Example jal Instruction

It would not be a disaster to return control to an instruction that does nothing. But sometimes clever programmers or clever compilers put something unexpected in the branch delay slot, so it is best not to pass control to it.

The diagram shows the execution of a jal instruction. The jal is at address 0x00400014. The return address is 0x0040001C which is the address of the jal plus eight. (The addu instruction there is just used as an example of what might be at the return address).

Here is how the jal instruction works in general:

jal sub    # $ra <― PC+4  $ra <― address 8 bytes away from the jal 
           # PC  <― sub   load the PC with the subroutine entry point

Here is how it works in this example. The entry point of sub is 0x00400100.

Fetch:      When the jal is fetched the PC has 0x00400014.

Increment:  The PC is incremented to 0x00400018.
            
Execute:    $ra <― 0x0040018+4 
            PC  <― 0x00400100

The nop instruction in the branch delay slot is executed. Then execution continues with the first instruction of the subroutine at 0x00400100. Control has been passed to the subroutine and the return address is in $ra.