Canon EF mount can't mount FD or FL mount lenses due to change in the mount distance. Unlike mirrorless companies that went shorter with registration distances Canon went LONGER. This means that FD lenses on a Canon EF mount DSLR body can't work without using an optical glass teleconverter type adapter.
Questions about using an FD mount. lens on a EF mount body READ THIS FIRST - Photo.net Canon FD Forum
Nikon has a more nuanced history. Some bodies can, some can't meter. Some bodies can, some bodies can't use screw drive lenses. Some require CPU's in their lenses to control aperture - some don't. It's really complex. There is a chart on this page:
Nikon Lens Compatibility that gives a lot more details. The most important stat to really look at is that PRE-AI lenses can't be used on a very large set of bodies and there are many bodies that can mount them but not meter them.
Pentax is simple. M42 via Adapter and K mount Preset type lenses with Av mode - focus wide open, stop down to meter and shoot. Green button for M & K with normally wide open apertures until shot exposure. A mode provides full aperture control in appropriate modes.
Pentax is this way partially because they never went the electronic aperture path and their MTF optimization etc was in body not in lens cpu. This is also why companies like Samyang have been able to provide aperture automation for Pentax due to the simplicity of that method of automation.
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Mirrorless does complicate the picture as FD lenses are greatly prized for the fact that there were so many orphaned with great glass and these went for a song for a long time. The cost has gone up over time as the best glass has been bought. The same can be said to a degree about Nikon Pre-AI lenses. Konica Hexanon, Topcon, etc. Digital mirrorless has given new life to a lot of old glass.