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)cds@atelabs.UUCP (05/27/87)
In article <335@cgcha.UUCP> whna@cgcha.UUCP (Heinz Naef) writes: > Device I/O Addr Range IRQ Comments > > COM3 3E8 to 3EF 5 *) steals IRQ and disallows use of LPT2 ^^^^ I'm only posting this because I've seen this mistake in a couple of postings recently and don't want others to act on mis-information. IRQ 5 is NOT dedicated to the printer (at least on an XT). It is dedicated to the fixed disk. The IRQ that is dedicated to the printer is IRQ 7. In truth, I don't think any "standard" printer adapter drivers actually use this interrupt under MS-DOS. This information is taken from the IBM XT Technical Reference. BTW, the fixed disk will stop working if you steal its interrupt. This is from personal experience. (When Microsoft first came out with a bus mouse they suggested you set the bus board jumper to use IRQ 5. Guess what that did to hard disk users.) -- Dave Shanks ..!tektronix!tessi!atelabs!cds AT&E Laboratories cds@atelabs.UUCP 1400 NW Compton Suite 300 (503) 690-2000 Beaverton, OR 97006