[rescue] Parallel ports [was Re: Slightly OT: ?Bad Cap Saga]

Jonathan C. Patschke jp at celestrion.net
Thu Aug 21 11:40:44 CDT 2008

On Thu, 21 Aug 2008, Geoffrey S. Mendelson wrote:

> On Thu, Aug 21, 2008 at 03:13:48AM -0500, Jonathan C. Patschke wrote:
>> Drivers?  If it's on the southbridge I/O, it's on the LPC bus, which is
>> just a modern version of ISA (the physical interface differs, but not in a
>> way visible to software).  To software, it's at the same old spot it's
>> been since the PC/AT.
> But what actually supports it? As time goes on the old drivers disappear
> and are never replaced. Microsoft did that in mass quantities with XP SP2
> (32 bit), removed more in the 64 bit version and even more in Vista.

It's a parallel port.  It's at offset 0x370 of wherever the ISA I/O space
lies.  As far the software (including the OS) is concerned, it's not on
the other side of a sea of strange I/O chips (as it is in the case of a
USB "printer" port).  There isn't any "support" software necessary.  You
write to that address (with the "OUT" opcode that's ON THE PROCESSOR), and
you get lines wibbling on the other side.

If you can write to that area of I/O space, you have access to the
parallel port.  Who gives a damn if Vista has a driver for it?  Anyone
with the DDK could roll one in less time than it takes to make a good
potroast.  If you're a reasonably fast learner, that includes the time
necessary to download the DDK.

> How long will it be that old operating systems won't boot on the "bare
> metal" and some sort of VM or emulator will be needed?  Once that
> happens things stop working, especially device drivers that need to run
> on the real hardware, that isn't there any more.

If it means getting rid of the "BIOS", that'll be Just Fine.

But if there's still an LPC bus, there will probably be a parallel port on
it, at address 0x370.  If there's no LPC bus, the x86 CPU will still have
an OUT opcode for byte-at-a-time I/O, and those lines will be tied to
something, or they'll be floating, and you can do your I/O by soldering a
shift register, a buffer, and a Google-logoed BPL level-converter onto the
board, and plugging your printer into a ham radio!

> As an example, look at the floppy driver for MacOS X. It never officialy
> existed and a user supported effort died off relatively quickly because
> no new computers were made with them.

I still use floppy disks on OS X 10.5.4 with the drivers built into the
operating system.

Jonathan Patschke | "There is more to life than increasing its speed."
Elgin, TX         |                                   --Mahatma Gandhi
USA               |

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