Mail Archives: opendos/2000/11/22/11:03:14
On Thu, 16 Nov 2000 Patrick Moran wrote:
> As far as I know, you can only have 3 printer ports and 4 comm
> ports with DRDOS without special drivers for more. These drivers
> generally come with multi serial port cards. I suppose the same
> would be true for parallel cards. But with all the stuff already
> loaded into memory for a server, who needs more drivers!:-)
While this is true in general, Multiuser DOS as well as
DR-OpenDOS 7.02 & DR-DOS 7.02 make an exception of the rule:
Multiuser DOS usually provides an LPT4: device similar to
the LPT1:..LPT3: devices and also allows to redefine the
default device for PRN: (which is usually the same as LPT1:).
It also provides AUX1:..AUX8: (or more?) devices for serial
ports.
DR-OpenDOS 7.02 introduced the LPT4: device as well, but this
was unfortunately removed with DR-DOS 7.03, since reportedly it
caused compatibility problems with EGA cards. I doubt, removing
LPT4: could have fixed any problems with EGA cards, but if, the
problem for sure was with the EGA card, not with the implementation
of the LPT4 device:
LPT4: worked similar to LPT1:..LPT3:, that is, IBMBIO.COM would
pass the request down to the ROM-BIOS. (It might be interesting to
know, that IBMBIO's LPT4: support did not cost a single byte of
resident memory, while it would, of course, cost a few hundred bytes
for an external driver to implement all the necessary device driver
functions.) If the ROM-BIOS would know how to handle a fourth
printer, it would succeed. If not, you would get an error message
as if for a non-existing printer port or off-line printer
(depending on the ROM-BIOS). IBMBIO.COM does not directly talk
to the hardware.
Well, fact is, most ROM-BIOSes do NOT know how to handle a
fourth printer, because the ROM-BIOS usually looks up the
corresponding I/O base addresses at the start of segment 40h.
With PCs and PC/XTs there have been 4 WORDs to hold these
addresses such as 3BCh, 378h, and 278h (there is no standard-
address for a fourth printer, but some add-on cards can be
jumpered to 270h - just to give you an example). The BIOS Data
Area also holds a variable named "BIOS Equipment" which contains
bit flags indicating the *count* of serial and parallel ports and
other devices. However, the problem is that many ROM-BIOSes store
wrong information in there, as IBM's original definition of
the flags was quite confusing (see Ralf Brown's Interrupt List
INTER61 MEMORY.LST for details)...
With the introduction of the PC/AT the entry for the fourth
printer was reserved for other purposes (and usually is set to 0).
PS/2 machines used it to store a pointer to the Extended BIOS
Data Area (XBDA), this use has been adapted by most other current
machines since a few years.
Alright, since the ROM-BIOS does not know of any parallel ports
on non-standard addresses (such as 270h) it will not find any
additional ports during its initial scan for them.
But under DR-DOS 7.02 you can still add support for a fourth
printer by loading a tiny TSR, that extends the ROM-BIOS INT 17h
and redirects output to port 4 to an I/O address internally known
to that driver (if the segment 40h entry for the 4th parallel
port is not used for other purposes (that is, zero), you could
store the I/O address there, so other programs have a chance to
get the address, too). You do not need to implement the whole LPT4:
device as this is provided by IBMBIO.COM itself.
If your ROM-BIOS does not discard requests for port 4 and
attempts to look up the I/O address in the traditional entry
in the segment 40h, you do not even need to load such a TSR, all
you need is to use the CONFIG.SYS LPT4= directive provided with
DR-OpenDOS 7.02+. The syntax is:
LPTn=[x][,t] or LPT4=x[,t]
whereby n=1..3 logical port number
x=1..4 logical port number
| 200..3Fx immediate port address (in hex)
t=timeout value in fractions of a second (right now, I'm
not sure about the unit, but it could well be 1/18 sec.)
(The LPT4= directive is a special case, as you cannot give the
timeout value without also giving the x value for security
reasons. Otherwise it works identical to LPT1=..LPT3=.)
So, in above example, inserting:
LPT4=270,10
in CONFIG.SYS would be sufficient to fully enable the LPT4: device
in DR-DOS 7.02. If you give invalid I/O addresses or the corresponding
entries in the BIOS Data Area are already in use for other purposes,
you will receive an error message.
These directives can also be used to chance the logical order or
assign other physical ports than those found by the ROM-BIOS. The
ROM-BIOS will continue to use the chanced entries, e.g.
LPT1=378
LPT2=278
LPT3=3BC
or, in this case (1=3BCh, 2=378h, 3=278h), just
LPT1=2 (1=378h, 2=378h, 3=278h)
LPT2=3 (1=378h, 2=278h, 3=278h)
LPT3=3BC (1=378h, 2=278h, 3=3BCh)
if you want to swap some of your ports.
Please note, that while the LPT4: device itself was removed from
DR-DOS 7.03, the LPTn= and LPT4= directives are not. Even LPT4=
is still functional, but you will need to provide the LPT4: device
by a device driver (DEVICE=).
(Future issues of DR-DOS may possibly reintroduce the LPT4: device,
but then it will only be linked into the visible device chain, when
the CONFIG.SYS LPT4= directive was also used. Otherwise the LPT4:
will be non-visible to external components (that is, not show up
with MEM /A). This would fix any possible compatibility problems
while still providing the maximum possible support for any count
of parallel ports used in a system.)
Similar directives named COMn=[x][,t] exist for serial ports:
They are useful in particular if your old ROM-BIOS only finds
COM1: and COM2:, while you have more serial ports installed.
Or, on modern machines with accellerated SVGA cards, the
previous standard-address for COM4 (2E8h) is conflicting with
the video card hardware, and the ROM-BIOS will still not find
any serial ports on non-standard addresses (that is, not 3F8h,
2F8h, 3E8h, 2E8h), so you can use COM4= to point the BIOS
to that I/O address... (for example COM4=260)
Furthermore, you can change the default assignment of PRN: (to
LPT1:) and AUX: (to COM1:) by using the PRN= and AUX= directives.
Their syntax is:
PRN=0 | 1..3 | 4
AUX=0 | 1..4
The default when not using these directives is 0, which enables
a "high-speed" bypass of the redirection code (saving a few clock
cycles). If your default printer is on LPT3:, but you do not
want to change the logical order of ports for some reason,
you could still use
PRN=3
to redirect your default printing to LPT3: instead of LPT1:
(Please note, that due to the ill-advised removal of the LPT4:
device, you must not use the PRN=4 value with DR-DOS 7.03+.
This may hang the machine.)
For further info on these (and other undocumented directives
please have a look at the README.TXT file in the IBMBIOA3.ZIP
archive available from the "OpenDOS" pages on my web-site.)
Hope it helps,
Matthias
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Matthias Paul, Ubierstrasse 28, D-50321 Bruehl, Germany
eMail: <Matthias DOT Paul AT post DOT rwth-aachen DOT de>
Web : http://www.rhrz.uni-bonn.de/~uzs180/mpdokeng.html
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