Re: documentation for fs redirector

["Rafał Cygnarowski" <zswi@pers.pl>]

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Hi!

Dnia czwartek, 15 listopada 2007, Mike McCarty napisał:
> Without actually seeing the defective code from DOS and your
> attempted workaround, it's impossible to tell. I am an old hand
> at MSDOS and assembler, though not much with the undocumented
> I/Fs. However, if you want you can shoot me some code and I'll
> have a look. Perhaps we can work this out via e-mail.
>
> A TSR may not be able to do what you want, unless it is rather
> "smart". CX may be clobbered so far up that there's no way
> for you to fix it, except in a DOS version specific manner,
> if at all. One possibility is to patch DOS at install. IOW,
> find the defective code, and patch over it, possibly jumping
> out into your own code in a couple of places. It may not be
> possible to fix this w/o clobbering something else that DOS needs,
> so the patches might be significant. IOW, you might need to
> trap multiple interrupt vectors and save information for more
> than one level of DOS, along with flags indicating what was
> taking place at the time CX got clobbered, and restore the
> values the various levels of DOS need.

This is fragment of dosemu code called after int2f AX=112E (just before 
quitting):

if (LOW(state->eax) == MULTIPURPOSE_OPEN /* 2E */) {
	u_char *stack_ch, *stack_cl;

	// original CX value when int21 ah=6c00 is called
        u_short in2e_fileattr = sda_ext_attr(sda); 

	// normally this should be enought
	SETWORD(&(state->ecx), out2e_result);

	// but I try to modify pushed CX on stack
	stack_ch = (u_char *) (Addr(state, ss, esp) - (784 + 6));
	stack_cl = (u_char *) (Addr(state, ss, esp) - (784 + 7));
	if (
        	((in2e_fileattr & 0x00ff) != *stack_cl) &&
		((in2e_fileattr & 0xff00) != *stack_ch)
	) {
                // sanity check
		error("Wrong stack offset for MULTIPURPOSE_OPEN!"			
			"Expected value: 0x%04x. Value on stack 0x%02x%02x\n",
                        in2e_fileattr, *stack_ch, *stack_cl);
		leavedos(1);
	}

        *stack_ch = (out2e_result & 0xFF00) >> 16;
        *stack_cl = out2e_result & 0x00FF;
}

I use this program to test if CX was set correctly:

#include <stdio.h>
#include <io.h>
#include <dos.h>
#include <conio.h>
#include <errno.h>

#define u_short unsigned int
#define u_char unsigned char

int open2e(u_short openmode, u_short fileattr, u_short fci, char *filename, 
int *respond)
{
	int __fd = -1, res;

	asm {
		mov ax, 0x6c00
		mov bx, openmode
		mov cx, fileattr
		mov dx, fci
		mov si, filename
		int 0x21
		jc blad
	}

	asm	mov res, cx;
	asm	mov __fd, ax;
	*respond = res;
	return __fd;

blad:
	asm	mov errno, ax;

	return -1;
}

int main(void)
{
	int fd, res;
	char *filename = "F:\\TEST.TXT";

	clrscr();

	fd = open2e(0x0042, 0x002a, 0x0001, filename, &res);

	if (fd == -1) {
		perror("open2e");
	} else {
		printf("2e respond: 0x%04x\n", res);
		printf("fd == %d\n", fd);
		write(fd, "write test", 11);
		close(fd);
	}

	getch();

	return 0;
}

If you need more do not hesitate to ask.

Best regards,
-- 
Rafał Cygnarowski
rafi@pers.pl

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