VTL-2 part 5, Transliterating to 6809

 

16K Color Computer 1
by Wikimedia contributor Bilby,
licensed under CC BY 3.0
via Wikimedia Commons

Well, the transliteration of the MC-10 version of VTL-2 in 6801 assembler to 6809 assembler on the Tandy Color Computer actually went pretty quickly once I made time, using the relationships between the 6800/6801 assembly language and runtime and the 6809 assembly language and runtime that I describe in my post on the software differences between the three CPUs. 

But then I found myself using the assembly language equivalent of poking telltales to the screen to figure out where I'd fallen asleep at the wheel.

There was only one place, really, where I had inverted the transfer from B to A in the character output routine, or was it from A to B in the keyboard input? Something like that.

And it handled basic expressions, but wandered off in the ether any time I tried to type in a program. After going back over the transliteration with a fine-toothed comb and finding nothing (and falling asleep doing it) for several days in a row, then using more telltales to the screen to pinpoint where it was dying, I decided to look back in the commented disassembly of Color Computer extended BASIC. 

It was only a matter of a half an hour to finding the problem.

Color Computer BASIC uses a number of variables in the direct page. 

I had dodged a few of the variables down around $C0 by starting the direct page variables at $D0. But there is a variable at $E2 used by the IRQ handler routine as a counter, and by starting there I was using $E2 as the SAVLIN variable, which is essentially the most used variable during program editing.

But $00E2 is the place where the CoCo BASIC IRQ response return was counting screen refresh interrupts (60 times a second).

So I moved everything down to start at $C4, just after the PIA mask variable, and the variable list ended right before $E2. 

So I didn't have to move any variables, and I didn't have to optimize the copy routines to be sensible and use X and Y together instead of X and the SRC and DEST variables in the DP.

And I can type in the test program I've been using:

10 A=0
20 A=A+1
30 ?=A
40 ?=""
50 #=(A<11)*20
60 ?="DONE"

and list it by typing 

0 

and hitting Enter. And I can run it by typing 

#=1 

and hitting Enter.

The source code is at https://osdn.net/users/reiisi/pastebin/8705. Copy or download it from there. 

If you want to assemble it to run in 32K, fix the ORG before COLD. It needs less than 1K of RAM for the code. With another 1K to the end of RAM for the BASIC stack it borrows and as a buffer between BASIC and VTL-2, set the ORG at 2K  before the end of RAM.

You'll need LWTools to assemble it -- either download it or use mercurial or git to clone it, then compile it and copy the executables into your preferred place for user-local executables. 

Then use the following command line or something similar to assemble the source:

lwasm --list --symbols -f decb -o VTL_6809_coco_translit.bin VTL_6809_coco_translit.asm

This will give you a .bin file that XRoar can load from XRoar's File menu. (Sometime I'll put a screenshots here. Until then, refer to the MC-10 post.) 

Run XRoar with the command line with

xroar -machine coco -ram 16k &

for a 16K RAM Color Computer configuration. It will look a lot like what I show of XRoar running the MC-10 emulation in the MC-10 post.

If you load it from the .bin file, you'll need to type

EXEC &h3800

at the BASIC prompt to get VTL-2 running after loading it.

Another way to load it is to convert it to a .cas format and load it from the Tape Control dialog, as I described for the MC-10. You can convert the .bin file to a .cas file with a command like 

bin2cas.pl -o VTL-2.CAS -C -l 0x3800 -e 0x3800 VTL_6809_coco_translit.bin

To get the bin2cas.pl tool, look for it under CAS Tools on the the dragon page on 6809.org: 

https://www.6809.org.uk/dragon/

If you specify the load and execute addresses in the bin2cas command line as above, you'll need to type 

CLOAD
EXEC

at the Color Computer BASIC prompt.

You should be able to modify this source code to run on other 6809 computers with a little thought, especially if you walk through my posts on getting the 6800 and 6801 source running on the MC-10 and on EXORsim.

You should also find some interesting challenges left for the interested reader, such as modifying it to use the Y register in the edit routines, and to work with the DP moved out of BASIC's working area. Such things as that.

I think I'm off to other interesting projects now.

(Maybe. Not sure whether I want to go back and see if I can get fig-Forth running right on the 6809 first or whether I want to try my hand at writing my own VTL source, or a VTL-like language of my own design, etc. Or go back to trying to work on novels for a while. I probably need to get XRoar set up for GNU-debugging before anything else.)

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I have written up a post on VTL expressions, here: https://joels-programming-fun.blogspot.com/2022/09/short-description-vtl-2-expressions-very-tiny-language-p1.html , which will help in testing and otherwise making use of the language. I should also shortly have a post up introducing programming in VTL-2.

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I now have my work on VTL-2 up in a private repository:

https://osdn.net/users/reiisi/pf/nsvtl/wiki/FrontPage

There is a downloadable version of VTL-2 for the Tandy MC-10 (6801) in the stock 4K RAM configuration in there, with source, executable as a .c10 file, and assembly listing for reference. Look for it in the directory mc10:

https://osdn.net/users/reiisi/pf/nsvtl/files/

I haven't wrapped it up for the Color Computer yet, expect that later. But you can see the latest source I'm working on for the Color Computer in the source tree:

https://osdn.net/users/reiisi/pf/nsvtl/scm/tree/master/

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