About the image

On 17.08.2010, at 13:27, stephane ducasse wrote:

>>> Hi, I have just began to work at this low level and I have 2 questions:
>>
>> Nice. But you need to give us a little more context. What are you trying to do?
>
> Bootstrap

Building an image from first principles? Nice :)

> and shaking/documenting the hidden assumptions on the way.

Excellent!

>>> - What is the minimal object structure I should implement from VM perspective?
>>
>> Normally you do not have to implement this, as the interpreter is shared between all platforms.
>
> yes we know. Nicolas read the chapter of tim but we wanted to check if this is still up to date.
> My original question was more: where can I find the class object VM interpretation (that the first slot should be superclass, format....)
> because I remember that we could not add an instance variable into behavior when we did traits.

Ah. That's a more concrete question :)

I thought the three inst vars in Behavior was all the VM knew and cared about? I'd consider everything else a bug ;)

>>> - Where are defined the 3 formats of objects representation?
>>
>> Not sure what you mean. ObjectMemory>>formatOf: defines the class format bits in the header.
>
> Yes we saw that.
>
>> Or do you mean the 3 header formats? Read the class comment of ObjectMemory.
>>
>> Also, there is a nice explanation in section A.1 of Tim's chapter in the NuBlue book:
>>
>> http://www.rowledge.org/tim/squeak/assets/OE-Tour.pdf
>
> Another question was where can we get a description of the image format.
> I was planning to read the VM C code included the generated one.

The image is just a header plus a memory dump of the object memory. The header is written in Interpreter>>writeImageFileIO: and read in Interpreter>>readImageFromFile:HeapSize:StartingAt:.

- Bert -

On 8/17/2010 6:24 PM, Eliot Miranda wrote:

On Tue, Aug 17, 2010 at 5:38 AM, Bert Freudenberg <[hidden email]

<mailto:[hidden email]>> wrote:
   I thought the three inst vars in Behavior was all the VM knew and
   cared about? I'd consider everything else a bug ;)


That's right.  See all implementors of initializeClassIndices:

BTW, you /can/ add an inst var to Behavior.  Try and evaluate

Object subclass: #Behavior
instanceVariableNames: 'superclass methodDict format extra'
classVariableNames: 'ObsoleteSubclasses'
poolDictionaries: ''
category: 'Kernel-Classes'

Works for me in a 4.1 derived image.

The problem is in Interpreter>>classNameOf: aClass Is: className which uses the Class' name ivar directly. This causes several "interesting" issues with the SmartSyntaxBlaBlaPlugins (such as JPEGReadWriterPlugin).

Cheers,
 - Andreas

On 8/18/2010 2:21 AM, Bert Freudenberg wrote:

On 18.08.2010, at 04:48, Andreas Raab wrote:

On 8/17/2010 6:54 PM, Eliot Miranda wrote:

In the VW VM I added code to choose a class in the
specialObjectsArray (Array) and search it for a byte object with the
characters 'Array' and use that as the class name index.  Similarly its
class was searched for an object equal to Array and that slot was used
as the thisClass index.  I see I was too lazy to do that in
StackInterpreter>>initializeInterpreter but I should, and should
introduce classNameIndex and thisClassIndex and use them in place of the
hard-coded constants.  That way one can save immediately after
adding/removing the behavior inst var and have the VM get things right
on start-up.

That's one option. An alternative is to provide the class name index via the splObjsArray and default to 5 if not present.

I like Elliot's option better. Not having to maintain that index means there is one thing less to worry about. If the added flexibility of an explicit index is needed later, it could still be added at that point.

I'm not entirely sure about that. You do have to maintain the information that after adding an ivar you'll have to save and restart the image or else strange things will happen. This effect is nowhere documented. With the index you could automate the process (using the ClassBuilder notification) so that if Class chnages shape it updates the splObj entry automatically.

Cheers,
 - Andreas