NIOS I custom instructions

You can't use the 11 bit K register to pass an argument to a function: the K register is cleared after all instructions (except the PFX and PFXIO instructions used to set it), and the PFX opcode is not allowed in the branch delay slot. Thus, the K register will always be cleared by the CALL instruction invoking your routine.

Besides, using the PFX op-code restricts you to compile-time constant values (unless you want to resort to self modifying code), while using a normal register allows the value to be set a run time.

In assembly speak, the term "immediate value" refers to a constant that is included as part of the op-code itself.