diff --git a/sources/brainfuck.dylan b/sources/brainfuck.dylan
index 1269e67..7cf6ae8 100644
--- a/sources/brainfuck.dylan
+++ b/sources/brainfuck.dylan
@@ -3,18 +3,6 @@ Synopsis: Brainfuck core
 Author: Fernando Raya
 Copyright: GPLv3
 
-define constant $memory-size
-  = 30000;
-
-define constant <memory-cell>
-  = <integer>;
-
-define constant <memory>
-  = limited(<vector>, of: <memory-cell>, size: $memory-size);
-
-define constant <memory-pointer>
-  = <integer>;
-
 define sealed class <bf> (<object>)
   slot bf-pp :: <program-pointer> = 0,
     init-keyword: program-pointer:;
@@ -22,7 +10,8 @@ define sealed class <bf> (<object>)
     init-keyword: memory-pointer:;
   constant slot bf-program :: <program>,
     required-init-keyword: program:;
-  constant slot bf-memory :: <memory> = make(<memory>, fill: 0),
+  constant slot bf-memory :: <memory>
+    = make(<memory>, fill: 0, size: $default-memory-size),
     init-keyword: memory:;
 end;
 
diff --git a/sources/brainfuck.lid b/sources/brainfuck.lid
index 51dae78..b1bc4f3 100644
--- a/sources/brainfuck.lid
+++ b/sources/brainfuck.lid
@@ -1,5 +1,6 @@
 Library: brainfuck
 Files: library.dylan
+       memory.dylan
        instruction.dylan
        comment-instruction.dylan
        io-instruction.dylan
diff --git a/sources/memory.dylan b/sources/memory.dylan
new file mode 100644
index 0000000..9bd30de
--- /dev/null
+++ b/sources/memory.dylan
@@ -0,0 +1,16 @@
+Module: brainfuck-impl
+Synopsis: Brainfuck memory 
+Author: Fernando Raya
+Copyright: GPLv3
+
+define constant $default-memory-size
+  = 30000;
+
+define constant <memory-cell>
+  = <integer>;
+
+define constant <memory>
+  = limited(<vector>, of: <memory-cell>);
+
+define constant <memory-pointer>
+  = <integer>;