Question

4. Explain the purpose of jump-and-link(jal) instruction. (10%)

Answer 1

Question 4 :

Solution:-

The Jump and Link instruction usually known as JAL instruction.These instructions are mainly used for jumping to a subprogram or function and also to return from the subprograms.The JAL perform an unconditional jump to the address specified and store the address of the next instruction in the return address register or $ra register. The next instruction refers to the instruction specified in the next address after jal.

The purpose of JAL instructions and jump instructions are almost similar because both instructions jump to the address provided except that, in jal instruction it saves the address of the next instruction in the $ra register.Hence,JAL instruction allows a subroutine or a function to return to the main program or simply we can say that ,it allows a subroutine to return to the main program that responsible for the call to that function.Thus linking the subprogram to the main program.

For example,

Consider a main program with the label say mainprog and a subroutine with label subprogram :

Here ,

• 'jal subprogram' instruction will call the subroutine.
• Subroutine will get executed and on the instruction - ' jr $ra' ,the control will go to the main program since flow of execution return from the subroutine to the caller.

Another example explaining the detailed flow of control over main program and subroutine is given below :

The diagram explains how the flow of execution happens while using jal instruction.The flow of execution described in step by step is given below :

• The mainprog starts at location 00000000.
• The return address register will be stored with address of the next instruction (instruction after jal) .That means, $ra = 00000008
• Now the subprog get executed and after the execution of each instruction the control finally enters to the instruction jr $ra .
• jr instruction is used to return from the subprog to the caller which is the mainprog.
• The next instruction going to execute is the instruction at the location which is stored in return address register which means instruction at location 00000008 .