diff --git a/README.md b/README.md
index cf682b3..5f34760 100644
--- a/README.md
+++ b/README.md
@@ -13,15 +13,20 @@ suceeding pseudo-UTF-8 character is encoded as follows:
 * `1111110H 10HHHxxx 10xxxxxx 10xxxxxx 10xxxxxx	10xxxxxx`
 * `11111110 10HHHHxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx`
 
-The first four bytes determine the type:
-
-* `0`: Value is a register.
-* `1`: Value is immediate.
+The first four bytes determine the type. The LSB high bit determines
+if the encoded value is a register (`0001`) or immediate (`0010`).
+The second bit from LSB determines if the value should be treated
+as a signed 32 bit two's compliment number (`001X`) or should be
+treated as an unsigned 32 bit number (`000X`).
 
 All other values are reserved. Overlong values are allowed, and for some
 argument values they are necessary. All lines are terminated by a byte
 of all zeros.
 
+## Assembler
+
+The macro assembler is Python (see the asm directory).
+
 ## Design Philsophy
 
 Creole is small but not minimal. It is easy to add instructions, and