uP port mapping

Something I've wondered about:

Suppose we have a c program running on some little uP, and it has some
integer variable value, 8 or 12 bits or something, and wants to drive
a parallel DAC off-chip.

The msb...lsb bits of the variable obviously have to get to the right
pins of the DAC.

So, in general, how does one pick the physical i/o port pins on the
uP, to get the order right? The PCB layout is easiest if we just wire
the DAC to the handiest port pins.

One could test and bit-bang each bit and port individually, and then
strobe the DAC, but that's inelegant.

In the RP2040 chip, one can apparently write to a register in a
PIO/state machine block, where each bit of the register can be
assigned to drive a physical port pin. I think there are some
constraints on the selected pins.