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I have tried *everything* in my power to get a *simple* little program to run and then have the
timer exit the routine back to the main routine. Okay, that doesn't make much sence so here is
a better explanation...
-save old INT 1CH pointer from IVT
-set INT 1CH to my handler
-jump to Start
-Start: show '!' over and over... Jump back to Start (endless loop).
-My INT 1CH handler will exit the Start: routine and go back to the main code and exit to DOS.
Okay, I know this program sounds silly but this is just an example of something "bigger" im trying
to make but my example below shows the "smaller" problem im having.
#The problem:#- The first and second time I run my program it works great. Once the timer ticks it
exits to DOS with no problem. The third time I run it, it exits to DOS and then stalls. WHY?
First thing I checked was the stack but I can't seem to find a problem there. I *do* have code
in the example to clean up the 8254(8253) timer state using MOV AL,20H : OUT 20H, AL but
that doesn't seem to do the trick either (it does help though).

Here is the assembly (NASM) code. I have only tested it under DOSEMU but I'm sure it fails
under Bochs and real DOS also. (I'll test ASAP).

              ORG       100H
              BITS      16

jmp Init ;goto startup

Done: ;comes here when timer "ticks"
        mov     ah, 0Eh
        mov     al, 'D'
        mov     bx, 0007h
        int     10h ;Print a 'D' for Done (for debug)
        int     20h ;Exit to DOS, 4C00H I21 works also


Init: ;Starts here...
              XOR       AX, AX
              MOV       ES, AX ;IVT start
              MOV       BX, [ES:1CH * 4 + 0]
              MOV       [OldINT1C + 0], BX ;Save old INT 1C
              MOV       BX, [ES:1CH * 4 + 2]
              MOV       [OldINT1C + 2], BX ;...
              MOV       [ES:1CH * 4 + 0], WORD NewINT1C
              MOV       [ES:1CH * 4 + 2], CS
              JMP       Start ;Our endless loop code...

OldINT1C:     DW        0
              DW        0

NewINT1C:

              XOR       AX, AX
              MOV       ES, AX

              MOV       BX, [CS:OldINT1C + 0]
              MOV       [ES:1CH * 4 + 0], BX
              MOV       BX, [CS:OldINT1C + 2]
              MOV       [ES:1CH * 4 + 2], BX ;Resume old original INT 1C handler

              SUB       SP, 4 ;Pop IP and CS, we don't need them here..
              PUSH      CS ;Put our CS on stack
              PUSH      Done ;Put our IP on stack

              MOV       AL, 20H
              OUT       20H, AL ;INT done
              
              IRET ;Return to Done:

Start: ;endless code , only exits when INT 1C wants us to.
        mov     ah, 0Eh
        mov     al, '!'
        mov     bx, 0007h
        int     10h ;Teletype '!' over and over....
        jmp     Start





THANKS. I need the help. I've been trying to get this routine to work for days now.
Once I figure it out I can't wait to continue programming my project.
TTTHHHAAANNNKKKSSS.

Well it's prety obvios.
First:When you set the interrupt vector, you should turn off interrupts or use a special BIOS function.
Second:I realy don't like this "Pop IP and CS, we don't need them here..". I would sugest to use a spin lock, in the loop where you type "!". And instead of changing CS and IP(int10 is probobly not threadable, so once you interrupt it's execution, you will stall when a char("D") is about to be printed), you would clear the value which is checked in the spin lock.
Anton