Question

If you need to keep the value held in $1, what is a safe way to call the subroutine other that might use this register, i.e what do you need to do before (and after) you jal other?

Answer 1

There are two ideas behind a subroutine.

1. You should be able to call the subroutine from anywhere.
2. Once the subroutine is complete, it should return back to the place that called the subroutine.

For making a subroutine call is jal which means "jump-and-link".

jal takes a label as its operand. This label is an address in memory for a subroutine. The assembler translates the label to an address.

To jump to that address really means to update the PC (program counter) to the address of the subroutine. The PC is a hidden register that holds the address of the current instruction being run.

The jal instruction saves the return address in register $r31. This register is also called $ra (where "ra" means return address).