How to change your Squeak to 2 tag bits and more immediate classes

<title>How to change your Squeak to 2 tag bits and more immediate classes</title>

<h1>How to change your Squeak to 2 tag bits and more immediate classes</h1>

<blockquote>
    Hans-Martin Mosner<br>
    &lt;<a href="mailto:hmm@heeg.de">hmm@heeg.de</a>&gt; (at work),<br>
    March 20, 1997
</blockquote>
To do this, you need to be able to re-create your Squeak VM from the Smalltalk
sources in Squeak. I've tried to make the process as easy as possible, and
the changes to the VM should applicable even if you have modified it with
your own primitives.
<p>
There are 4 files:

<hr>

<ul>
    <li><a href="VM-2TagBits.cs">VM-2TagBits.cs</a><br>
    This changes your VM. You should file it into the image in which you
    generate your VM, and then build a new VM. Be sure to backup everything,
    as this change is radical!<br>
    The new VM will have an image format version of 6503 as opposed to 6502,
    so you won't be able to run images saved by this VM with any older VM.
    The new VM will be able to read old images, but the images won't run
    correctly unless <b>SmallInt-2TagBits.cs</b> has been filed in first.
    The reason for this is that the old SmallIntegers are converted to the
    new format by simply shifting them 1 bit to the left. If they are not
    in the appropriate range, their value will change.<br>
    In the new VM, primitives 126 and 127 are defined to support more immediate
    classes. They are used by <b>MoreImmediateClasses.cs</b>.
    <li><a href="SmallInt-2TagBits.cs">SmallInt-2TagBits.cs</a><br>
    This scans the image for SmallIntegers that would not fit into the range
    that SmallInts will have after the 2-tag-bit conversion. These SmallIntegers
    are converted into LargeIntegers. The file-in also changes the SmallInteger
    minVal and maxVal methods to check for the actual range ofSmallIntegers
    on image startup. An image prepared with these changes will still be
    ok to use with an older VM, but when you want to save it for conversion
    by a 2-tag-bit VM, you should execute "<code>SmallInteger prepareFor2BitTags</code>"
    before saving to make certain that no SmallIntegers will be incorrectly
    converted.
    <li><a href="MoreImmediateClasses.cs">MoreImmediateClasses.cs</a><br>
    The code in this file-in supports the creation of more immediate objects.
    Currently it consists of 2 primitives that are used to convert between
    SmallIntegers and other immediate values, plus somecode to register Immediate
    classes with the CompactClasses Array (which will be extended from 31
    to 47 entries for this purpose). 
    <li><a href="SmallPoint.cs">SmallPoint.cs</a><br>
    This is an example of how the mechanism can be used. It modifies the
    methods in class Point so that only the getters and setters for x and
    y access the instance variables directly. Then a new subclass SmallPoint
    is defined. You can use instances of the class just like normal points,
    but the VM is not yet aware of them. Be warned that this has not been
    fully tested!
</ul>

<hr>

SmallCharacters and other funny and useful things are left as an exercise
for the reader...
<p>
Have fun with this. I hope it will make it into the next Squeak release.
<p>
Hans-Martin