IO

Pausing I/O on the ISA bus

(from Linux Device Drivers http://www.xml.com/ldd/chapter/book/ch08.html)

Some platforms -- most notably the i386 -- can have problems when the processor tries to transfer data too quickly to or from the bus. The problems can arise because the processor is overclocked with respect to the ISA bus, and can show up when the device board is too slow. The solution is to insert a small delay after each I/O instruction if another such instruction follows. If your device misses some data, or if you fear it might miss some, you can use pausing functions in place of the normal ones. The pausing functions are exactly like those listed previously, but their names end in _p; they are called inb_p, outb_p, and so on. The functions are defined for most supported architectures, although they often expand to the same code as nonpausing I/O, because there is no need for the extra pause if the architecture runs with a nonobsolete peripheral bus.

PCI

There are also some more general docs on linux and pci programming:

• [Linux Device Drivers, 2nd Edition] [pdf]
  • [Chapter 15: Overview of Peripheral Buses]
• [The Linux Kernel API: PCI Support Library]

A: yes, have a look at:

 https://mail.rtai.org/pipermail/rtai/2003-October/005352.html

Parallel port

http://www.beyondlogic.org/spp/parallel.htm

http://people.redhat.com/twaugh/parport/html/parportguide.html

From the mailinglist:

outb(value,address)

Standard parport (SPP) registers are at three consecutive
addresses:

base+0 = data register (read/write)
base+1 = status register (read only)
base+2 = control port (read/write)

legacy ports have the base at 0x378 (LPT1) and 0x278 (LPT2).

When the port is in output mode, a 1 in the data register
will send that data line to ~3V, a 0 to about 0V.

Your LED could be wired such that it goes on when the line
goes to 0.

You can (on PS/2 or EPP ports) switch the data lines into
input mode by setting bit 5 in the control register. The
input lines go high-impedance and their state will be 
mirrored when reading the data register.

Here is an excellent paper about driving the parallel port:

http://www.beyondlogic.org/spp/parallel.htm

The examples are meant for DOS, so you need to translate:

inportb(address)         is inb(address)
outportb(address,data)   is outb(data,address)