Re: Build your own Forth for Microchip PIC: the nature of metacompilation : Forth

Date: Sat, 14 Jul 2007 08:47:51 -1000
Organization: FORTH, Inc.
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Reply-To: Elizabeth D Rather <eratherXXX@forth.com>
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none Byron Jeff wrote:
....
> After some further reading of Brad's, Stephen's, and especially
> Elizabeth's material, I believe I'm starting to grasp the nature of
> metacompilation in Forth. The summary is that words are structured to
> compile themselves and are organized into vocabularies that changes the
> function of words depending on vocabulary search order.
>
> Each word in the TARGET vocabulary needs 4 data elements in its definition:
>
> 1) target address of the word
> 2) possibly the length of the code

Haven't found this necessary, really. If you're downloading a newly
compiled definition, you have its starting address and the address of
the next unused location, and the difference is the size of your new
code. If you're building an image for later download, you track the
image's size.

> 3) flags (such as INLINE and NOT TRANSFERRED (to target))

Well, some definitions may be inline-able, but that's a property of the
INTERPRETER part, not the target version. And we keep a global switch
that governs whether we're transferring definitions as they're compiled
vs. building an image for download, so no need to track individual
definitions.

> 4) the actual code definition
>
> this set would represent the parameter field(s) of the definition.
>
> The code field would simply pull the target address of the word, create
> a CALL instruction out of it (presuming not inline) and stick that
> instruction at the end of the code definition currently being compiled.
>
> It's starting to make more sense.

Good.

> A question: is there a standard way to get an image of a vocabulary to
> and from a file?

No. In the first place, a vocabulary is a logical entity whose
components are not in any particular physical location. Traditionally,
Forth is compiled from source to executable form. You may wish to save
an image of your kernel or app for flashing at the start of a
development session, but not individual vocabularies.

Cheers,
Elizabeth

--
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Elizabeth D. Rather (US & Canada) 800-55-FORTH
FORTH Inc. +1 310-491-3356
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