diff --git a/bootfiles/18e/boot20-x86.lisp b/bootfiles/18e/boot20-x86.lisp
new file mode 100644
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+++ b/bootfiles/18e/boot20-x86.lisp
@@ -0,0 +1,189 @@
+;;;
+;;; Cross-compile bootstrap file for adding heap overflow checking for x86.
+;;;
+;;; Use this as the cross-compile script for cross-build-world.sh.
+;;;
+
+(in-package :cl-user)
+
+;;; Rename the X86 package and backend so that new-backend does the
+;;; right thing.
+(rename-package "X86" "OLD-X86")
+(setf (c:backend-name c:*native-backend*) "OLD-X86")
+
+(c::new-backend "X86"
+   ;; Features to add here
+   '(:x86 :i486 :pentium
+     :stack-checking
+     :heap-overflow-check
+     :linkage-table
+     :mp
+     :gencgc
+     :conservative-float-type
+     :hash-new :random-mt19937
+     :linux :glibc2 :glibc2.1
+     :cmu :cmu18 :cmu18d
+     )
+   ;; Features to remove from current *features* here
+   '(:x86-bootstrap :alpha :osf1 :mips
+     :propagate-fun-type :propagate-float-type :constrain-float-type
+     :openbsd :freebsd
+     :long-float :new-random :small))
+
+;;; May need to add some symbols to *features* and
+;;; sys::*runtime-features* as well.  This might be needed even if we
+;;; have those listed above, because of the code checks for things in
+;;; *features* and not in the backend-features..  So do that here.
+
+(pushnew :heap-overflow-check *features*)
+(pushnew :heap-overflow-check sys::*runtime-features*)
+
+;;; Compile the new backend.
+(pushnew :bootstrap *features*)
+(pushnew :building-cross-compiler *features*)
+(load "target:tools/comcom")
+
+;;; Load the new backend.
+(setf (search-list "c:")
+      '("target:compiler/"))
+(setf (search-list "vm:")
+      '("c:x86/" "c:generic/"))
+(setf (search-list "assem:")
+      '("target:assembly/" "target:assembly/x86/"))
+
+;; Load the backend of the compiler.
+
+(in-package "C")
+
+(load "vm:vm-macs")
+(load "vm:parms")
+(load "vm:objdef")
+(load "vm:interr")
+(load "assem:support")
+
+(load "target:compiler/srctran")
+(load "vm:vm-typetran")
+(load "target:compiler/float-tran")
+(load "target:compiler/saptran")
+
+(load "vm:macros")
+(load "vm:utils")
+
+(load "vm:vm")
+(load "vm:insts")
+(load "vm:primtype")
+(load "vm:move")
+(load "vm:sap")
+(load "vm:system")
+(load "vm:char")
+(load "vm:float")
+
+(load "vm:memory")
+(load "vm:static-fn")
+(load "vm:arith")
+(load "vm:cell")
+(load "vm:subprim")
+(load "vm:debug")
+(load "vm:c-call")
+(load "vm:print")
+(load "vm:alloc")
+(load "vm:call")
+(load "vm:nlx")
+(load "vm:values")
+(load "vm:array")
+(load "vm:pred")
+(load "vm:type-vops")
+
+(load "assem:assem-rtns")
+
+(load "assem:array")
+(load "assem:arith")
+(load "assem:alloc")
+
+(load "c:pseudo-vops")
+
+(check-move-function-consistency)
+
+(load "vm:new-genesis")
+
+;;; OK, the cross compiler backend is loaded.
+
+(setf *features* (remove :building-cross-compiler *features*))
+
+;;; Info environment hacks.
+(macrolet ((frob (&rest syms)
+	     `(progn ,@(mapcar #'(lambda (sym)
+				   `(defconstant ,sym
+				      (symbol-value
+				       (find-symbol ,(symbol-name sym)
+						    :vm))))
+			       syms))))
+  (frob OLD-X86:BYTE-BITS OLD-X86:WORD-BITS
+	#+long-float OLD-X86:SIMPLE-ARRAY-LONG-FLOAT-TYPE 
+	OLD-X86:SIMPLE-ARRAY-DOUBLE-FLOAT-TYPE 
+	OLD-X86:SIMPLE-ARRAY-SINGLE-FLOAT-TYPE
+	#+long-float OLD-X86:SIMPLE-ARRAY-COMPLEX-LONG-FLOAT-TYPE 
+	OLD-X86:SIMPLE-ARRAY-COMPLEX-DOUBLE-FLOAT-TYPE 
+	OLD-X86:SIMPLE-ARRAY-COMPLEX-SINGLE-FLOAT-TYPE
+	OLD-X86:SIMPLE-ARRAY-UNSIGNED-BYTE-2-TYPE 
+	OLD-X86:SIMPLE-ARRAY-UNSIGNED-BYTE-4-TYPE
+	OLD-X86:SIMPLE-ARRAY-UNSIGNED-BYTE-8-TYPE 
+	OLD-X86:SIMPLE-ARRAY-UNSIGNED-BYTE-16-TYPE 
+	OLD-X86:SIMPLE-ARRAY-UNSIGNED-BYTE-32-TYPE 
+	OLD-X86:SIMPLE-ARRAY-SIGNED-BYTE-8-TYPE 
+	OLD-X86:SIMPLE-ARRAY-SIGNED-BYTE-16-TYPE
+	OLD-X86:SIMPLE-ARRAY-SIGNED-BYTE-30-TYPE 
+	OLD-X86:SIMPLE-ARRAY-SIGNED-BYTE-32-TYPE
+	OLD-X86:SIMPLE-BIT-VECTOR-TYPE
+	OLD-X86:SIMPLE-STRING-TYPE OLD-X86:SIMPLE-VECTOR-TYPE 
+	OLD-X86:SIMPLE-ARRAY-TYPE OLD-X86:VECTOR-DATA-OFFSET
+	))
+
+(let ((function (symbol-function 'kernel:error-number-or-lose)))
+  (let ((*info-environment* (c:backend-info-environment c:*target-backend*)))
+    (setf (symbol-function 'kernel:error-number-or-lose) function)
+    (setf (info function kind 'kernel:error-number-or-lose) :function)
+    (setf (info function where-from 'kernel:error-number-or-lose) :defined)))
+
+(defun fix-class (name)
+  (let* ((new-value (find-class name))
+	 (new-layout (kernel::%class-layout new-value))
+	 (new-cell (kernel::find-class-cell name))
+	 (*info-environment* (c:backend-info-environment c:*target-backend*)))
+    (remhash name kernel::*forward-referenced-layouts*)
+    (kernel::%note-type-defined name)
+    (setf (info type kind name) :instance)
+    (setf (info type class name) new-cell)
+    (setf (info type compiler-layout name) new-layout)
+    new-value))
+(fix-class 'c::vop-parse)
+(fix-class 'c::operand-parse)
+
+#+random-mt19937
+(declaim (notinline kernel:random-chunk))
+
+(setf c:*backend* c:*target-backend*)
+
+;;; Extern-alien-name for the new backend.
+(in-package :vm)
+(defun extern-alien-name (name)
+  (declare (type simple-string name))
+  #+(and bsd (not elf))
+  (concatenate 'string "_" name)
+  #-(and bsd (not elf))
+  name)
+(export 'extern-alien-name)
+(export 'fixup-code-object)
+(export 'sanctify-for-execution)
+(in-package :cl-user)
+
+;;; Don't load compiler parts from the target compilation
+
+(defparameter *load-stuff* nil)
+
+;; hack, hack, hack: Make old-x86::any-reg the same as
+;; x86::any-reg as an SC.  Do this by adding old-x86::any-reg
+;; to the hash table with the same value as x86::any-reg.
+(let ((ht (c::backend-sc-names c::*target-backend*)))
+  (setf (gethash 'old-x86::any-reg ht)
+	(gethash 'x86::any-reg ht)))