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

1 files changed, 39 insertions, 0 deletions

diff --git a/sim/mips/README.Cygnus b/sim/mips/README.Cygnus
index 06c5e43..5b646ef 100644
--- 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
 -------------------------------------------------------------------------------