🚀 go-pugleaf

In a previous article jhs@hpfinote.HP.COM (John Stanback) writes:
>I'm trying to install a mouse event handler function using TurboC 1.5
>with the medium memory model.  The mouse handler is installed using
>INT 33, function 12.  The references I've seen to doing this use
>an area in the PSP to store the mouse event status and access this
>memory referencing _CS in the mouse handler.  This seems to work in
>the small memory model, but not in the medium.  How can I store the
>mouse event status using the medium memory model????   Also, I would
>like to be able to access the programs global variables from the
>mouse handler.  Is this possible???

Yes, it is possible because I have done it.  You may not like my solution
but it works.  If all you are interested in is working then listen up.

The problem comes from the fact that the _PSP doesn't seem to work right
in the medium and large models.  And, when referencing _CS, you are in
a different segment than in the small model.  The example programs that
came with the mouse for TurboC *CHEAT* and use small model.  Anyone could
do it in small model, sheesh!  The main program can setup the buffer area
fine in the end half of the PSP area.  The interrupt handler cannot find
it because it uses a different code and data segment.

So, there must be someway to get it to use the same area.  Well, this is
exactly why you were using the PSP area in the first place.  Viscous
circle right?  Global variables won't cut it because you get a different
_DS and _CS when the interrupt handler is called.  It can't find your
global variables.

What I decided to do was use some globally known memory locations.  Since
8088 doesn't have a "zero page" like on 6502 machines, the closest thing
was the Interrupt Vectors.  These are modifiable and they come in groups
of 4 bytes.  The easiest one to pick is the "switch to Basic" interrupt.
Who needs it anyways, especially if you are in TurboC anyways!  heh heh.
Ok, so I forget which it was, but assume it is 0000:0064 for now.  Before
installing the handler, make a far pointer to it  MK_FP (0x0000, 0x0064)
and then stuff into it the address of the PSP, or the _CS for the _MAIN.

Also, in the interrupt handler, make a far pointer to it and extract
the value from there.  Then, make another far pointer out of THAT value
and you have the address you want.  You can either save the segment or
both the seg & offset.  Just be consistent.  Be careful about what
interrupt vector you steal because some of them are important.  Others
are not so important like the Basic interrupt.


--
Jeff Capehart 		Internet: jdc@beach.cis.ufl.edu
University of Florida	UUCP:   ..!uflorida!beach.cis.ufl.edu!jdc