bluelbo@ucscb.ucsc.edu (Blue Elbow) (08/17/90)
Topic: Turbo Pascal 5.5 Overlays AND
Turbo C++ 1.0 Overlays
---at the same time!----
I am writing a word processor in Pascal, and have a spell
checker written in C. The two are successfully integrated
but not with overlays.
Problem: I don't use the stacks properly so the Pascal
overlay manager crashes while 'walking' the stack.
Solution: Use the stack properly. But to do this, I need
to know EXACTLY what the overlay manager expects to see on
the stack.
My view:
Stack (size is 16K bytes)
SS:4000 Bottom of Stack ------------
3ffe 0000 bottom
3ffc 0000 ------------
3ffa -BP- original BP
3ff8 -pushed call proc_1
3xxx parameters-
3xxx -return (program)
3xxx address- ------------
3xxx 3ff8 (BP) previous BP
xxxx -local
xxxx variables-
xxxx -pushed call proc_2
xxxx parameters-
xxxx -return (proc_1)
xxxx address- -----------
xxxx -BP- previous BP
xxxx etc
OK, the BP stack frame is a _MUST_ for the overlay manager.
Fine. But the spell checker is an external program
(CSPELL.EXE) which requires a DOS.Exec() call. Once the
exec gives control to CSPELL, Pascal is dormant and C has
its own seperate stack. Now the trouble becomes obvious.
The word processor must regain control for displaying prompt
windows, updating the screen, submitting words for spell
verification, etc. Pascal must regain control of its stack
and put it in a form recognizable by its overlay manager.
xxxx -return (my.spellchk)
xxxx address- -------------
xxxx -BP- previous BP
xxxx -EXEC call DOS
xxxx parameters-
xxxx -return (my.invoke)
xxxx address- -------------
xxxx -BP- previous BP
xxxx -EXEC local
xxxx variables-
xxxx -FLAGS- int 21h
xxxx -return (DOS.Exec)
xxxx address- ------------
xxxx ???????????
Does int 21h use any of my local Pascal stack? Or if I
figure out where the next free space in the stack is, can
I just use it? Will the overlay manager look at the
-return address- to DOS.Exec and know what it is? How can
I verify what I am doing is correct?
Side notes for those wondering: The C program creates its
own stack, much like the pascal code has its own.
Communication between the two is initiated by a parameter
on the command tail (line) passed to CSPELL.EXE, for
example CSPELL 1234ABCD87 which says that a data structure
is located at physical memory 1234:ABCD, with checksum 8
and not checksum 7. Once the address is verified with the
checksum, CSPELL's main() sets a struct pointer to that
address, stores where the C stack, BP, SP, and pointers
to important C spell functions are, and does a far call to
the Pascal code via an address (supplied by the Pascal code)
in the structure. The pascal code's job is now the user
interface of the spell checking, and does far calls back to
the C code for spelling verification, correction, and
additions to a personal dictionary. Once completed, the
Pascal does a far return (RETF, opposite of far call) to
return to the C code, which terminates the spell checker
and then main() does a return(0) which ends the exec() call.
Thus the C program has terminated and only the Pascal code
is in memory. (I have taken care of heap problems in Pascal
by moving down the free list and limiting the heap to 8K...
hopefully not that much required by spelling corrections,
etc.)
Now if that didn't confuse you completely, and you have a
useful, intelligent answer, or worthwhile input, please
respond. I am requesting this help of utmost urgency.
Thanks!
- Dave (Blue Elbow)
--
David R. Van Wagner bluelbo@ucscb.ucsc.edu
P.O. Box 211 ...!ucbvax!ucscc!ucscb!bluelbo
Mount Hermon, CA 95041 work: (408) 459-8811
USA (408) 335-7578 FAX: (408) 459-8853