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)
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.
+
projects / crowdsupply.git / commitdiff