25.4.2. Functionality of Isolated I/O

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.

The concept of programmed I/O is explained here, where the CPU repeatedly checks (or polls) the status of an input/output device to see if it is ready to transfer data. This continuous checking consumes CPU time, preventing it from performing other tasks.

Imagine a teacher continuously checking if a student is ready to answer a question. While the teacher is focused on that one student, they can't help other students, leading to inefficient use of their time.

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, I/O module does not inform CPU directly.

The programmed I/O process follows specific steps: the CPU first requests I/O operations. The I/O module performs the necessary actions and sets a status bit based on the device's readiness. The CPU checks this status bit at regular intervals to see if it should proceed with the data transfer. It’s important to note that the I/O module does not actively inform the CPU when the device is ready; instead, the CPU must keep checking.

Think of it like ordering food at a restaurant. You place your order with the waiter (CPU) and then keep checking back to see if your food is ready. The kitchen (I/O module) doesn't signal you directly when your food is ready; you have to ask around until it arrives.

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. So that means, we have to have device address so this is the one point how we are going to give the addresses of the I/O devices.

Whenever the CPU wants to communicate with an I/O device, it must address that device, similar to how we use street addresses to send mail. Each device is assigned a unique binary address, allowing the CPU to know exactly where to send instructions or data.

It’s like sending a letter. You have to know the specific address of the person you are writing to. If you send it to the wrong address, it won't reach them. In computing, addressing ensures that instructions reach the correct I/O device.

So, basically some control commands we have to issue just to initiate the process like that already have explained. If we want to print something in the printer we should send some control signal to the printer to initiate it or maybe to bring the printer head to the appropriate position.

Control commands are necessary to commence I/O operations. For example, to print a document, the CPU sends commands to the printer to prepare for printing, positioning the print head and ensuring the printer is ready. This step is crucial before any data transfer occurs.

Before a concert, the stage crew has to set up the equipment and test it to make sure everything works. Similarly, the CPU must prepare the I/O device before it begins transferring data.

So, 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 in to the output device.

When transferring data, there are two main modes: reading and writing. Reading involves pulling data from an input device (like a keyboard or sensor), while writing involves sending data to an output device (like a printer or monitor). Understanding these modes is pivotal for developing effective I/O operations.

Think of it like a library. When you borrow a book, you are 'reading' from the library—the book (input). When you return the book, you are 'writing' back to the library, giving them back the information (output).

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.

There are two primary methods for mapping I/O devices: memory mapped I/O and isolated I/O. In memory mapped I/O, some memory addresses serve both RAM and I/O devices. In isolated I/O, distinct address spaces are used for I/O devices, allowing more flexibility in device identification.

Consider a store with two aisles: one for food and one for electronics. If it's memory mapped I/O, you might stock both items on the same shelf. In isolated I/O, each item has its specific aisle, making it easier to find and organize.

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.

In isolated I/O, the system uses control signals—specifically the IO/M signal—to indicate whether the address on the bus refers to a memory location or an I/O device. This distinction is crucial for the proper functioning of the system, as it directs the CPU to the correct operation depending on the context.

Imagine a post office that sorts letters based on their destination—either to a home (memory) or to a business (I/O). The sorting signals ensure that every piece of mail arrives at the correct destination without mix-ups.

In case of isolated I/O we are going to get a bigger I/O space, because equal number of memory location and equal number of I/O devices, but in case of memory mapped we are going to reserve some of the memory addresses for identifying the I/O devices.

Using isolated I/O allows the system to have a larger number of I/O devices compared to the memory mapped method, where only a portion of address space can be assigned to I/O devices. This gives isolated I/O an advantage in terms of scalability and flexibility in device connections.

Imagine a city with more lanes (isolated I/O) versus a small road with strict one-way traffic (memory mapped I/O). The city can accommodate more vehicles (devices), whereas the small road can only handle a limited amount.

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