So, basically in the units what is in the unit summary, we will first discuss about the unit summary before going into the depth of the unit. Basically as you already know the control transfer instructions are of two type conditional and unconditional sorry unconditional and conditional as we shown in the cases in the slide. So, what is the unconditional jump instruction, an unconditional jump instruction is very simple like jump go to that memory location where the corresponding instruction is there, maybe you call a function so that is an unconditional jump. And what is the conditional jump, when you look at these flags and decide what to do like jump on zero to some memory location 3030 where the next instruction is there. It will jump only if the zero flag is set, so that way we can differentiate the means control transfer instruction as conditional and unconditional.

As we have already discussed in the last three units that basically the three steps that is the first three steps basically involve fetching of the instruction. So, in this case what happens in the step one basically we load the program into the bus, PC into the bus and we give it to the memory address register, so that the corresponding instruction can be fetched. We make the main memory in read mode and we get the value of the program or the instruction in the memory data register in this third stage basically.

So, basically that is what is the thing and then we say that but here there are slight difference in case of a jump instruction or transfer instructions. In general type of instructions, we take the value of the program counter to the bus, put it in the address register, and then wait for the corresponding instruction to be obtained in the memory data register. And also what we do, we also try to increment the value of PC to the next value to point to the next instruction. But in this case we do a slightly separate step over here that what we do? We actually also stop the incrementing of the PC here, but also we store as we will see the requirement what is the necessity. We also put the temporary value of PC into a temporary register Y.

Now, in this case, see previous case we have seen how to add, how to store from memory to the registers, but here actually it is something going to a jumps space. For example, let us take the instruction called jump unconditional to say 3030, then we want to jump to that memory location that is what is the execution part here. So, in this case, what we have to do, we have to somehow load the value of program counter in the next stage to 3030 that is what is the execution to be done.

The offset basically is a part of the instruction register, which actually goes to the CPU, CPU bus basically. So, what is an offset, offset is present value of PC − M and you take the general the positive value out of it. So, if you assume that the present value of the PC is equal to 10 for the time being let us assume that is equal to 10, so your offset will be 3030 this 10 − 3030 the positive you are going to take it.

Now, what do you do the offset is set the offset value is this one and it is saying the input value of the CPU bus is the offset which is the second operand to the ALU. That means, what we are going to generate the jump address, and we are going to load it to the PC, that is what is the job of the fourth step.

So, after that is already told that is nothing but value of PC − M this gets cancelled and you get the M. And basically this one you will now update Z will update to program counter, so you are now jumping to that instruction and you are actually executing it.