28.4.3. Steps in Memory Fetch Operation

Now, we see step wise basically what happens, now we will again deal with we have already discussed a similar example beforehand, but now we will see in more depth of the different instructions, even registers and the formats. So, as I told you. So, this is the first instruction to be executed. So, the PC is going to have the value of this one value of the memory location of the first instruction.

The first step in understanding the memory fetch operation involves recognizing how instructions are executed in cycles. When a program starts, the Program Counter (PC) holds the memory address of the first instruction to be executed. As the CPU executes instructions, the PC gets updated to point to the next instruction. This sequential fetching is crucial for the program's flow.

Think of the Program Counter as a page marker in a book. As you read (execute) each page (instruction), you move the marker to the next page to know where to go next. If the marker jumps around (like with a conditional or branching instruction), it can change the order in which you read the book.

So, in this case. So, this instruction no as I told you we are assuming that this is a 16 bit size. So, each of this is memory location has a 4 instructions. So, only one word can be taken to the memory buffer register or the instruction register and your job is done for example, in this case this is a single address instruction.

Once the PC indicates the address of the instruction, the CPU fetches the instruction from the memory. In this case, the architecture has a 16-bit instruction size where each memory location can store two instructions (4 bits each in this case). The CPU uses the Memory Buffer Register (MBR) to temporarily hold the fetched instruction while it is decoded and executed.

Imagine a library where books are organized in specific sections. Each section can hold a number of pages (instructions). When you want to read a page, you grab the book (fetch the instruction) and hold it (store it in the memory buffer) before flipping it open to read (execute the instruction).

The last 2 are basically data transfer and this is the arithmetic. First F what happens the PC is pointing to this, this instruction is going to the memory buffer register and as we know that because of this addressing format each memory location has a single instruction. So, need not worry directly take the value of memory buffer register to instruction register, it will decode it and it will find out that it is asking to load the value of FF0 to accumulator.

After fetching the instruction and storing it in the MBR, the instruction is transferred to the Instruction Register (IR) for decoding. The CPU identifies the operation it needs to perform, in this case loading a value from a specific memory address (e.g., FF0) into the accumulator, which temporarily holds data for processing.

Think of decoding an instruction like reading a recipe. Once you fetch (get) the recipe card (instruction), you need to read it to understand which ingredients to use and what steps to follow. When the recipe tells you to add an ingredient to a bowl (accumulator), you carry it out by following the instructions.

So, there is a synchronization issue that is a handshaking that you say that I want to read it read some memory location. So, I give the value in the memory address register, and then the memory address register via the address bus is connected to the memory, then you say that read so the read signal is made, but how much time I should wait before I take the value from the memory buffer register.

Synchronization is vital to ensure that the CPU can communicate effectively with memory. Once a read operation is initiated, the system must wait for a control signal indicating that the read operation is complete and the data is ready in the Memory Buffer Register (MBR). This process ensures data integrity and prevents the CPU from fetching stale or incorrect data.

Imagine placing an order at a restaurant. After you place the order (initiate the read command), the waiter (control signal) must ensure that your food is ready before serving it. If you try to take your order too early, you might get an incomplete or wrong dish!

Once the memory function is complete, the value is stable in the memory buffer register, now it can be read to the accumulator it can be read to the instruction register and so forth.

After the completion of the read operation, the CPU must confirm that the fetched data is stable and valid before proceeding. Data is then transferred from the MBR to the appropriate registers (accumulator or IR) for execution. Ensuring that values are stable and correct is critical to avoid errors in processing.

Think of it as if you've received a package in the mail. You don't open it (transfer data) until you've checked that it's delivered and in good condition. Only then can you use the contents for your intended purpose without any problems.

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