The `jalr` Instruction

A jal instruction always calls the same subroutine. Here is how main would usually call the first subroutine:

jal sub1

But what if you wanted the same instruction to call different subroutines depending on circumstances? This is where a jump table is useful. The table contains a list of subroutine entry points.

To call a subroutine, copy its address from the table into a register. Now use the following instruction:

jal r      # $ra <― PC+4  (the address 8 bytes away from the jal) 
           # PC  <― $r    load the PC with the address in $r
           # a branch delay slot follows this instruction

This works just like the jal instruction except the address of the subroutine now comes from a register. The return address (to the caller) is put in register $ra as with jal.

QUESTION 5:

Here is a section of main. Fill in the blanks so that control is passed to sub1:

          .text
main:     
          lw      $t0,     # get the first entry point in the Jump Table
          
          jalr             # pass control to sub1
          
          li      $v0,10          # return to OS
          syscall

          .data
sub1add:  .word   sub1            # Jump Table
sub2add:  .word   sub2

Answer:

The jalr Instruction

QUESTION 5:

The `jalr` Instruction