What is an instSpec?

> On Tue, Aug 11, 2020 at 2:26 PM David T. Lewis <[hidden email]> wrote:
>
> > One of the interesting aspects of Squeak is how a Class in the image is
> > used to make new object instances in the Squeak VM object memory.
> >
> > I collected some of my recent notes on this topic and put them on the
> > swiki at http://wiki.squeak.org/squeak/6646
> >
> > Dave
> >
>
> You are right that some values in the instSpec are unused. However, the
> instSpec is only used to initialize each object's format bits. Some of the
> bits are used to store the size of values that are smaller than a full 32
> bit word. The size in the object header always refers to full words, so the
> actual size in e.g. bytes is sizeInWords * 4 - sizeFromFormatBits.
>
> So you can't use format code 9 to mean "indexable short (16-bit) field"
> because codes 8-11 are already used for 8-bit fields of differing sizes
> between whole words. Format 8 means all 4 bytes of the last word are used,
> format 9 means only 3 bytes of the last word are actually used, format 10
> means 2 bytes are used, and format 11 means only one byte is used. (Or the
> other way around, I forgot)
>
> To add 16 bit fields you need 2 unused format codes, one fore even and one
> for odd length.
>
> For 64 bit fields you would only need a single code.
>
> Unless you find another unused bit in the object header we don't have
> enough bits to represent both 16 and 64 bit arrays. One way may be to not
> ever allow odd 16 bit arrays I guess?
>