[Edit: this is the latest version of Fleet Forth's assembler.

Attachment:

Fleet Forth Assembler.zip [10.67 KiB]
Downloaded 77 times

The original version is three posts down

Forth assemblers with structured conditionals have served me well over the years, but I've recently noticed that they can have their shortcomings. I'd like to start a discussion on how to improve an assembler with structured conditionals.
I have three examples of the word (EXPECT) from my Forth's kernel. At the start of the loop if the y-register equals the value in N, the goal is to branch to the JSR to FFCF, one of the routines in the C64 kernel.
The first two examples use various control flow stack manipulators, CS-ROT,CS-SWAP, etc. to manipulate the control flow information.

The final example was some prototyping for another approach. It makes use of the four unused bytes at 3FC-3FF. The idea is that there could be an array of available memory locations for control flow data. The words to access this memory could be:

Used as:




Any comments?
I know this approach doesn't look very Forthy, and would appreciate it if anyone has other ideas to improve an assembler with structured conditionals.

_________________
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?

(EXPECT) really isn't that long at 46 bytes of code ( not counting the CFA ).

They look like another way to do structured conditionals in the assembler.
Notice the first example ( they all compile the same code ). It's from my Forth's kernel. There are three WHILE,s in the loop but only two exit the loop. By use of control flow manipulation with CS-ROT the first WHILE, is resolved by THEN, so it branches to the 0FFCF JSR, instruction.
The next example avoids having a WHILE, that doesn't leave the loop. The first WHILE, is replaced with IF, Since WHILE, is just:

The second WHILE, places its control flow data under the control flow data for IF,. THEN, resolves IF, but CS-SWAP is needed to place WHILE,'s control flow data under BEGIN,'s.
The third example uses an experimental approach to place the control flow data for IF, in a temporary location and later place it back on the stack for THEN,.
Which of the three is easiest to follow? Is there a better way to enhance an assembler with structured conditionals?

By the way, the code is actually two BEGIN, loops with a common starting point. CS-DUP duplicates the control flow data for BEGIN, so it is still available after 0= UNTIL,. I did this so the WHILE,s would branch out of both loops. Again, I'm not sure how well that reads. Since I plan to release my Forth and its accompanying source, I would like the code to be as easy to follow as possible without rewriting it to make it [the assembled code] bigger.
I'd also like to improve my assembler.

[Edit: one clarification]

I wonder if there is a lack of interest in Forth assemblers with structured conditionals or if Ragsdale's is the only one with which most Forth programmers are familiar? Here is the source for mine.
Any comments, questions, suggestions?
[Edit: It's released under the LGPL version 2 or later. Do I need to include a copy of the license?]
[Edit: Included license]

The problem with Forth assemblers isn't necessarily the syntax (as off-putting as it may be to some), but the fact that it's in Forth in the first place.

If it were just a stand alone program that takes a text file and spits out an assembled binary, then it's simply a different syntax rather than an entirely different tool chain.

and, yes, you should include a copy of the license in the ZIP file.

_________________
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?

That's the basic problem, though.

Forth assemblers (and when I say "Forth assemblers", I'm talking the typical Forth Assembler most of us are familiar with rather than some edge case) are for writing words for a Forth environment, rather than large assembly language programs. So, they're not, typically, designed for some of the problems that regular assemblers are designed for.

Notably, Forth assemblers aren't set up to manage or work with libraries and object files. They don't need a linker. They also don't offer things like cross references, reference tables (labels and their assignments), etc.

Early Forth assemblers were also, naturally, limited by the BLOCK structure of Forth source code, which is why much of it is written with several instructions on a single line vs one instruction per line in traditional assemblers.

Forth assemblers are single pass assemblers as well.

Basically consider someone writing Prince of Persia using a Forth assembler.

There's nothing to stop someone from writing a full boat, stand alone, full featured assembler using the RPN syntax. But, in the end I don't think it would save much in terms of the assembler code, or how wide of an appeal it would have in the end.

_________________
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?

Well, Ragsdale's assembler (which includes structured conditionals) has been around since Day One -- ie, FIG Forth for 6502, which was 1980 or thereabouts. Said assembler very likely has room for improvement, but any revised or alternative version has some catching up to do when it comes to garnering attention.

There has, however, been a fair amount of discussion here on this forum. It's not all tidily arranged in one thread, unfortunately -- that can't 100% be expected, given the way discussions evolve!

In addition to this thread, here are two others with pertinent remarks:

This and nearby posts in the Petil thread pertain to CASE statements.
Self Modifying Code (perhaps too broad a title, as you didn't mention the intended context of structured conditionals)

BTW, I'll quickly inject a comment of my own. Ragsdale's assembler (and yours, I think) check to see that conditionals are properly paired, which means (for example) that BEGIN leaves both an address and an error-check code on stack and UNTIL removes these two items. Usually that's fine, but in certain situations the error-check code becomes a nuisance, leading to extra SWAPs ROTs etc. I daresay there's no perfect solution, but perhaps it'd be best if the error-check codes (but not the addresses) got pushed & popped from their own separate stack. (This is a variation of an idea Garth suggested elsewhere.)

-- Jeff

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Well, there's this example:

http://www.forth.org/fig-forth/fig-forth_68000.pdf

This is an implementation of Fig-Forth written in an RPN assembler (itself written in Forth). Though if you look at it, you'll see it's more traditional assembler than the typical utility in a Forth system to build assembler based words.

AT A GLANCE, this listing is almost unreadable, to me. By that I mean I would have to give it some thorough study to get past the syntax that they are using. But if you notice, this assembler does NOT offer structured conditionals or loops (this is readily apparent in the FIND word, where you see FIND1, FIND2, etc. labels.

I only mention this as an example of the momentum that typical assembler syntax has, being pretty common across CPUs and instructions sets.

RPN as an expression representation has utility to folks, particularly in calculators as it just works better on going calculations. But having as assembler of this scope relying on RPN, is really designed as a benefit to the computer, not the developer. I don't think I would necessarily enjoy working with the assembler syntax presented here.

But, then, we can get used to anything.

I enjoy postfix assembly. I'm curious, you've mentioned how bad that metacompiler was, could that have colored your perception of the assembler?

Ah but that's not the case with the metacompiler's assembler ( which is also postfix ). When I was developing my metacompiler, I added a feature that I never actually needed ( it was easy to add ). The metacompiler has a word SUB which is used to write subroutines a code word might need. SUB creates a label ( on the host, of course ) but does not create any header or code field on the target. It then switches to the metacompiler assembler vocabulary. Here is a short test routine I wrote last night.

I know it's not much, even for an example but, it shows that it is possible to use the metacompiler's assembler to write stand alone programs.
This short piece was saved with the metacompiler word TSAVE and loads at address $C000 or decimal 49152 ( an address familiar to Commodore 64 users who also programmed their machines ). The screen shot shows the short BASIC program that uses this routine. The screen shot was taken during the test. Yes it completed successfully.
This has given me food for though about the metacompiler. Currently, the source is compiled with MLOAD or MTHRU but, writing MINCLUDE, a metacompiler version of INCLUDE will be easy to write but, there may be a better way. I'll have to go through the metacompiler code and clean it up and maybe rethink some of the metacompiler design.

Cheers,
Jim

P.S. BOTTOM! could be renamed ORIGIN.

Sorry about that, I kinda got sidetracked on that one. The disscussion digressed to a discussion on fixing that ugly hand calculated offset in my example SMC. My assembler uses structured conditionals, so I'm biased that way. In addition to the SMC use by NEXT, I was hoping to see more examples of SMC in assembly, in any notation, or high level Forth, although I've never had an occasion to use it in high level Forth. Still, it would have been interesting to see some examples. There's chitselb's example here but, I really would have liked to see more examples.

That's why I've got CS-DUP CS-DROP CS-SWAP and CS-ROT which also work with high level Forth conditionals. I just treat the address and security number as a single unit.

_________________
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?

I don't usually align on the opcode, but rather the first character on a line as shown below:

Is high level Forth so different?