[monocasa]

On pretty much anything Linux is running on, full words are loaded into registers before a multiply begins. Even for say, something like x86 where a multiply can have a memory argument and say that it straddles a cache line boundary so you could get a portion of the word, the system still splits it into load to (temporary) register, execute mul, and store to memory micro-ops.

[SAI_Peregrinus]

Correct. There are a few cases where the operands don't fit into a single machine word, the most notable being many cryptographic operations. Particularly RSA and ECC, which involve multiple-precision arithmetic.

There are also non-Linux cases, mostly microcontrollers. EG the Arm Cortex M0 doesn't have a hardware multiplier, the M0+ does.

And then there's that one guy who got Linux running on an 8-bit AVR by emulating a 32-bit ARM and running it on that[1]. I'd consider this a silly edge case. Too fun not to mention though.

[1] https://dmitry.gr/?r=05.Projects&proj=07.%20Linux%20on%208bi...