19.2.1. Instruction Fetch Process

So, first let us take an example called 𝐴𝐷𝐷 𝑅1,𝑀 that is you are taking the memory location value 𝑅1 dumping it to 𝑅1 after adding it to whatever the content in 𝑅1 if you look forget about last class we have discussed that these are the series of control instructions, what are the signals involved?

In this chunk, the instruction ADD R1, M is introduced. This instruction involves adding the content of the memory location (denoted as M) to the value in register R1. The concept emphasizes understanding what data values are involved, where they come from, and how they are modified. The overall functionality of ADD operation is to take values from memory and manipulate them in the CPU registers (in this case, updating R1).

Imagine you have a register (R1) that represents your savings in a bank account. The value from a memory location (denote M), like an incoming paycheck, is added to this account every time. The ADD instruction can be thought of as the command that tells the bank to update your account balance.

So, they are all will be generated. So, now, first we will say if we remember that first was program counter out that is the 𝑃𝐶 value will be fed to the memory address register and then you will make it select zero, Add, 𝑍. This part corresponds to incrementing of the value of 𝑃𝐶.

This chunk delves into the control signals that play a crucial role in executing the instruction. The Program Counter (PC) is initially responsible for pointing to the current instruction's address in memory so it can be fetched. The signal 'Program Counter out' indicates that the current value (address) stored in the PC is to be sent to the Memory Address Register (MAR), while also preparing for the addition of a constant (often used for increment). The output from the ALU (Arithmetic Logic Unit) is then stored back into the PC to get ready for the next instruction.

Think of the Program Counter as a pointer on a recipe book that shows you which step you are currently on. When you follow a step (fetching an instruction), you note the page number (PC) and move on to the next one (incrementing the PC) after completing the instruction, ready to pick up from where you left off.

Now, we have to put it in 𝑃𝐶 that now 𝑃𝐶 is updated. Now, we have to it’s important we are waiting for an external signal this is an external signal. So, this is an external signal.

After updating the PC with a new value, the control logic must pause until it receives a particular external signal, often indicating that the data has been successfully fetched or that certain conditions have been met. This external signal is crucial for the control sequence to proceed safely to the next step.

This step is akin to waiting for a delivery confirmation before proceeding with a plan. Until you receive word that your package has arrived at home (the external signal), you won't move on to the next activity that depends on receiving that item.

So, all these like 𝑃𝐶,𝑀𝐴𝑅,𝑟𝑒𝑎𝑑,𝑠𝑒𝑙𝑒𝑐𝑡 0,𝐴𝐷𝐷,𝑍 they are the control signals which you have to generate and then 𝑊𝐹𝑀𝐶 is something on which you have to wait till you can go to the third stage.

This chunk communicates the importance of various control signals during the instruction fetch cycle. Signals like read, select, and add are generated based on the state of execution and govern how the processor retrieves and processes data from the memory. The WFMF signal here is especially significant as it determines when the processor can proceed to the following step, ensuring the sequential nature of instruction execution.

Imagine a traffic system where various signals (e.g., green light, stop sign) dictate when cars can move through an intersection. Just as the traffic must wait for a green light to proceed, the CPU (through its control signals) has to wait for the WFMF signal to ensure that the correct timing of operations is maintained.

Whenever the memory is ready you can go to the next state. So, whenever it says 𝑀𝐹𝐶; that means, I am done. So, then you can go for here actually 𝑀𝐷𝑅 is 1; that means, you are going to generate the signal that I want to read the memory.

This chunk explains the transitions between different states in a finite state machine model used in CPU control. The shift from one state to another is contingent on the readiness of the memory indicated by the MFC (Memory Function Complete) signal. When the memory is ready, the CPU can proceed to interact with the Memory Data Register (MDR) to read the necessary data from memory, a crucial part of fetching an instruction.

This scenario can be likened to a chef waiting for all the ingredients to be prepared before starting to cook (next state). If a crucial ingredient is ready and available (memory ready indicated by MFC), only then does the chef move forward with the recipe.

For state three nothing you have to do just you have to wait for the next clock pulse which will make the state equal to 4.

Here, the text describes the process in state three where the CPU merely waits for the next clock pulse to increment the state counter, reflecting the tightly coupled nature of instruction execution times with clock cycles in a finite state machine. A pulse leads to further transitions allowing the instruction fetch continues in a prescribed sequence.

Imagine a running race where runners can only take their mark (progress from one state to another) on the sound of a starting gun (the clock pulse). Until heard, they remain in place, awaiting the signal to move again.

Here instruction register out will go to the memory address register in, because you have to fetch another operand from the memory if you remember your instruction is ADD R1, M the next instruction is available in the memory location.

After fetching the instruction, it’s critical to load the next operand from memory, which is facilitated through the Instruction Register (IR) directing data to the Memory Address Register (MAR). This helps to retrieve necessary data required for the execution of the current instruction, maintaining a seamless flow.

Think of it like a store manager (Instruction Register) directing an employee (Memory Address Register) to fetch specific items from the stockroom (memory). Once one item is fetched, they need to prepare to go back and fetch the next item (next operand).

Whenever it says 𝑀𝐹𝐶; that means, I am done. So then, you can go for here actually 𝑀𝐷𝑅 is 1; that means, you are going to generate the signal that I want to read the memory.

This segment reflects on the finalization of the instruction fetch cycle, reaffirming that after the necessary signals are sent for reading instruction data, feedback from the memory must confirm successful data retrieval. The instruction fetch process is crucial as it sets the stage for the next steps of instruction execution.

Returning to our chef analogy, this phase is akin to the chef confirming that all ingredients are on the counter and ready before proceeding to cook. The process cannot continue without verification that everything is in place (memory acknowledgment).

Finally, what is to be emphasized it’s very easy to understand only some clocks are there first is this machine is invoked that is the initial state condition enabling is 𝐼𝑁𝑆 that is based on the opcode only when the signal is one you can go to the corresponding state machine.

This wrap-up encapsulates core aspects of the instruction fetch process, accentuating the simplicity and the role of clock signals in facilitating transitions through states depending on the 'INS' opcode. Each sequence in a control unit can be meticulously outlined, highlighting how instructions are associated with specific micro-steps.

Consider this whole process like a well-coordinated relay race; each participant (control signals) only runs (steps) when the baton (clock signal) is passed. Each runner knows their role (instruction) and timing based on the overall race plan (control unit sequence).

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