25.5.1. Execution of Device Service Routine

So, basically it is a sensing of status, then using the read write command, then transferring of data; this is the way that we are going to do, but here what is the problem that we have first? CPU needs to wait and poll, it is checking it continuously. So, it cannot do any other work. So, there is a wastage of CPU time.

This chunk introduces the concept of Programmed Input/Output (I/O). The CPU checks the status of a device by continuously polling it to see if it is ready for data transfer. This polling creates a delay because the CPU is occupied with checking the device status and cannot perform other tasks. As a result, this leads to inefficiency since CPU time is wasted while waiting for the device to become available for data transfer.

Think of the CPU like a person trying to talk to a friend who is busy on a phone call. Instead of doing other things, this person keeps trying to get their attention by shouting their name. This waiting creates unnecessary stress and wasted time, much like the CPU waiting for the I/O device.

So, basically this is now programmed I/O details now you just see these are the simple steps; first one CPU requests to I/O operation that in processor we have shown it, I/O module performs operation, then by looking into the state of the devices I/O module sets the status bit, CPU checks status bits periodically.

This chunk explains the steps involved in programmed I/O. First, the CPU issues a request for an I/O operation. The I/O module then performs the requested operation, checking the state of the device and setting a status bit to indicate whether the device is ready. The CPU periodically checks these status bits to determine if it can proceed with the data transfer. This illustrates the cyclical nature of the process, where the CPU and I/O module are in constant communication through status checking.

This is similar to a waiter in a restaurant checking on food orders. The waiter first asks the kitchen if the meal is ready (CPU requesting operation). The kitchen then cooks the meal and indicates when it is ready (setting the status), and the waiter keeps checking back to see if the food can be served (CPU checking status).

So, already I have mentioned that we need some commands I/O commands. So, first the issues for that I/O command is like that how to identify the module.

In this chunk, control commands necessary for I/O operations are discussed. The first task is to identify which I/O module the CPU will interact with, necessitating a device address. This address is akin to a home address for postal delivery, ensuring that data reaches the correct device.

Imagine you are sending a birthday gift. You need the recipient's home address to ensure the package gets to the right location. Similarly, in computing, the CPU needs the specific address of the I/O module to send and retrieve data correctly.

One is this is the mode of transfer, whether read basically we say that we are taking something from the input device and write we are saying that we are going to put some information into the output device.

This chunk delves into the modes of transfer during I/O operations. There are generally two modes: 'read' and 'write'. In the read mode, the system fetches data from input devices, while in write mode, it sends data to output devices. Understanding these modes is crucial as they dictate how data flows between devices and the CPU.

Consider a library where you 'read' books (checking them out) and 'write' new entries in the library record (donating books). The modes of transfer in computing work similarly, directing data flow in and out of storage.

So, basically, this is the addressing scheme. So, what is the address? It is very much similar to the address of a memory location which contains 0s and 1s ok; that is all.

In this chunk, the addressing scheme of I/O devices is explained. Each I/O device is assigned a unique address, similar to memory locations. This unique address ensures that the CPU can access and communicate with the correct device without confusion.

Imagine each device in a city (like traffic lights, fire stations, hospitals) has a unique address. If you want to send a message to the fire station, you need to know its specific address. In a computer, unique addresses for I/O devices serve the same purpose, directing commands precisely.

So, in that particular case, we are having two different ways of doing it. One is your memory mapped I/O and second one is your isolated I/O.

This chunk introduces two techniques for mapping I/O devices to CPU memory: memory-mapped I/O and isolated I/O. In memory-mapped I/O, both memory and I/O devices share the same address space, limiting the number of devices that can be connected. In isolated I/O, separate address spaces are used for memory and I/O, allowing for a larger number of devices to connect to the CPU.

Think of a college campus. In a combined campus, classrooms and offices share the same building (memory-mapped I/O), but in segregated campuses, classrooms are in one building and offices in another (isolated I/O). This separation in isolated I/O allows for more offices to be accommodated without confusion.

Now how to identify whatever addresses that we have put in this particular address bus. It is an address of a memory location or it is an address of an I/O devices.

This chunk explains the importance of control signals in distinguishing between memory and I/O addresses. A specific control signal, IO/M̅, is used to determine whether the address on the address bus refers to a memory location or to an I/O device. This differentiation is crucial for correct data operation.

It's like a reception desk in a large hotel that sorts visitors based on whether they are guests (memory) or delivery personnel (I/O devices). The reception uses specific signals (like guest cards) to direct each visitor to the appropriate location.

So, this is the way we are going to transfer information in programmed I/O and for that we need the addressing of the devices and we need the command.

The final chunk summarizes how information transfer occurs in programmed I/O. It emphasizes the necessity of correctly addressing devices and issuing commands to facilitate successfulcommunication between the CPU and I/O devices. This systematic approach ensures efficient operation of data transfers.

Consider a chef who uses a recipe book to prepare meals. The recipes are like the commands, guiding the chef (CPU) on what ingredients (data) to use, and the kitchen (I/O devices) is where everything comes together. The chef must follow the correct recipe (commands) and know where to find ingredients (addresses) for successful meal preparation (data transfer).

--