whna@cgcha.UUCP (Heinz Naef) (05/22/87)
In DOS 3.3, four asynchronous ports (COM1 to COM4) seem to be supported.
The only problem is, that COM3 and COM4 cannot be accessed if DOS 3.3 is
running on an IBM XT or AT. The reason seems to be the parameter checking
done in the prologue of the BIOS routine RS232_IO.
There are two (may be more) principal ways to get this problem solved:
(1) Get an upgraded version of the ROM BIOS that corresponds to the
facilities of the new DOS version.
--> Does anyone know about a new ROM BIOS chip for the IBM XT or AT?
(2) Develop a TSR (terminate and stay resident) program which will
logically replace the BIOS routine RS232_IO and will allow to
refer to COM1 to COM4, as follows:
Device I/O Addr Range IRQ Comments
COM1 3F8 to 3FF 4 as implemented in ROM BIOS
COM2 2F8 to 2FF 3 as implemented in ROM BIOS
COM3 3E8 to 3EF 5 *) steals IRQ and disallows use of LPT2
COM4 2E8 to 2EF 2 *) IRQ 2 vector is classified as RESERVED
*) the IRQ numbers are based on third party expansion hardware available
for XT and AT systems.
--> Did anyone already try to support at least COM3 in this way?
Any comments are highly appreciated.
Regards,
Heinz Naef (whna@cgcha.UUCP)