8.1.4. Dependency and Sequencing

So, in this fetch stage so, PC is equal to you are keeping the value of the memory address register can I actually merge this 2 in a single time step not possible, because in the first unit the value of the program counter will go to the memory address register, you give some time give one unit of time for that, then the memory address register will be read, now the memory will know that I have to supply the data which is actually the instruction from the address, which is given in the memory address register.

In the fetch stage of an instruction cycle, the program counter (PC) holds the address of the next instruction to be executed. Initially, the PC's value is loaded into the memory address register (MAR). This process requires time, as the CPU needs to read the address and prepare to fetch the instruction. Because fetching cannot happen instantaneously, these actions must occur in separate time steps.

Think of it like a librarian (the CPU) receiving a request for a book (the instruction) from a patron (the program counter). The librarian first writes down the requested book's reference (loading the address into the MAR) and then has to go to the shelf to find the book. This takes time, as the librarian can't fetch the book until they've recorded the request.

So, if you are free at hand then actually I can reuse the PC that indicates I am incrementing it by one. So, memory, taking the data from the memory to the memory buffer register and program counter increment these 2 micro instructions you can do it in time steps 2, because they are 2 non-dependent micro instructions.

Once the value from the PC is loaded into the MAR, the PC is free to perform another task, specifically to increment by one to point to the next instruction. This action can safely occur simultaneously with transferring the fetched instruction data into the memory buffer register (MBR) as both instructions are independent of one another and do not interfere with each other.

Imagine the librarian finished writing the request and is now free while waiting for the book to be fetched from the shelf. While waiting, they can start organizing the next request. In computing, this simultaneous processing speeds up instruction cycles.

So, what is clock grouping proper sequence should be maintained you cannot alter the sequence that is I am getting the data; before I put the registers in address that cannot be done.

Clock grouping refers to the careful arrangement of micro instructions in a sequence that respects their dependencies. It’s essential not to alter the order of operations; for instance, you cannot read data from a register while simultaneously writing to it. Ensuring that all operations happen in the correct sequence prevents conflicts and potential errors, ensuring the CPU runs efficiently.

Consider a recipe: you can’t bake a cake before mixing the ingredients. If you try to do operations out of order, like reading and writing to a register simultaneously, it’s like attempting to add flour when the eggs are still uncracked—it simply doesn’t work.

So, if it is a simple immediate mode of addressing then we require 3 steps 1 2 and 3.

Immediate addressing means that the data is directly specified in the instruction itself. In such cases, fetching an instruction requires fewer steps—usually about three time steps. In contrast, if the addressing mode is non-immediate, like direct or indirect addressing, more steps are required to first retrieve the address and then go back to fetch the data from that address, potentially extending the instruction cycle significantly.

Think of immediate addressing like having a recipe right in front of you (the instruction). You only need to read it and proceed. Non-immediate addressing, however, is like needing to first look up the recipe online before getting to your kitchen; it requires more steps to fetch the necessary information before moving ahead with the cooking.

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