How a Jump Works

When a program is executing, its instructions are located in main memory. Each instruction has an address.

Each machine cycle executes one machine instruction. At the top of the machine cycle, the PC (program counter) contains the address of an instruction to fetch from memory. The instruction is fetched into the processor and is prepared for execution.

In the middle of the machine cycle the PC is incremented by four so that it points to the instruction that follows the one just fetched. Then the fetched instruction is executed and the cycle repeats. The machine cycle automatically executes instructions in sequence.

When a jump instruction executes (in the last step of the machine cycle), it puts a new address into the PC. Now the fetch at the top of the next machine cycle would fetch the instruction at that new address. Instead of executing the instruction that follows the jump instruction in memory, the processor would "jump" to an instruction somewhere else in memory.

However, it takes an extra machine cycle before the change in the PC takes effect. Before the PC changes, the instruction that follows the jump instruction in memory is fetched and executed. After that instruction executes, the next instruction to execute is the one that was jumped to. The instruction that follows a jump instruction in memory is said to be in the branch delay slot.

The reason for this delay is that MIPS is pipelined. Logically, instructions are executed one after another in sequence. However, in order to gain speed, the processor works on several sequential instructions simultaneously. These instructions are in an instruction pipe. When a jump instruction is about to change the PC, the work already started on the instruction that follows it is allowed to finish.