Category : Alternate Operating Systems - Quarterdeck DesqView, CP/M, etc
Archive   : QWHITE13.ZIP
Filename : COM3N4.TEC

Output of file : COM3N4.TEC contained in archive : QWHITE13.ZIP

ID:CM COM Ports and DESQview
Quarterdeck Technical Note #145 Filename: COM3N4.TEC
by Stan Young CompuServe: COM3N4.TEC
Last revised: 2/10/92 Category: DV

Subject: How DESQview handles COM3 and COM4. Explains why a single IRQ can't
be shared by multiple COM ports. Suggests IRQs that can be used.
Mentions FOSSIL drivers as a possible solution to the "COM port

Q: Why doesn't DESQview support more than COM1 and COM2?

Q: How can I run more COM ports on my machine in DESQview?

In DESQview's Change a Program menus, DESQview allows you to limit the
COM ports that the BIOS reports to be available in the window. The choices
are 1, 2, Y and N. This does not mean, however, that DESQview does not
support the use of COM3 and COM4: It does.
DESQview allows you to specify whether the BIOS will report that only
COM1 or COM2 exists so that it can attempt to make the system appear to have
only the port (1 or 2) that you have specified. This is done so that ill-
behaved programs that clear both ports on startup, making the assumption that
they are the only program running on the machine, will not interfere with a
program that is already using the other port. These programs do not typically
interfere with COM3 or COM4, so if you are using one of these ports, you can
set "Uses serial ports" to either N or Y. It makes no difference.
All DESQview does is clear the bytes in the BIOS Data area where the BIOS
stores I/O addresses of the serial ports. These bytes are 40:0 and 40:1 for
COM1, 40:2 and 40:3 for COM2, 40:4 and 40:5 for COM3, and 40:6 and 40:7 for
COM4. You may view the contents of this portion of memory in Manifest's First
Meg/BIOS Data screen. When "Uses serial ports" is set to N, 40:0-40:3 appear
to contain 0 in this window; when "Uses serial ports" is set to 1, 40:2 and
40:3 (where the I/O port address of COM2 resides) appear to contain 0 in this
window; when "Uses serial ports" is set to 2, 40:0 and 40:1 (where the I/O
port address of COM1 resides) appear to contain 0 in this window. DESQview
never changes the apparent contents of 40:4-40:7 (where the I/O addresses of
COM3 and COM4 reside.) Programs that do not rely on the BIOS reporting the
addresses of the serial ports (and many do not) will not be "fooled" by this
strategem of DESQview, and will do what they will with the serial ports. If a
program in one window interferes with the use of another serial port on a
separate IRQ (see below) in another window, and trying to segregate the serial
ports to their separate windows with the "Uses serial ports" feature of
DESQview does not solve the problem, then you should inform the makers of the
culprit program.


However, there is a key issue that is important to understand when trying
to use ports beyond 2 in a multitasking system. There are two parts to serial

1. The software side, which is the BIOS COM port identification.
2. The hardware side, which is the hardware interrupt (IRQ) that is
being used.

A good analogy for understanding the importance of this is to think of
serial port communications as a telephone system. The COM port represents the
actual telephone unit (the extension, if you will). The hardware interrupt, or
IRQ, represents the line out that connects to the phone company. COM1 and
COM2 have their own dedicated lines (IRQ4 and IRQ3, respectively). This is
the standard configuration in the PC environment, though not absolutely
When you add a COM3 or COM4, they must be assigned an IRQ. You have two
options: You can assign them their own separate IRQ, or you can assign them
to one of the IRQs that are already in use (3 or 4). However, if you assign a
new COM port to an IRQ that is already assigned to another port, it is like
adding another telephone extension to an existing line. You will be able to
call out from either extension, but you will not be able to call out from both
extensions at the same time -- for that, you need a dedicated line.
Most of the hardware devices (modems, mice, plotters, scanners or
whatever), that allow you to define them as a port above 2 unfortunately do so
by reusing IRQ3 or IRQ4. This works well when using the ports from free-
standing programs, one at a time, but in multitasking systems these setups are
generally unsatisfactory, as users of multitasking systems want to be able to
use the ports simultaneously.
If the hardware that supplies your additional COM ports allows you to
configure the port to a IRQ, other than IRQ3 or IRQ4, that is free on your
machine -- say IRQ2 or IRQ5 -- then that situation would be like a phone with
another line out, and you would be able to use it simultaneously with COM1 and
COM2. Otherwise, you are limited to using these ports serially, not


There are some systems that are now accessing multiple ports through the
use of FOSSIL drivers and special multi-port hardware (sometimes with its own
on-board processor). A full discussion of FOSSIL drivers is beyond the scope
of this technical note, but, briefly, this is how they work:

A driver is loaded in the CONFIG.SYS file which sets up
communications with the hardware supplying the ports. The hardware
may contain 4, 8, or even more ports. The hardware usually uses
ONLY one standard hardware IRQ, usually IRQ3 (used by COM2 in
standard configurations). The driver is then addressed by the
communications software (which must know how to address the FOSSIL
driver) as though it were a series of COM ports (again, 4, 8, or
more). When communications to one of the ports is received, the
driver multiplexes through the single IRQ to communicate with the
board which, since it knows how to talk to the driver, can
determine to which physical port on the board the data is to be

We have a number of users who use systems with FOSSIL drivers to run
multiple ports on electronic bulletin board systems and for CAM process
control. To implement a multi-port system using FOSSIL drivers, however, you
need three things: The hardware, the FOSSIL driver, and communications
programs that are specifically written to use the FOSSIL driver.
For more information about FOSSIL drivers, obtain a file called
FOSSIL.ZIP, available on many bulletin board systems around the country.

*This technical note may be copied and distributed freely as long as it*
*is distributed in its entirety and it is not distributed for profit. *
* Copyright (C) 1990-2 by Quarterdeck Office Systems *
************************ E N D O F F I L E *************************