24.1. External Devices and Their Types

So, these are basically human readable devices; like screen, printer, keyboard and like that.

Human readable devices are output and input devices that make it easier for users to interact with computers. Examples include screens that visually display information, printers that create physical copies of documents, and keyboards that allow users to input text and commands. These devices convert computer output into formats that are comprehensible by humans, thereby enabling effective user interaction with computers.

Think of human readable devices like a restaurant menu. The menu presents food options in a way that customers can understand and decide upon, just as a screen displays information for users to read and a keyboard allows them to input their choices.

So, we are having some devices which are machine readable. So, this machine readable devices are basically used for monitoring and controlling purposes. One simple example is your fingerprint.

Machine readable devices interact with computers in a way that is not directly comprehensible to humans but facilitates monitoring and control. An example is a fingerprint scanner, which allows users to unlock devices by scanning a physical attribute, thus enhancing security. Other examples include various sensors that collect data and transmit it to a computer for processing.

Consider a security system in a building where only authorized personnel can access restricted areas. The fingerprint scanner acts like a key that only recognizes specific fingerprints, allowing the system to decide who can enter, similar to how machine readable devices make decisions based on data input.

So, we can have a hierarchy of the memory. So, first we can talk about the registers inside a processor. Then next level is your main memory, then next level is you can say hard disk.

Storage devices are categorized in a memory hierarchy, which consists of multiple levels including the smallest but fastest, registers inside the processor; next, the larger but slower main memory; and finally, the hard disk, which has the largest capacity and is the slowest. This hierarchy allows computers to manage data efficiently by placing frequently used data in faster memory locations while keeping less frequently accessed data in slower, larger storage.

Think of a filing cabinet in an office. The most frequently used documents are in an easily accessible drawer (registers), less commonly used files are in a larger section nearby (main memory), and older or less urgent paperwork is in a storage room (hard disk). This organization makes it quicker to find necessary documents based on need.

So, this is the way we can just look for the hierarchy. So, these are external devices where we can store our information.

The I/O module is responsible for managing communication between the processor and external devices. It controls timing, manages CPU communication, handles data buffering to accommodate speed differences between devices, and can detect errors in data transfer. This modular design helps the CPU to communicate efficiently with various types of devices without direct involvement during every data transaction.

Imagine a post office that works as an I/O module. It accepts letters from individuals (data from devices), sorts and organizes them (buffers data), and sends them to the respective addresses (transferring to the CPU), also making sure that all letters are properly stamped and ready to be sent as error detection.

So, here we are going to discuss about those three different techniques the way we are going to transfer our information; one is your programmed I/O, second one is your interrupt driven and third one is your direct memory access or DMA.

Data transfer methods can be categorized into three main types: programmed I/O, where the CPU actively waits and checks if a device is ready; interrupt-driven I/O, where the CPU can continue working until notified by the device; and Direct Memory Access (DMA), which allows devices to transfer data to memory without CPU intervention. Each method has its advantages and is suited for different scenarios.

Compare the data transfer methods to a waiter in a restaurant. In programmed I/O, the waiter constantly checks if the food is ready, wasting time. In interrupt-driven I/O, the waiter continues to serve other customers until the chef shouts that the food is ready. In DMA, the chef serves the food directly to the customer’s table without waiting for the waiter, speeding up service.

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