From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Sun, 14 Feb 2021 12:34:38 +0000 (+0000)
Subject: add extra FSM explanatory comments to TestIssuer
X-Git-Tag: convert-csv-opcode-to-binary~231^2~10
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;ds=sidebyside;h=a3097ba93ed558d704c0e4eb11b93d08f532bf65;p=soc.git

add extra FSM explanatory comments to TestIssuer
---

diff --git a/src/soc/simple/issuer.py b/src/soc/simple/issuer.py
index ad9b1dd7..8c69b115 100644
--- a/src/soc/simple/issuer.py
+++ b/src/soc/simple/issuer.py
@@ -260,6 +260,11 @@ class TestIssuerInternal(Elaboratable):
         # then "holds" information, combinatorially, for the core
         # (as opposed to using sync - which would be on a clock's delay)
         # this includes the actual opcode, valid flags and so on.
+
+        # this FSM performs fetch of raw instruction data, partial-decodes
+        # it 32-bit at a time to detect SVP64 prefixes, and will optionally
+        # read a 2nd 32-bit quantity if that occurs.
+
         with m.FSM(name='fetch_fsm'):
 
             # waiting (zzz)
@@ -335,7 +340,11 @@ class TestIssuerInternal(Elaboratable):
                 with m.If(fetch_insn_ready_i):
                     m.next = "IDLE"
 
-        # decode / issue / execute FSM
+        # decode / issue / execute FSM.  this interacts with the "fetch" FSM
+        # through fetch_pc_ready/valid (incoming) and fetch_insn_ready/valid
+        # (outgoing).  SVP64 RM prefixes have already been set up by the
+        # "fetch" phase, so execute is fairly straightforward.
+
         with m.FSM():
 
             # go fetch the instruction at the current PC