From: Ron Grunwald <RGRUNWAL AT wasp DOT cs DOT cowan DOT edu DOT au>
Organization: Edith Cowan University
To: djgpp AT sun DOT soe DOT clarkson DOT edu, Elmar Vogt <elmar DOT vogt AT rzmail DOT uni-erlangen DOT de>
Date: Thu, 16 Nov 1995 16:44:24 GMT+800
Subject: Re: A peculiar question: Extra Segments & DJGPP...?
Reply-To: r DOT grunwald AT cowan DOT edu DOT au

Alright, here are my opinions to this.

> The method of choice is then to use interrupt x10, function (dec)19
> - write a string to screen. Still on the right track?
> 
> From C, this is usually done with int86x(19, &register_in,
> &register_out, &segments). Correct?

No. Try

register_in.h.ah=19;
int86x(0x10, &register_in, &register_out, &segments);

> 
> x10 asks for a pointer to the string to be displayed. Fine. Alas,
> the way I understood it, its address is given by a combination of
> Extra Segment ES and Base Pointer BP, where ES is defined in segments,
> and BP in register_in.
> 
> Say, *STRING is the string I want to display, then STRING holds the
> corresponding address- yes? Then, all I have to do is convert that value
> to a long int, divide it by 16 and pass the result to ES, then
> taking the modulo of the long int and passing this to BP. Agreed?

Definitely not. You have to remember that this refers to programming 
under real mode with 16-bit memory segments. Under DJGPP you are 
programming in protected mode, where the concept of segmentation is 
vastly different. The ptr. contained in STRING is a 32-bit linear 
addr. The segment value you're after is actually the Selector 
defining your data segment. Memory segments are defined in Descriptor 
tables, and the Selector is an index to a particular entry in a 
Descriptor table.

> Unfortunately, whenever I try to do so, I get a segment violation upon
> calling the interrupt.

This is not surprising if you applied the algorithm you mentioned 
before.

> Now, where did I go astray?

As for what to do now, it'll be a pretty lengthy task if you want to 
do this from scratch, and I'm not quite sure how to do this yet 
either.
Djgpp comes with a whole bunch of _go32_dpmi_... functions to handle these sorts of 
problems. You'll be much better off solving your problem by looking 
at these first. They're contained in DPMI.H.


Regards, Ron.




 ********************************************************************
 | Author.............. Ron Grunwald                                | 
 | Internet............ r DOT grunwald AT cowan DOT edu DOT au                     |
 | Phone............... (09)273 8027 or (09)273 8468                |
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 | Department.......... Computer Operations and Systems Management  |
 | Division/Faculty.... Information Technology                      | 
 | Institute........... Edith Cowan University, Churchlands         | 
 | Location............ Perth, Western Australia                    |
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 "I don't have any solution but I certainly admire the problem!"