Re: Redirecting a DOS interrupt call out to Linux

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19.07.2013 19:58, Andrew Bird пишет:
On Thursday 18 July 2013 23:49:41 Stas Sergeev wrote:
18.07.2013 20:35, Andrew Bird (Sphere Systems) пишет:
Hi all,

	I've been playing with the Dosemu source code with the hope of replacing

an old TSR with a native Linux daemon. Currently a DOS app I have no
for makes an interrupt call to a TSR with data, the TSR works with it and
the result is returned.

I had in mind to replace it with something like this:
1/ Old DOS app makes call to interrupt
2/ New Dosemu interrupt handler that collects the data given to it by the
DOS app and pushes it into a message queue on the host.
3/ A Linux daemon that reads the incoming message, does the processing and
returns the result via a second message queue.
4/ The new Dosemu interrupt handler then reads the result from the second
message queue, places it in memory and returns.
5/ The DOS app sees the result and is unaware of any difference.

My first step was to hook in a new skeleton Dosemu interrupt handler, but
it doesn't seem to be being called and the DOS app thinks the TSR is not
installed. I'm using Dosemu 1.2.2 for historical reasons, but I expect
the problem is similar on 1.4.0 etc.

I figured all I'd need to do would be to add a new function to
src/base/async/int.c and reference it.

/* this is the handler for INT 76 calls from DOS to the test interface */
int test_int (void) {

	ds_printf("INT76: we have been called\n");
	return 1;


/* add reference to it in setup_interrupts() */
interrupt_function[0x76] = test_int;

Can anybody tell me what I'm missing? How does the interrupt vector table
get built?
Firstly, the interrupt handling is much different in 1.2
and 1.4. Secondly, you need to make sure the interrupt
vector points to BIOSSEG:INT_OFF(i) (see bios_setup() in setup.c)
and that no one have hooked it from DOS (or, at least, chains
it back to the original address).
Alternatively you can make dosemu to try and intercept the int0xNN
instruction. In this case you won't need to worry if someone have
hooked that vector under DOS. For that you will need to modify
the can_revector() function.
Note that this all applies to 1.4. I have no idea how 1.2 worked,
this was too long ago.
Additionally, curent git allows you to sleep inside the interrupt
handler of dosemu, while the DOS is still running. This will likely
allow you to do the thing within one interrupt call, instead of 2.
Hi Stas,
	Thanks for the help. For the time being I've switched to latest git
master whilst I experiment. It seems the reason the handler wasn't being
called initially was that the DOS program was checking to see the memory
location matched what it expected in the Interrupt table. If it wasn't correct
then it just bailed out. Swapping the SETIVEC to match that address cured the
problem for the DOS programs, but of course I need my interrupt routine to
live there.
THat's why I suggested you to set it to BIOSSEG:INT_OFF(i).
This is exactly a hlt address, which will then call the handler
from interrupt_function[i][NO_REVECT].

  So it seems I can't use the normal Dosemu mechanisms for adding
I don't think this is the right conclusion.

  I don't want to code my mods in assembler, as you can imagine.
I noticed that the int33 mouse was doing something similar, so I added a
little code to the bios.S assembler file, to just add a 'hlt' at the correct
address. I've then modified the hlt handler to call my function just like
int33_post(). Inside there I have a di_printf() and a fake_iret(), and it's
being called fine now, I just have to add the function meat.

	Are there any downsides to this approach?
Aside from the redundant work you did - none.
Please do

git checkout devel

to get even more recent dosemu, and you'll find that even
int33 is no longer doing what you describe.
There is already a hlt at BIOSSEG:INT_OFF(i) which should
do what you need, and your interrupt vector was probably
initialized to NULL instead, so the DOS prog didn't want to
use it.
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