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But it is much better to follow a subroutine linkage convention such as is about to be discussed.) . not just $ra. Like all jump and branch instructions, the jr instruction is followed by a branch delay. The diagram shows the subroutine returning to the return address that was loaded into $ra by the jal instruction in the caller.
18 Aug 2008 If you look through the disassembly listing you will see that only the JAL and JR instructions are used. The appending NOP satisifies a delay slot requirement for the pipelined MIPS processor. Consider the following code snippet: 9D000040 0F40001F jal 0x9d00007c 9D000044 00000000 nop 9D000048
3 Mar 2012 The address in $ra is really PC+8. The instruction immediately following the jal instruction is in the "branch delay slot". It is executed before the function is entered, so it shouldn't be re-executed when the function returns. Other branching instructions on the Mips also have branch delay slots. The delay slot is
Each cycle executes one machine instruction. This is because: (i) returning from the subroutine to the jal instruction would be a disaster (since it would execute again, sending control back to the subroutine), and (ii) the instruction following the jal is a branch delay slot.
What happens if a jal call is made while in a subroutine? return address overwritten with a new return address must place the return address onto the stack (memory). It turns out that the return address is PC + 8, not PC + 4 jump instructions have to be followed by a branch delay slot instruction purpose of avoiding stalling in
It would not be a disaster to return control to an instruction that does nothing. But sometimes programmers or compilers put something clever 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
Each cycle executes one machine instruction. (This is a somewhat simplified view, but sufficient for now). The jal instruction does the following in the execute phase of the machine cycle: jal sub # $ra <? PC+4 (the address 8 bytes away from the jal) # PC <? sub load the PC with the subroutine entry point # a branch delay

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January 2025