Re: documentation for fs redirector

Rafał Cygnarowski
2007-11-15T18:52:19Z

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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,