Summary of Hardware and Software Concepts.

Summary

An operating system is primarily a resource manager, so the design of an operating system is intimately tied to the hardware and software resources the operating system must manage.

A PCB is a hardware component that provides electrical connections between devices at various locations on the PCB. The mainboard is the PCB to which devices such as processors and main memory are attached.

A processor is a hardware component that executes a stream of machine-language instructions.A CPU is a processor that executes the instructions of a program; a coprocessor executes special-purpose instructions (such as graphics or audio) efficiently. In this book, we use the term “processor” to refer to a CPU. Registers are high-speed memory located on a processor that hold data for immediate use by the processor. Before a processor can operate on data, the data must be placed in registers. The instruction length is the size of a machine-language instruction; some processors support multiple instruction lengths.

Computer time is measured in cycles; each cycle is one complete oscillation of an electrical signal provided by the system clock generator. Processor speeds are often measured in GHz (billions of cycles per second).

The memory hierarchy is a scheme for categorizing memory, which places the fastest and most expensive memory at the top and the slowest and least expensive memory at the bottom. It has a steep, pyramidal shape in which register memory occupies the hierarchy’s top level, followed by L1 cache memory, L2 cache memory, main memory, secondary storage and tertiary storage.A system’s main memory is the lowest data store in the memory hierarchy that the processor can reference directly. Main memory is volatile, meaning it loses its contents when the system loses power. Secondary storage, such as hard disks, CDs, DVDs and floppy disks, persistently store large quantities of data at a low cost per unit storage.

A bus is a collection of thin electrical connections called traces that transport information between hardware devices.A port is a bus connecting two devices. I/O channels are special-purpose components devoted to handling I/O independently of the computer system’s main processors.

A peripheral device is hardware that is not required for a computer to execute instructions. Printers, scanners and mice are peripheral devices; processors and main memory are not.

Some hardware exists specifically to improve performance and simplify the design of operating systems. Computer systems generally have several different execution states. For user applications, the subset of instructions the user may execute in user mode precludes, for example, the direct execution of input/output instructions. The operating system ordinarily executes with most trusted user status in kernel mode; in kernel mode, a processor may execute privileged instructions and access resources to perform tasks on behalf of processes. Memory protection, which prevents processes from accessing memory that has not been assigned to them (such as other users’ memory and the operating system’s memory), is implemented using processor registers that can be modified only by privileged instructions. Most devices send a signal called an interrupt to the processor when an event occurs. The operating sys- tem can respond to a change in device status by notifying processes that are waiting on such events.

Programmed I/O (PIO) is a technique whereby a processor idles while data is transferred between a device and main memory. By contrast, direct memory access (DMA) enables devices and controllers to transfer blocks of data to and from main memory directly, which frees the processor to execute software instructions.

Interrupts allow hardware to send signals to the processor, which notifies the operating system of the interrupt. The operating system decides what action to take in response to the interrupt.

A computer contains several types of clocks and timers. An interval timer is useful for preventing a process from monopolizing a processor. After a designated interval, the timer generates an interrupt to gain the attention of the processor; as a result of this interrupt, the processor might then be assigned to another application. The time-of- day clock keeps track of “wall clock time.”

Bootstrapping is the process of loading initial operating system components into memory. This process is per- formed by a computer’s BIOS.

Plug-and-play technology allows operating systems to configure newly installed hardware without user interaction. To support plug-and-play, a hardware device must uniquely identify itself to the operating system, communicate with the operating system to indicate the resources and services the device requires to function properly, and identify the driver that supports the device and allows soft- ware to configure the device (e.g., assign the device to a DMA channel).

Caches are relatively fast memory designed to increase program execution speed by maintaining copies of data that will be accessed soon. Examples of caches are the L1 and L2 processor caches and the main memory cache for hard disks.

A buffer is a temporary storage area that holds data during I/O transfers. Buffer memory is used primarily to coordinate communication between devices operating at different speeds. Buffers can store data for asynchronous processing, coordinate input/output between devices operating at different speeds or allow signals to be delivered asynchronously.

Spooling is a buffering technique in which an inter- mediate device, such as a disk, is interposed between a process and a low-speed or buffer-limited I/O device. Spooling allows processes to request operations from a peripheral device without requiring that the device be ready to service the request.

Three types of programming languages are machine, assembly and high-level languages. Machine languages consist of streams of numbers (ultimately reduced to 1s and 0s) that instruct computers how to perform their most elementary operations. A computer can understand only its own machine language. Assembly languages represent machine-language instructions using English-like abbreviations. High-level languages enable programmers to write instructions that look similar to everyday English and that contain common mathematical notations. High-level languages accomplish more substantial tasks with fewer statements, but require translator programs called compilers to convert high-level language programs into machine language. C, C++, Java and C# are examples of high-level languages.

Today, high-level languages tend to fall into two types, structured programming languages and object-oriented programming languages. Structured programming is a disciplined approach to creating programs that are clear, correct and easy to modify. Pascal and Fortran are structured programming languages. Object-oriented programs focus on manipulating objects (nouns) to create reusable software that is easy to modify and understand. C++, C# and Java are object-oriented programming languages.

APIs allow a program to request services from the operating system. Programs call API functions, which may access the operating system by making system calls.

Before a high-level-language program can execute, it must be translated into machine code and loaded into memory. Compilers transform high-level-language code into machine code. Linkers assign relative addresses to different program or data units and resolve external references between subprograms. Loaders convert these addresses into physical addresses and place the program or data units into main memory.

Most computers contain firmware, which specifies software instructions but is physically part of a hardware device. Most firmware is programmed with microprogramming, which is a layer of programming below a computer’s machine language.

Middleware enables communication among multiple processes running on one or more computers across a net- work. Middleware facilitates heterogeneous distributed systems and simplifies application programming.