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Re: [PATCH] Cygwin: Provide more COM devices
Brian Inglis writes:
> On 2019-12-14 11:38, Achim Gratz wrote:
>
> [Sorry, thought I'd sent this, it was backgrounded!]
>
> What are the distinctions between /dev/sd[a-c][a-z], /dev/sdd[a-z], and
> /dev/sd[a-z] appearing in parts of devices.cc?
/dev/sd is the base name for disks, letters denote distinct units (a..dx
for a total of 128 units) and numbers the partitions (starting from 1,
with no number addressing the whole device), just like Linux.
The first 16 partitions of each drive have a fixed device major number
(out of 8) and mangle the drive / partition into the lower nibbles of
the minor. Any drive with higher partition numbers gets mapped into 25
higher device numbers where the bits for the unit number spread across
major/minor and the remaining 48 partitions make up the lower part of
the minor. I'd rather had kept the original numbering scheme and packed
each set of 16 extra partitions into one more major number for a total
of 24 extra, but that's what we have now.
> There are 127 each cons,nst,pty,ptym,st,ttyS entries allocated for potential
> devices, which will not exist on most systems.
You'll proably won't find a system with so many tapes, not. But the way
Windows deals with USB devices means that you can easily have several
dozen of (virtual) COM ports, since it creates a distinct number for
each device / USB port combination you've ever used. The limit is
probably even higher than the 128 ports Cygwin tries to support.
> Note that GPT supports 128 partitions per device.
But Cygwin only supports 64.
> Are there systems using more than 32 of any supported device?
I have systems that are way past COM64 and I seem to remember having
seen one with a COM port past 127.
> Are there documented Windows I/O device addressing limits?
You could have 256 since Win98 (not for DOS applications, though), but
that IIRC was always a limitation of the control panel, not the
underlying platform. Not sure if Win10 changed something in that
regard, but probably not.
Regards,
Achim.
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