第6章インフラストラクチャーの抽象化層：オペレーティングシステム

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Device drivers are specialized software components that enable higher-level computer programs to interact with hardware devices. These drivers provide a software interface to hardware devices, allowing operating systems and other computer applications to access hardware functions without needing to know precise details about the hardware being used.

• At the heart of any computer is the CPU, the primary component responsible for executing instructions. Interestingly, CPUs themselves do not typically require external device drivers for direct operation, as the core management of CPU resources is a fundamental role of the operating system.

• Memory operates under the direct management of the operating system, which allocates and manages the system’s memory resources. RAM can be accessed randomly and used for storing data temporarily while a computer is running. While standard RAM modules—both dynamic and static RAM—do not necessitate distinct drivers, specialized memory hardware, such as flash memory devices, including solid-state drives (SSDs), USB flash drives, and memory cards, interact with the system through file system drivers that manage the organization and access of stored data.

• Storage devices, encompassing a broad range of hardware from traditional hard disk drives (HDDs) to modern SSDs and removable storage media, require device drivers to facilitate data read/write operations.

• Network connectivity relies on an array of device drivers designed to manage the protocols and hardware functions of network interfaces.

A device register is the interface a device presents to a programmer where each I/O device appears in the physical address space of the machine as a memory address. The operating system reads and writes device registers to control the device. There are three types of device registers: status, command, and data. The status register provides information about the current state of the device (e.g., read), the command register issues a command to the device (e.g., writing a value to the register), and the data register is used to transfer data between the computer and the device. Device registers use bits to service three purposes: parameters provided by the CPU to the device, status bits provided by the device, and control bits set by the CPU to initiate operations. Figure 6.8 provides an example of the content of each of a device register’s bits.

The behavior of device registers is different from that of ordinary memory locations. For example, the start operation bit may always read as 0, and the bits may change from one status to another (i.e., the operation complete bit may change without being written by the CPU).

The OS uses the device register to communicate with an I/O device as follows:

1. The CPU writes device registers to start the operation.

2. The CPU polls the ready bit in the device register.

3. The device sets the ready bit when the operation is finished.

4. The CPU loads the buffer address into a device register before starting the operation to define where to copy data read from disk.

5. Fast storage media devices move data directly to/from physical memory via direct memory access (DMA); other devices require the intervention of the CPU via programmed I/O or interrupt initiated I/O.

6. Interrupts allow the CPU to do other work while devices are operating, and the OS figures out which device interrupted.

Pretty much anyone needs to print a document these days. It is therefore important to understand how an operating system enables us to print documents so easily. When an application wants to print a document, it hands that task off to the operating system. The operating system sends instructions to the printer’s drivers, which then send the correct signals to the printer.

Being able to access the network is another critical need in today’s business world. It is therefore important to understand how OSs facilitate access to the network and how they control it. When a user uses an application that interacts with the Internet, the application sends messages from process to process using the OS’s transport layer socket API. These messages are then split into packets within the operating system and eventually passed to a network interface card device, which transmits them to the Internet.

Figure 6.9 depicts the device register for a keyboard. Note that it has 2 bytes (16 bits). The data is in the first byte, and the second byte includes all zeros (i.e., from bit 8 to bit 15). When the user starts typing, the ready bit, which is bit number 15, sets to 1.