From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Sat, 5 Sep 2020 20:43:04 +0000 (+0100)
Subject: add comments on MSR read
X-Git-Tag: semi_working_ecp5~177
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=d798c4edb292da21095a25025068938774991208;p=soc.git

add comments on MSR read
---

diff --git a/src/soc/simple/issuer.py b/src/soc/simple/issuer.py
index 58c9769b..ff525e98 100644
--- a/src/soc/simple/issuer.py
+++ b/src/soc/simple/issuer.py
@@ -226,7 +226,7 @@ class TestIssuer(Elaboratable):
                     comb += self.imem.f_valid_i.eq(1)
                     sync += cur_state.pc.eq(pc)
 
-                    # initiate read of MSR
+                    # initiate read of MSR.  arrives one clock later
                     comb += self.state_r_msr.ren.eq(1<<StateRegs.MSR)
                     sync += msr_read.eq(0)
 
@@ -237,9 +237,9 @@ class TestIssuer(Elaboratable):
 
             # dummy pause to find out why simulation is not keeping up
             with m.State("INSN_READ"):
-                # one cycle later, msr read arrives
+                # one cycle later, msr read arrives.  valid only once.
                 with m.If(~msr_read):
-                    sync += msr_read.eq(1)
+                    sync += msr_read.eq(1) # yeah don't read it again
                     sync += cur_state.msr.eq(self.state_r_msr.data_o)
                 with m.If(self.imem.f_busy_o): # zzz...
                     # busy: stay in wait-read