23.5.2. Next Steps in Learning

Now, we are coming to the indirect phase of instruction execution that is an interrupt... instruction service routine nothing but another code itself or a code module with some instruction is a jump.

An interrupt is a signal that temporarily halts the ongoing execution of instructions so that the processor can address a specific condition or event, such as input from an I/O device. When an interrupt occurs, the processor saves the current state (like the Program Counter or PC) to a stack, allowing it to resume from that exact point after servicing the interrupt. The processor then jumps to an Interrupt Service Routine (ISR), which is a special block of code designed to handle the specific condition that generated the interrupt.

Imagine you are reading a book when your phone rings. You save your place in the book (saving your state), answer the call (servicing the interrupt), and then go back to the page you were reading (resuming execution) once the call is finished.

After no interrupt can offer where the instruction is being executed... instruction has been completed then we check whether the interrupt is there or not.

Instructions in a program are processed in a cycle of fetching, decoding, executing, and storing results. After each instruction is executed, the processor checks for any interrupts. Interrupts are checked only after an entire instruction cycle is completed to avoid complications. If an interrupt is detected, the processor will save the current state and execute the ISR; otherwise, it simply moves on to the next instruction.

Think of it like cooking a dish: you chop vegetables, check if you need to season anything (interrupt check), and once you finish chopping (complete instruction), you either start cooking (next task) or deal with any urgent issues (interrupt).

Now, this something very interesting. Now basically there is something called a ICC that is nothing but instruction cycle code... 00 means instruction fetch, 01 means decode, 10 means execute, and 11 indicates an interrupt.

The Instruction Cycle Code (ICC) is a two-bit code that tracks the current phase of instruction execution. The different codes indicate whether the processor is fetching an instruction (00), decoding it (01), executing it (10), or servicing an interrupt (11). This system allows the processor to manage its operations efficiently and ensures that it knows exactly which phase it is in at any given moment.

Think of ICC like a traffic light: green means go (fetching instructions), yellow means slow down and prepare to stop (decoding), and red means stop and deal with emergencies (interrupts). Understanding the color of the light at any moment helps the driver make safe driving decisions.

So, in a nutshell you start with the ICC 00 code is instruction fetch... make the code as 11 and again after servicing the interrupt make it 00 now that is what is the idea.

The sequence starts with the ICC set to 00 for an instruction fetch. If an immediate operand is present, it moves to execute it directly (setting ICC to 10). If the operand requires fetching data, it goes to 01 to fetch it first. After data handling, the system checks for any interrupts; if one is present, it sets the ICC to 11 to service it. Once processing the interrupt is complete, the cycle returns to instruction fetching (ICC set back to 00).

Picture a classroom setting: when the teacher gives a lesson, that's like fetching an instruction (ICC 00). If a student needs clarification or has a question (interrupt), the teacher stops the lesson (sets ICC 11), addresses the question, and then continues teaching (returns to ICC 00).

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