From: NoEmailAds AT execpc DOT com (Chris Giese)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Handle software interrupts with DPMI
Date: Tue, 25 Sep 2001 01:50:24 GMT
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Eli Zaretskii <eliz AT is DOT elta DOT co DOT il> wrote:

>On Mon, 24 Sep 2001, Chris Giese wrote:
>
>> Suppose I want to write a DOS (DPMI) program to run Linux apps.
>> This program must handle the Linux syscalls (INT 80h).
>
>You might wish to think again.  You will need to emulate those system
>calls with the functionality available to DJGPP programs on DOS or
>Windows.  However, some (many?) of the important calls have no
>equivalent, and cannot be emulated.  For example, page-level
>protection, signals, threading, and other important components of a
>Linux executable--all these need system calls you have no hope
>emulating.

Then I will try to write an emulator for my own OS, which is
much more primitive than Linux. I can delete the parts of the
API that I can't emulate, put the tasks in relocatable format
(ld -r --no-undefined ...) so I don't have to worry about
paged memory management, etc.

>> I can write a DJGPP program that issues and handles software
>> interrupts, but how do I get the values of the registers
>> when the INT instruction occured?
>
>The way to do this with DJGPP programs is to install a handler for
>protected-mode software Int 80h.  See the documentation of the library
>function __dpmi_set_protected_mode_interrupt_vector, and the related
>sections of the DPMI Spec.

I read through the DPMI 0.9 spec and a bunch of DJGPP demo code,
but I didn't find what I wanted. Maybe I have a mental block...

Here is a portion of my code:

/* For now, syscall_int() just does a hex dump of the stack.
I want to see 'A', 'B', 'C' etc. on the stack -- these are
the register values at the time of the INT instruction */

static void syscall_int(int the_stack)
{	dump((unsigned char *)&the_stack, 512); }

int main(void)
{	_go32_dpmi_seginfo old_syscall_vector, new_vector;

/* save old vector */
	_go32_dpmi_get_protected_mode_interrupt_vector(SYSCALL_VECT,
		&old_syscall_vector);
/* install new handler */
	new_vector.pm_selector = _my_cs();
	new_vector.pm_offset = (unsigned long)syscall_int;
	_go32_dpmi_allocate_iret_wrapper(&new_vector);
	_go32_dpmi_set_protected_mode_interrupt_vector(SYSCALL_VECT,
		&new_vector);
/* make a system call */
	__asm__ __volatile__(
		"int %0\n"
		:
		: "i"(SYSCALL_VECT),
		"a"(0x41),	/* EAX='A' */
		"b"(0x42),	/* EBX='B' */
		"c"(0x43),	/* ECX='C' */
		"d"(0x44),	/* EDX='D' */
		"S"(0x45),	/* ESI='E' */
		"D"(0x46));	/* EDI='F' */
	...

This code doesn't segfault, and the handler gets called
(I can see the hex dump of the stack), but I don't see
the register values I'm looking for...

>The problem with this is that an interrupt handler might be limited in
>what it can and cannot do, including in the (DOS/Windows) system calls
>it can safely invoke, and the amount of stack it can depend upon.

So, unless I write my own DOS extender, I'm doomed?

-- 
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