>Back in the day, the parallel port was awesome for hobbyist hardware hacking.
Also the MIDI-game port which was basically an ADC for the PC, meaning you could build your own gaming racing steering wheel using a volume knob screwed in the middle of a CD cake case and pedals using volume wipers, all connected to the MIDI port with no extra drivers needed. Good times.
The game port was amazing. You got 4 ADCs (for the X axis and Y axis of a joystick, 2 joysticks supported) and you got 4 GPIO bins for the "buttons" (2 buttons per joystick, again 2 joysticks supported with 1 port).
If you didn't care about burning CPU, you could bit bang these "button" inputs and interface simple home-brew electronics. The clock line attaches to one "button" pin, the data line attaches to another "button" pin. When button 1 is "pressed" sample the state of button 2! Now you are reading a data stream.
I used this trick to interface magstripe readers directly to computers back in the early 2000s, and even wrote an article for the first issue of O'Reilly's Make Magazine about it. While professional readers/writers were hard to get, cost $100+, interfaced to the parallel or serial port, and used proprietary software, this let me do it for around ~$20 in parts and use my own software. I had quite a lot of fun learning what was stored on the various tracks of the cards I had, like my student id.
ISA cards, like the soundblaster had the MIDI and joystick on the same connector. The joystick had up to 4 analog inputs, maybe that's what's meant here.
The PC game port is a nearest thing to intentional GPIO that PC has. Parallel port is almost always better for digital GPIOs, but it was not designed that way and the fact that it can be used like that (and extended several times into what in the end was SCSI-like protocol) is kind of an coincidence.
> The PC game port is a nearest thing to intentional GPIO that PC has.
Er, what? Not every IBM PC (clone) had a (sound) card with joystick ports, but almost every PC clone had (ISA, later PCI) extension slots. There were/are plenty of digital i/o adapter cards for those available, from plain, cheap 8255 based ones to those with slave CPUs (potentially more powerful than the host's CPU).
But then, perhaps I don't understand what you mean by 'intentional' GPIO.
Also the MIDI-game port which was basically an ADC for the PC, meaning you could build your own gaming racing steering wheel using a volume knob screwed in the middle of a CD cake case and pedals using volume wipers, all connected to the MIDI port with no extra drivers needed. Good times.