OSCLI problem in Basic

I’m having a problem with the OSCLI command in BASIC. I’m using it to call an external program; specifically Jeffrey Lee’s wave-file-playing application (from dream2-code.zip). The program is called succssfully, and works as it should, but the BASIC program terminates immediately afterwards. How can this be resolved?

NB I’ve tried smething similar in (Norcroft) C, using the system command, and there the program does retain control.

In a word, you can’t.

In a nutshell, every program believes it is located at address &8000. When you load a program from a program, that program is loaded at &8000 on top of the current program.

The reason system() works is because it copies the original program high up in memory (out of the way) and unlinks the various environment handlers, essentially to disconnect the program from RISC OS. It then calls the OS to run the requested program. Once that has been done, we return to the second half of system() which copies the original program back to &8000 and reinstates all of the various environment handlers and such.

OSCLI, on the other hand, does nothing of the sort. One program overwrites another.

It is possible to do this in BASIC but it requires a wodge of assembler and a knowledge of both BASIC and RISC OS.
Note that because of how BASIC works, you can’t play with PAGE to push the program up as there’s an amount of BASIC workspace that will still need to moved out of the way.

In a word, you can’t.

That’s two words. :-P

It is possible to do this in BASIC but it requires a wodge of assembler and a knowledge of both BASIC and RISC OS.
Note that because of how BASIC works, you can’t play with PAGE to push the program up as there’s an amount of BASIC workspace that will still need to moved out of the way.

Or you could take the infinitely simpler option and call Wimp_Initialise. This will make the Wimp do all the work for you. You don’t even need to call Wimp_Poll at any point – your program can still single-task, you just have to remember to call Wimp_CloseDown at the end.

There are, of course, caveats. You can no longer use command windows or TaskWindows to display output, and you can’t change screen mode, or issue any other VDU commands that RISC OS would normally clean up for you after you ‘press SPACE or click mouse to continue’. Also, you can only call other single-tasking programs this way, otherwise control will pass back to you before the child task has finished.

The latter problem can be worked around by using Wimp_StartTask instead of OSCLI, but then you have to write a poll loop to wait for the task to quit. It all depends on how comfortable you feel about RISC OS programming.

Ah. I had noticed that I could use OSCLI to call a * command (like CAT) without problems, but no doubt * commands are different from ‘ordinary’ programs in this respect. OS commands on UNIX-like systems are just the same as ordinary programs (ownership aside); clearly not on RISC OS.

Judging by the proposed solutions, I think it will be quicker to rewrite the program in question in C :-;

Martin said

You can no longer use command windows or TaskWindows to display output

If you think about it, in a TaskWindow you are already a Wimp task, so you can use Wimp_StartTask anyway. It is even possible to call Wimp_Poll from inside a TaskWindow, as long as you’re really careful about event masking.

Technically you can call Wimp_StartTask from outside the desktop too, but it has unfortunate side effects on the screen (which the Wimp really could have avoided) and won’t take effect until you return to the desktop.

TIL You can call Wimp_Poll from outside the desktop (without calling Wimp_Initialise first). That was sort of unexpected and rather unfortunate.

It’s only sort of unexpected, because technically “outside the desktop” is a Wimp task… but a special one.

Martin also said

Also, you can only call other single-tasking programs this way, otherwise control will pass back to you before the child task has finished.

That in itself isn’t necessarily a problem. Communication protocols are slightly more tricky though.

Having said that, you can send wimp messages that don’t need a reply with no trouble at all.