25.6.1. Questions Addressing Objectives

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.

In programmed I/O, the CPU checks the status of the I/O device continuously (this is called polling). While doing this, the CPU cannot perform other tasks, which leads to wasted time and efficiency reduction. Polling involves the CPU sending requests to the I/O device to see if it is ready for data transfer, which can delay the completion of other processes.

Imagine a worker (the CPU) constantly checking if a printer (the I/O device) is ready to print. While waiting, the worker cannot start other tasks, such as preparing the next report. This wasted waiting time can be very inefficient, just like how the CPU waits instead of doing useful work.

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, you now see that here I/O module is not going to inform the CPU directly that device is ready, but in case of interrupt driven in somewhere, in somewhere I/O module is going to inform this particular things to the processor.

The steps in programmed I/O are as follows: First, the CPU requests an I/O operation. Next, the I/O module performs the requested operation. Then, the I/O module updates a status bit to indicate whether the device is ready. The CPU periodically checks this status bit to see if it can proceed. In programmed I/O, the I/O module does not directly notify the CPU, unlike in interrupt-driven I/O systems where the module informs the CPU immediately.

Think of this process like a restaurant (CPU) that provides a menu (I/O operation request) to a waiter (I/O module). The waiter goes to the kitchen (the I/O module) to prepare the food. The waiter must keep checking back to see if the food is ready (status bit update) but does not tell the restaurant right away when it is ready. Instead, it waits until the restaurant checks back periodically.

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.

To communicate with I/O devices, the CPU needs to identify them using unique device addresses. This is similar to how each house on a street has a distinct address so that mail can be delivered correctly. Each I/O device must have a specific identifier so that the CPU can accurately send commands to the right device.

Consider a post office that must deliver mail to various neighborhoods. Each house (I/O device) has a unique address. If a letter is sent without a correct address (device address), it might end up at the wrong house. Similarly, the CPU ensures commands are sent to the correct I/O device by using specific addresses.

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 instructions sent from the CPU to the I/O devices to initiate actions. For example, before printing, the CPU must send a command that tells the printer to start preparing itself. These commands ensure that the I/O devices are ready to perform their functions before any data transfer takes place.

Imagine a conductor of an orchestra (the CPU) signaling musicians (I/O devices) to get ready before the performance. The conductor raises a baton (control command) to prepare the musicians to start playing. If the musicians don’t receive this signal, they won’t know when to begin.

Now how we are going to give this particular address, how we are going to map this particular input output devices. So, for that we have to look I/O mapping, so how we are going to map the I/O devices to the processor. So, that we can identify that particular device correctly.

I/O mapping techniques are strategies used to assign unique addresses to different I/O devices, allowing the CPU to interact effectively with them. There are two primary methods: memory-mapped I/O where I/O devices are assigned addresses within the memory space, and isolated I/O where devices have a dedicated addressing space, making it easier to differentiate between memory and I/O addresses.

Consider a city divided into two distinct areas: one for residential buildings (memory space) and another for businesses (I/O space). The city planner (CPU) must know where to find homes and where to find stores. In memory-mapped I/O, homes and stores share the same avenue; in isolated I/O, they have their separate streets. This organization helps the planner efficiently access both types of properties.

So, these are the issues, basically we have one is your addressing scheme, second one is what are the commands that we need to handle this particular input output devices and along with that I think we have already used those particular program I/O details.

In conclusion, effective I/O operations hinge on understanding the addressing scheme for devices, the commands required to communicate with them, and the programming involved in programmed I/O. Each aspect is vital to ensure smooth interaction between the CPU and external devices, thus fulfilling the objectives laid out for the unit.

Think of a successful team project: a clear communication plan (addressing scheme), assigned roles (commands), and a project timeline (programming details) are necessary for success. Just like these pieces work together for a team, addressing, commands, and programming are essential for successful I/O operations.

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