From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Fri, 24 May 2019 07:12:03 +0000 (+0100)
Subject: add priority picker docstring
X-Git-Tag: div_pipeline~1966
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=e63f2d5ed2f45b4d92c9203c171f8568461dec0a;p=soc.git

add priority picker docstring
---

diff --git a/src/experiment/compalu.py b/src/experiment/compalu.py
index bcd71b0c..1df0cc25 100644
--- a/src/experiment/compalu.py
+++ b/src/experiment/compalu.py
@@ -18,7 +18,7 @@ from nmutil.latch import SRLatch, latchregister
       The src1 and src2 registers and the operand can be latched in
       at this point
 
-    * Read request is set, which is ackowledged through the Scoreboard
+    * Read request is set, which is acknowledged through the Scoreboard
       to the priority picker, which generates (one and only one) Go_Read
       at a time.  One of those will (eventually) be this Computation Unit.
 
diff --git a/src/scoreboard/group_picker.py b/src/scoreboard/group_picker.py
index f02f8863..133b3720 100644
--- a/src/scoreboard/group_picker.py
+++ b/src/scoreboard/group_picker.py
@@ -2,6 +2,49 @@ from nmigen.compat.sim import run_simulation
 from nmigen.cli import verilog, rtlil
 from nmigen import Module, Signal, Cat, Elaboratable
 
+""" Group Picker: to select an instruction that is permitted to read (or write)
+    based on the Function Unit expressing a *desire* to read (or write).
+
+    The job of the Group Picker is extremely simple yet extremely important.
+    It sits in front of a register file port (read or write) and stops it from
+    being corrupted.  It's a "port contention selector", basically.
+
+    The way it works is:
+
+    * Function Units need to read from (or write to) the register file,
+      in order to get (or store) their operands, so they each have a signal,
+      readable (or writable), which "expresses" this need.  This is an
+      *unary* encoding.
+
+    * The Function Units also have a signal which indicates that they
+      are requesting "release" of the register file port (this because
+      in the scoreboard, readable/writable can be permanently HI even
+      if the FU is idle, whereas the "release" signal is very specifically
+      only HI if the read (or write) latch is still active)
+
+    * The Group Picker takes this unary encoding of the desire to read
+      (or write) and, on a priority basis, activates one *and only* one
+      of those signals, again as an unary output.
+
+    * Due to the way that the Computation Unit works, that signal (Go_Read
+      or Go_Write) will fire for one (and only one) cycle, and can be used
+      to enable the register file port read (or write) lines.  The Go_Read/Wr
+      signal basically loops back to the Computation Unit and resets the
+      "desire-to-read/write-expressing" latch.
+
+    In theory (and in practice!) the following is possible:
+
+    * Separate src1 and src2 Group Pickers.  This would allow instructions
+      with only one operand to read to not block up other instructions,
+      and it would also allow 3-operand instructions to be interleaved
+      with 1 and 2 operand instructions.
+
+    * *Multiple* Group Pickers (multi-issue).  This would require
+      a corresponding increase in the number of register file ports,
+      either 4R2W (or more) or by "striping" the register file into
+      split banks (a strategy best deployed on Vector Processors)
+
+"""
 
 class PriorityPicker(Elaboratable):
     """ implements a priority-picker.  input: N bits, output: N bits