It certainly is a hoot to test the limits of Forth. I recently thought of an idea to implement an assembly language terminology of self-modifying code. In this case though, a word would modify another word.

As an example, ERROR is only called a couple of times. Once by ?ERROR and once by R/W.

But ?ERROR is called 17 times and R/W is called whenever disk access is required.

My thought was if I modified ?ERROR to a different ERROR routine, then the original ERROR routine can still be used by R/W and ?ERROR will have a different ERROR routine.

For example, ?ERROR has this definition:

: ?ERROR SWAP IF ERROR ELSE DROP THEN ;

The goal is to change the CFA of ERROR to something else. This is what I came up with:

: error1 ." something different than the original word definition." cr quit ;
: modify ( modify cccc dddd eeee ffff) [compile] ' 0 ! [compile] ' 2 ! [compile] ' 4 !
[compile] ' 6 ! 0 @ begin 2+ dup @ 4 @ = if 6 @ swap ! quit else
dup @ 2 @ = then until ;

example: MODIFY ?ERROR ; ERROR ERROR1 will change the CFA of ERROR to the CFA of ERROR1 in the word ?ERROR.

cccc: ?ERROR ( is the word to change)
dddd: ; ( ?ERROR's delimiter: either ; or (;code)
eeee: ERROR ( gets the CFA of the old word to be replaced)
ffff: ERROR1 ( gets the CFA of the new word higher in memory)


This is just an exercise to see if it could be done. Discussion of practicality is welcome.

NOTES:
ERROR and ERROR1 cannot be the same name as -FIND will not find lower definition in memory.
Take note that memory addresses 0,2,4,6 are free for use with Forth in my system. Change as necessary for yours.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

My Forth does not have >BODY. Is there a definition anywhere?

Doing a google search is useless as it doesn't recognize the ">" sign.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Thanks. It is not about what you may or may not have mentioned. A couple of different codes that I found for DOER/MAKE had >BODY, so I gathered that DOER/MAKE was part of a newer Forth standard and was not likely to see an older definition that I could use.

I really thought >BODY was a more complex definition. I am actually disappointed in its definition now since it takes 3 extra characters more to type than its definition. But I understand it is more about what the word is supposed to represent.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Yes; but what goes the other direction? It's not PFA, because that takes the NFA to give you the PFA; and NFA goes the opposite direction. I'm looking in the fig-Forth assembly source code and the installation manual and I'm not finding a word to do that. I guess you're just supposed to know to use 2+.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

This kind of confusion is why I am kind of adopting a "best suited" method. Some words don't even have a PFA as the code is or can be redirected to elsewhere in memory.

My Forth breaks more if I use : PFA CFA 2+, so I rewrote it so that FIND returns the CFA, then to get the PFA it is just : PFA CFA @. Even with an STC forth, PFA, instead of being 3+, could be made to work with CFA @, without breaking STC or DTC portability.

I don't know, but I think this seems to be an easier port between different threaded Forths.

After reading some other topics and finally understanding them, I see that >BODY makes sense if only used with words without CODE.

Since >BODY can damage a code word, wouldn't it be more prudent to define >BODY to only allow a change to the body of a word if that word is a COLON word? Maybe something like this:

: >BODY ( cfa --- ) DUP @ [ ' DO @ ] = IF 2+ ELSE DROP # ERROR THEN ;

it seems to me that any word that can cause damage, or allow to cause damage, to another word (by human error) should have at least a little error checking.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?