm.
 For completeness, scoreboarding and explicit register renaming need
 to be properly and comprehensively investigted.
-More as it happens...
+
+# Scoreboarding
+
+[Scoreboards](https://en.wikipedia.org/wiki/Scoreboarding) are an
+effort to keep "score" of whether an instruction has all of its
+operands (and the hardware that uses them) ready to go, without conflict.
+Everything about the scheme, unfortunately, says that it is an incomplete
+mechanism.  Unlike the Tomasulo algorithm when augmented with a Reorder
+Buffer, there's no way to "undo" a completed operation: the operation
+proceeds and modifies registers or memory, in an out-of-order fashion,
+regardless of consequences.  If an exception occurred *tough*!
+
+Hence, it was quite hard to accept scoreboarding enough to understand it.
+It wasn't until
+[this message](http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2018-December/000223.html)
+that I realised that there is near-direct equivalence between the features
+of scoreboarding and the Tomasulo Algorithm.  It's not complete equivalence,
+because the Reorder Buffer is what keeps everything transactional and clean.
+However, at least the scoreboard does have direct equivalence to the
+Reservation Station concept: the scoreboard records whether the source
+registers are ready or not (and so does the Reservation Station).
+
+The problem is: there are far too many things missing from the scoreboard
+concept:
+
+* Register-renaming has to be done separately (the Tomasulo Reservation
+  Stations, in combination with the ROB, handle that implicitly).
+* Scoreboarding introduces the concept of "waffly exceptions"
+  (the ROB can even record exceptions that are only actioned if they
+  make it to the head of the queue).
+* Scoreboarding does not provide a means to do multi-issue (the ROB
+  does: you just put more than one entry per cycle into the buffer)
+
+# Next step
+
+The project is being run along ethical lines.  That in particular means
+unanimous decision-making.  Nobody gets to over-rule anyone else: everyone
+matters, and everyone's input matters.  So if I, personally, think that
+Tomasulo is better, it's up to me to keep on researching and evaluating
+and reasoning until I have convinced everyone else... or they have convinced
+me otherwise.
+
+I have it on good authority from some extremely comprehensive research
+that this is a hell of a lot better way to do decision-making than
+"mob-rule" voting.  "Mob-rule" voting (aka "democracy") basically
+*automatically* destroys the morale and discounts the will of the
+"minority".  No wonder democratic countries have "Minority Representation
+Political Groups", because it's heavily brainwash-ingrained into people
+living in such countries that the "Minority" view *shall* be disregarded;
+of *course* they feel the need to shout and get angry!
+
+Learning from these mistakes, which are often copied and reflected
+in Software Libre groups, is I feel very important, to try something
+different.  In previous Libre projects that I've run, I was the
+kind-of informal "technical lead", where I never actually defined any
+guidelines or rules about how the project should make decisions: basically
+I let people do what they liked unless it disrupted the project for
+everyone else.  I set myself up as the "arbitrator", so to speak.
+The way that this project will be run is very much new and experimental,
+even to me.  We'll see how it goes.
+