diff --git a/blk/001 b/blk/001
index c000d68..157785d 100644
--- a/blk/001
+++ b/blk/001
@@ -7,7 +7,7 @@ MASTER INDEX
 200 Z80 assembler             260 Cross compilation
 280 Z80 boot code             350 ACIA driver
 370 SD Card driver            390 Cross-compiled core
-439 Core words                480 AT28 Driver
+439 XPACKed core              480 AT28 Driver
 490 TRS-80 Recipe             520 Fonts
 550 TI-84+ Recipe
 
diff --git a/blk/439 b/blk/439
index 376b6de..b6e9c1c 100644
--- a/blk/439
+++ b/blk/439
@@ -1,12 +1,14 @@
-Core words
+XPACKed core
 
-These words follow cross-compiled words, but unlike them, these
-are self-bootstrapping and don't depend on the Cross Compiler.
-They will typically be included in source form right after a
-stage1 binary which will interpret it on boot and bootstrap
-itself to a full intepreter, which can then be relinked with
-the Relinker. There is no loader for these libraries because
-you will typically XPACK (B267) them.
+Most of Collapse OS' core words are cross compiled (B390).
+However, some of them are too dynamically referenced to be
+cross-compiled without great pain, so we XPACK (B267) them,
+that is, we put them in source form in the target's
+initialization section (see B89).
 
-440 core                       447 readln                   
-453 blk
+These words will be compiled into RAM at initialization, which
+is a bit wasteful both in RAM and in boot time, so we will
+typically relink (B120) that newly compiled binary and append
+it to our existing binary for optimal resource usage.
+
+Load range: 440-446