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Message from discussion Build your own Forth for Microchip PIC (Episode 838): Threading
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none Byron Jeff  
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 More options Jun 24 2007, 8:40 am
Newsgroups: comp.lang.forth
From: byron@upstairs.(none) (Byron Jeff)
Date: Sun, 24 Jun 2007 07:40:31 -0500
Local: Sun, Jun 24 2007 8:40 am
Subject: Re: Build your own Forth for Microchip PIC (Episode 838): Threading
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In article <1182664370.207959.205...@w5g2000hsg.googlegroups.com>,
Bruce McFarling  <agil...@netscape.net> wrote:

>On Jun 23, 6:32 pm, byron@upstairs.(none) (Byron Jeff) wrote:
>> 48 words. Ouch! When bootstrapping the primitives are critical.

>They can be reduced. Some by simply omitting them, when they are
>primitives because they cannot be synthesized from other words, but
>you do not require them. If there is no dictionary and no interpreter
>in the PIC, then that may increase the number of primitives that may
>be dispensed with.

Possibly. But you have to analyze the entire core tree to determine if a
word is in fact not needed.

>Others by synthesizing them from a simpler Forth model.

This is what I've been looking for. A factored primitive model that is
architecture independent along with a coding of core words from that
model.

For example:
---------

>A ... from data stack to an address pointer

A> ... from pointer to data stack.

A! ... store cell on stack at address in A
A@ ... fetch cell at address in A and write to stack

A+ ... increment value in A

>R ... from data stack to return stack, and

R> ... from return stack to data stack
-----------

Now that's an easily implementable model. Only one question:

1. From the description above for A! and the descriptions for ! and 2!
below the value to store is in A and the address is on the stack, right?
Something doesn't match up.

: DROP ( x -- ) >A ;
: DUP ( x -- x x ) >A A> A> ;
: OVER ( xa xb -- xa xb xa ) >R >A A> R> A> ;
: SWAP ( xa xb -- xb xa ) >R >A R> A> ;
: ROT ( xa xb xc -- xb xc xa ) >R >R >A R> R> A> ;
: @ ( a -- x ) >A A@ ;
: ! ( x a -- ) >A A! ;

\ two address units per cell
: 2@ ( a -- xl xh ) >A A@ >R A+ A+ A@ R> ;
: 2! ( xl xh a -- ) >A A! A+ A+ A! ;

\ one address unit per cell
: 2@ ( a -- xl xh ) >A A@ >R A+ A@ R> ;
: 2! ( xl xh a -- ) >A A! A+ A! ;

: 1+ ( x -- x+1 ) >A A+ A> ;
: 2+ ( x -- x+2) >A A+ A+ A> ;

: R@ ( -- x | R: x -- ) R> >A A> A> R> ;
: 2>R ( xl xh -- | R: -- xl xh ) >A >R A> >R ;
: 2R> ( -- xl xh | R: xl xh -- ) R> >A R> A> ;

>etc. ... presumably in this context, after getting the boostrap up and
>running, the more cumbersome definitions may be replaced.

Exactly.

Thanks.

BAJ


 
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