2014-05-07 13:16 GMT+02:00 Helmut Rohregger <helmut.rohregger at gmail.com>:

> Hi,
>
> I am interested in the squeak class format word, which is an instance
> variable of every squeak class object. I could not find much info about
> it...
>
> The low 2 bits are size bits - size of what? header?
> Behavior>>instSize shows how to fetch the number of instance variables for
> an object.
> Thats it...
>
> For example, the format for 'Object' is 2, for 'Array' its 6402, for
> 'MethodContext' its 29070.
> Can anybody tell me what additional info is stored in this format word?
>
> - Helmut
>
>
It's all in image.
Search for inst var write of format

ClassDescription>>superclass: aClass methodDictionary: mDict format: fmt
ClassBuilder>>privateNewSubclassOf: newSuper from: oldClass
ClassBuilder>>computeFormat: type instSize: newInstSize forSuper: newSuper
ccIndex: ccIndex
ClassBuilder>>format: nInstVars variable: isVar words: isWords pointers:
isPointers weak: isWeak

You will find from highest bits to lowest bits of the SmallInteger:

2 bits for instanceSize high bits
5 bits for compactClassIndex (there are 32 possiblie compact classes)
4 bits for instanceSpecification
6 bits for instanceSize low bits
1 bit = 0 unused (alignment for wordSize in 32bits)

Note that instanceSize = number of instance variables + 1.

In memory, there is one more bit for representing the SmallInteger:
1 bit = 1 (=SmallInteger)

Thus, the low 8 bits represent the size in bytes of a Word-oriented object
(variableWord: or pointers...)
(assuming the object has at most 63 inst.var. and assuming a 32bits image)
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