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diff --git a/subx/Readme.md b/subx/Readme.md
index 4f6f6c24..d898cde9 100644
--- a/subx/Readme.md
+++ b/subx/Readme.md
@@ -2,6 +2,17 @@
 
 Bytecode interpreter for a subset of the 32-bit x86 ISA.
 
+* only instructions operating on the 32-bit E*X registers
+* assuming a flat address space, completely ignoring segment registers
+* ignoring the carry flag; arithmetic operations always operate on signed numbers
+  (while bitwise operations always operate on unsigned numbers)
+
+Basically a minimum basis set that a compiler for Mu can target. This isn't
+meant to run arbitrary binaries, just those generated by our compiler,
+allowing better tests for the compiler, access to processor/memory state
+without getting bogged down in details of the ELF format, ABI, STABS debugging
+format, etc., etc.
+
 To run tests:
 
   ```