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Answer:

A no-op instruction is an instruction that has no effect. A common no-op instruction is sll $0,$0,0.


Altering the PC

Machine Cycle

Here is a sequence of instructions. The "load" and "add" represent typical instructions. The "jump" instruction shows the address we wish to put into the PC. (The actual MIPS instruction for this involves details that we are skipping for the moment.)

The last instruction, a no-op, fills the branch delay slot to give the PC time to change. Once started, the four instructions execute in an unending loop.

AddressInstruction
(details omitted)
PC just after this
instruction has executed
(at the bottom of the cycle)
.......................... 00400000
00400000 load 00400004
00400004 add 00400008
00400008 jump 0x00400000 0040000C
0040000C no-op 00400000
effect of the jump

A loop structure is created with the jump instruction. The intent of the jump instruction is to put the address 0x00400000 into the PC. However, this effect is is not seen until after the instruction in the branch delay slot has executed.


QUESTION 3:

(Thought question:) Is there anything in hardware that guarantees that the target of a jump instruction is an instruction (and not data)?