add priority picker docstring

diff --git a/src/experiment/compalu.py b/src/experiment/compalu.py
index bcd71b0cabc63d54565e5f21217ef488ae5f943c..1df0cc25d7319b35099040e863653a28a14507cc 100644 (file)
--- 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
 
       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.
 
       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 f02f88630e7080d6b3109c03d41a929f01605ab6..133b37203a473bfe3cfea73559c589987ccfedf3 100644 (file)
--- 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
 
 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
 
 class PriorityPicker(Elaboratable):
     """ implements a priority-picker.  input: N bits, output: N bits