DOS ain't dead - ctty/dup2 in DOS4

ctty/dup2 in DOS4 (Developers)

posted by ecm , Düsseldorf, Germany, 26.12.2025, 21:39

> In the DOS4 source code, I can see:
>
> cmd\command\tcmd2b.asm
>
> ICLLOOP: ; Close basic> handles > MOV AH,CLOSE > INT int_command > INC BX > LOOP ICLLOOP > POP BX ; Get handle > MOV AH,XDUP > INT int_command ; Dup it to 0 > MOV AH,XDUP > INT int_command ; Dup to 1 > MOV AH,XDUP > INT int_command ; Dup to 2 > MOV AH,CLOSE ; Close initial handle > INT int_command
>
> Now according to RBIL:
>
> https://www.ctyme.com/intr/rb-2931.htm
>
> BX should have the file handle. But I see 3 executions of Int int_command,
> and I don't see BX changing from 0, to 1, to 2, between executions.
>
> Any idea how this works?
>
> Note - I'm trying to get "ctty com1:" to work on PDOS-generic, and I
> don't really have the concept yet.
>
> Thanks. Paul.

The bx input is the same for all three, the device opened earlier. Before using the duplicate handle calls, the first three handles are closed so that the output handles are 0, 1, 2. Then the new handle is closed.

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l

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Complete thread:

ctty/dup2 in DOS4 - kerravon, 26.12.2025, 20:08 (Developers)
- ctty/dup2 in DOS4 - ecm, 26.12.2025, 21:39