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Operating System Fundamentals
68 IO Management Software From a software perspective, there are really two main elements to consider. The first is device
drivers, which are the operating system software components that interact with the devices controllers. The second element is the interrupt handler. This software is really part of the device driver, but it is responsible for issuing interrupt signals to the interrupt controller when a device requests access to the CPU. The IO software system functions in layers, as illustrated in Figure 5.3 (below. One layer is the device drivers. Each type of device shown below has its own controller, which is responsible for relaying instructions to all devices connected to it. Device drivers issue instructions from the operating system to the device controller, which in turn handles the actual functioning of the device. Another layer is the interrupts. When a device controller issues a request from a device to gain access to the CPU, the interrupt handler (part of the device driver) sends the interrupt signal to the interrupt controller. The interrupt is then relayed to the CPU, as outlined in the previous section.
Figure 5.3
I/O Software System Layers
Device Drivers
Device drivers are essential components of an operating system. They translate instructions from the operating system and other processes into instructions that are used to drive the device controllers. In order to handle controlling the input/output devices that are part of the computer system, device drivers are structured into two layers. The Upper Half of the device driver handles taking requests in from the operating system, and places them in a ―Shared
Requests List.‖ The Lower Half of the device driver handles taking requests from the shared requests list, and programs them for the device control to carryout the instructions. The
interrupt handler is part of the Lower Half of the device driver. It is used when the device controller calls for an interrupt request, and it handles the issuing of the interrupt request to the interrupt controller. Figure 5.4 (below) illustrates the structure of atypical device driver.

Operating System Fundamentals
69 Shared Data
Structures
Upper Half
Lower Half
Shared Request
List
Interrupt Handler
Device Controller
I/O Request from OS
Results
Interrupts

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