Would you like signs with those chars?

LDRx for B/H(/W on AArch64) zero-extends in general, yet short and int are still signed. Plus if you put an S after the R you get the sign-extending version of the instruction (which even exists for the LDRxT (AArch32) / LDTRx (AArch64) unprivileged loads). Thus nothing about the ABI is forced, or even made more efficient if a particular choice is made, by the instruction set here, just whether the mnemonics have an S in them or not.