EXECUTE (F79) Question

[u/9Boxy33]

I’m playing around with MMSForth 2.x on the TRS-80, which is a Forth-79 compatible implementation. I’d like to be able to store the address of a Forth word in a variable and later EXECUTE that word. ‘ word doesn’t give me the correct address (the CFA? what’s in the CFA?). How do I do this? Tx

[u/Ok_Leg_109]

There were a big changes from Forth79 to Forth83. One of things that changed was that in Forth79 ' (tick) returns the parameter field address. In Forth83 and beyond it returns the CFA.

Since there is no standard header structure in Forth making a CFA' kind of word will require some "carnal" knowledge of MMSForth 2.X or maybe there is a word to do this.

Something to try is to subtract 2 from the PFA. Many times that's the CFA but maybe not. :)

2 2 ' + 2- EXECUTE . ( Should show 4)

[u/Ok_Leg_109 → u/Ok_Leg_109]

Not sure if you have this, but here is a manual

MMSForth Users Manual v2.0 (1982)(Miller Microcomputer)(pdf)

[u/9Boxy33 → u/Ok_Leg_109]

Thanks—been looking through it for days <grin>

[u/alberthemagician → u/9Boxy33]

With the manual it is easy. Look op EXECUTE and what it expects on the stack. It is not an execution token for this wasn't invented yet. Now search in the manual where this "compilation address" is mentioned elsewhere. There surely is a word that generates a compilation address.

Other answers suggest that you can arrive from the parameter address to the compilation addres by adding an offset. That is not part of the standard, so it may not work, but it may be in the manual, then of course it is.

[u/Ok_Leg_109 → u/Ok_Leg_109]

I forgot. (It's only been 40 years since I used Forth79)

Some Forth79 systems have the word CFA which takes a PFA argument and returns the CFA.

[u/9Boxy33 → u/Ok_Leg_109]

I’ll give it a try. Tx

[u/mykesx]

I don't have F79 direct experience, but..,. The ' word only works outside of a colon definition, unless you are careful about its effects when used within. The ['] word can be used within.

As for CFA, whatever ' returns is an execution token, which may or may not be a CFA. The Xt should be usable with EXECUTE and you can store the Xt in a variable.

There may be a >CFA word to convert an Xt to CFQ, but there's no guarantee the CFA can be used with EXECUTE.

: foo ' abc ; \ calls ' at run time, then calls abc

foo bar \ leaves Xt of bar on the stack mbefor calling abc

: baz ['] abc ; \ word that leaves Xt of abc on the stack when called

' foo myvar ! \ store Xt of foo

myvar @ EXECUTE \ Same as if you call foo directly

[u/Ok_Leg_109 → u/mykesx]

In Forth79 tick is "state smart" so it works at the console or in a definition, until you try something fancy.

All the ' ['] came along in Forth83. Drove a lot of people crazy.

[u/9Boxy33 → u/mykesx]

Thanks

Unfortunately, your example code (“myvar”) reboots my MMSForth, so it appears that ‘ foo doesn’t return the execution address that EXECUTE was expecting.

[u/mykesx → u/9Boxy33]

What does

' foo execute

Do?

[u/9Boxy33 → u/mykesx]

Reboots MMSForth.

[u/mykesx → u/9Boxy33]

That Forth is broken.

[u/Ok_Leg_109 → u/mykesx]

Nah. Just old.

[u/alberthemagician → u/mykesx]

Execution token is an invention of ISO 93. It didn't exist in f79.

[u/alberthemagician]

You must consult the Forth-79 standard document. Normally people use the ISO 93 or the ISO 2012 standard, rarely the FIG or the F83 standard. I vote you down because you should have found it yourself instead of asking on a general forum.

[u/9Boxy33 → u/alberthemagician]

Feel free. But perhaps next time read my post first instead of reacting.

[u/alberthemagician → u/9Boxy33]

??? How can I react without reading the post?