I have now installed the 64 bit version of PiOS on an RPi4B device.

If I install Lazarus/FPC on that can it create applications both for 32 and 64 bit RPi operating systems?

Or do I have to install two versions of Lazarus and/or FPC to be able to do that?

It would be nice to have only one Lazarus but both 64 and 32 bit FPC compilers and be able to select for which I want to build the application inside Lazarus itself.

fpcUpDeluxe allows to install additional FPC folder. with 'crosses' - for ex, I have additional FPC on Linux with crosses for Windows, Linux ARM, *BSD x32+x64, macOS x64+ARM. In the IDE you can specify that new FPC installation. Using IDE 'build modes' you can make N new modes, one mode pear each CPU/OS case.
Alex

OK thanks, then I have to use fpcupdeluxe to install Lazarus + FPC then. I will try that as a first on the new RPi4 then.

then I have to use fpcupdeluxe to install Lazarus + FPC then.

Lazarus can be old. Only FPC need to be installed from Deluxe. Old Lazarus - in it, specify path for new FPC.
Alex

I asked on the Lazarus forum as well, my first post is here: https://forum.lazarus.freepascal.org/index.php/topic,34645.msg496844.html#msg496844

It seems like the fpcupdeluxe available for armhf on GitHub does not run in PiOS 64 bit. It is a 32 bit application...

NOTICE!
I was alerted to using the wrong version of fpcupdeluxe, it should be the aarch64-linux version!!!

With this version downloaded it tstarts running OK and I can configure and start building. But it fails after about 10 minutes complaining about this:

make[1]: *** [Makefile:2682: packages_smart] Error 2 make: *** [Makefile:2837: build-stamp.aarch64-linux] Error 2 fpmake.pp(60) Error: Error while linking
fpmake.pp(60) Fatal: There were 1 errors compiling module, stopping Fatal: Compilation aborted

fpcupdeluxe: ERROR: FPC Native Installer (BuildModule: FPC): Error running /usr/bin/make for FPC failed with exit code 512
. Details: .

ERROR: Fpcupdeluxe fatal error !
Sequencer (FPC): Failure running fpcupdeluxe: error executing sequence FPC Sequencer (Default): Failure running fpcupdeluxe: error executing sequence Default

I refer the discussion over to the Lazarus forum (see link above).

Bo Berglund wrote

I refer the discussion over to the Lazarus forum (see link above).

Just a quick follow-up here:
It seems like the fpc sources have not been updated with the solution to the bug discussed here:

https://gitlab.com/freepascal.org/fpc/source/-/issues/39295

THis causes the build of Lazarus using fpcupdeluxe with the latest fpc 3.2.2 to fail because of a problem with __libc_csu_init during linking.

Apparently it is fixed in non-released sources of fpc which we are waiting for...