Document existence of old (gencode) and new (igen) MIPS ISA simulators.

diff --git a/sim/mips/README.Cygnus b/sim/mips/README.Cygnus
index 06c5e43383083dcf64ec7d765525fca0d59a60b5..5b646eff794b36a0757fad88f97cf6ac502f495f 100644 (file)
--- a/sim/mips/README.Cygnus
+++ b/sim/mips/README.Cygnus
@@ -1,3 +1,42 @@
+This directory contains two very different simulators:
+
+       o       gencode (old)
+
+               Gencode.c outputs a single monolithic file that is
+               #included by interp.c
+
+       o       igen (new)
+
+               The *.igen files are used as inputs to ../igen/igen.
+               A number of separate, fairly modula files, are created.
+
+The new simulator has a number of advantages:
+
+       o       builtin support for multi-simming (single simulator
+               image supporting a number of different instruction
+               set architectures).
+
+       o       Easier maintenance. The input files are not confused
+               by an intermixing with the generator code.
+
+gencode continues to exist so that old architectures can be emulated.
+*.igen should be used when adding new architectures or adding
+instructions to an existing ISA.
+
+Known bugs?
+
+A mips16 simulator cannot be built using igen.  A custom mips16
+engine.c needs to be written.
+
+In mips.igen, the semantics for many of the instructions were created
+using code generated by gencode.  Those semantic segments could be
+greatly simplified.
+
+
+----
+
+Old README.Cygnus ...
+
 > README.Cygnus
 -------------------------------------------------------------------------------