Another example is QNX, a real-time operating system for embedded systems. The QNX Neutrino microkernel provides services for message passing and process scheduling. It also handles low-level network communication and hardware interrupts. All other services in QNX are provided by standard processes that run outside the kernel in user mode.

Perhaps the best-known illustration of a microkernel operating system is Darwin, the kernel component of the macOS and iOS operating systems. Darwin, in fact, consists of two kernels, one of which is the Mach microkernel. We will cover the macOS and iOS systems in further detail in Section 2.8.5.1.

One benefit of the microkernel approach is that it makes extending the operating system easier. All new services are added to user space and consequently do not require modification of the kernel. When the kernel does have to be modified, the changes tend to be fewer, because the microkernel is a smaller kernel. The resulting operating system is easier to port from one hardware design to another. The microkernel also provides more security and reliability, since most services are running as user—rather than kernel—processes. If a service fails, the rest of the operating system remains untouched.

The main function of the microkernel is to provide communication between the client program and the various services that are also running in user space. Communication is provided through message passing, which was described in Section 2.3.3.5. For example, if the client program wishes to access a file, it must interact with the file server. The client program and service never interact directly. Rather, they communicate indirectly by exchanging messages with the microkernel.

We have already seen that the original UNIX system had a monolithic structure. As UNIX expanded, the kernel became large and difficult to manage. In the mid-1980s, researchers at Carnegie Mellon University developed an operating system called Mach that modularized the kernel using the microkernel approach. This method structures the operating system by removing all nonessential components from the kernel and implementing them as user-level programs that reside in separate address spaces. The result is a smaller kernel. There is little consensus regarding which services should remain in the kernel and which should be implemented in user space. Typically, however, microkernels provide minimal process and memory management, in addition to a communication facility. Figure 2.15 illustrates the architecture of a typical microkernel.

Layered systems have been successfully used in computer networks (such as TCP/IP) and web applications. Nevertheless, relatively few operating systems use a pure layered approach. One reason involves the challenges of appropriately defining the functionality of each layer. In addition, the overall performance of such systems is poor due to the overhead of requiring a user program to traverse through multiple layers to obtain an operating-system service. Some layering is common in contemporary operating systems, however. Generally, these systems have fewer layers with more functionality, providing most of the advantages of modularized code while avoiding the problems of layer definition and interaction.

Each layer is implemented only with operations provided by lower-level layers. A layer does not need to know how these operations are implemented; it needs to know only what these operations do. Hence, each layer hides the existence of certain data structures, operations, and hardware from higher-level layers.

The main advantage of the layered approach is simplicity of construction and debugging. The layers are selected so that each uses functions (operations) and services of only lower-level layers. This approach simplifies debugging and system verification. The first layer can be debugged without any concern for the rest of the system, because, by definition, it uses only the basic hardware (which is assumed correct) to implement its functions. Once the first layer is debugged, its correct functioning can be assumed while the second layer is debugged, and so on. If an error is found during the debugging of a particular layer, the error must be on that layer, because the layers below it are already debugged. Thus, the design and implementation of the system are simplified.

An operating-system layer is an implementation of an abstract object made up of data and the operations that can manipulate those data. A typical operating-system layer—say, layer M—consists of data structures and a set of functions that can be invoked by higher-level layers. Layer M, in turn, can invoke operations on lower-level layers.

The monolithic approach is often known as a tightly coupled system because changes to one part of the system can have wide-ranging effects on other parts. Alternatively, we could design a loosely coupled system. Such a system is divided into separate, smaller components that have specific and limited functionality. All these components together comprise the kernel. The advantage of this modular approach is that changes in one component affect only that component, and no others, allowing system implementers more freedom in creating and changing the inner workings of the system.

Despite the apparent simplicity of monolithic kernels, they are difficult to implement and extend. Monolithic kernels do have a distinct performance advantage, however: there is very little overhead in the system-call interface, and communication within the kernel is fast. Therefore, despite the drawbacks of monolithic kernels, their speed and efficiency explains why we still see evidence of this structure in the UNIX, Linux, and Windows operating systems.

The Linux operating system is based on UNIX and is structured similarly, as shown in Figure 2.13. Applications typically use the glibc standard C library when communicating with the system call interface to the kernel. The Linux kernel is monolithic in that it runs entirely in kernel mode in a single address space, but as we shall see in Section 2.8.4, it does have a modular design that allows the kernel to be modified during run time.

An example of such limited structuring is the original UNIX operating system, which consists of two separable parts: the kernel and the system programs. The kernel is further separated into a series of interfaces and device drivers, which have been added and expanded over the years as UNIX has evolved. We can view the traditional UNIX operating system as being layered to some extent, as shown in Figure 2.12. Everything below the system-call interface and above the physical hardware is the kernel. The kernel provides the file system, CPU scheduling, memory management, and other operating-system functions through system calls. Taken in sum, that is an enormous amount of functionality to be combined into one single address space.

The simplest structure for organizing an operating system is no structure at all. That is, place all of the functionality of the kernel into a single, static binary file that runs in a single address space. This approach—known as a monolithic structure—is a common technique for designing operating systems.

A system as large and complex as a modern operating system must be engineered carefully if it is to function properly and be modified easily. A common approach is to partition the task into small components, or modules, rather than have one single system. Each of these modules should be a well-defined portion of the system, with carefully defined interfaces and functions. You may use a similar approach when you structure your programs: rather than placing all of your code in the main() function, you instead separate logic into a number of functions, clearly articulate parameters and return values, and then call those functions from main().

As is true in other systems, major performance improvements in operating systems are more likely to be the result of better data structures and algorithms than of excellent assembly-language code. In addition, although operating systems are large, only a small amount of the code is critical to high performance; the interrupt handlers, I/O manager, memory manager, and CPU scheduler are probably the most critical routines. After the system is written and is working correctly, bottlenecks can be identified and can be refactored to operate more efficiently.

The advantages of using a higher-level language, or at least a systems-implementation language, for implementing operating systems are the same as those gained when the language is used for application programs: the code can be written faster, is more compact, and is easier to understand and debug. In addition, improvements in compiler technology will improve the generated code for the entire operating system by simple recompilation. Finally, an operating system is far easier to port to other hardware if it is written in a higher-level language. This is particularly important for operating systems that are intended to run on several different hardware systems, such as small embedded devices, Intel x86 systems, and ARM chips running on phones and tablets.

Early operating systems were written in assembly language. Now, most are written in higher-level languages such as C or C++, with small amounts of the system written in assembly language. In fact, more than one higher-level language is often used. The lowest levels of the kernel might be written in assembly language and C. Higher-level routines might be written in C and C++, and system libraries might be written in C++ or even higher-level languages. Android provides a nice example: its kernel is written mostly in C with some assembly language. Most Android system libraries are written in C or C++, and its application frameworks—which provide the developer interface to the system—are written mostly in Java. We cover Android's architecture in more detail in Section 2.8.5.2.

Policy decisions are important for all resource allocation. Whenever it is necessary to decide whether or not to allocate a resource, a policy decision must be made. Whenever the question is how rather than what, it is a mechanism that must be determined.

We can make a similar comparison between commercial and open-source operating systems. For instance, contrast Windows, discussed above, with Linux, an open-source operating system that runs on a wide range of computing devices and has been available for over 25 years. The “standard” Linux kernel has a specific CPU scheduling algorithm (covered in Section 5.7.1), which is a mechanism that supports a certain policy. However, anyone is free to modify or replace the scheduler to support a different policy.

The separation of policy and mechanism is important for flexibility. Policies are likely to change across places or over time. In the worst case, each change in policy would require a change in the underlying mechanism. A general mechanism flexible enough to work across a range of policies is preferable. A change in policy would then require redefinition of only certain parameters of the system. For instance, consider a mechanism for giving priority to certain types of programs over others. If the mechanism is properly separated from policy, it can be used either to support a policy decision that I/O-intensive programs should have priority over CPU-intensive ones or to support the opposite policy.

One important principle is the separation of policy from mechanism. Mechanisms determine how to do something; policies determine what will be done. For example, the timer construct (see Section 1.4.3) is a mechanism for ensuring CPU protection, but deciding how long the timer is to be set for a particular user is a policy decision.

Specifying and designing an operating system is a highly creative task. Although no textbook can tell you how to do it, general principles have been developed in the field of software engineering, and we turn now to a discussion of some of these principles.

There is, in short, no unique solution to the problem of defining the requirements for an operating system. The wide range of systems in existence shows that different requirements can result in a large variety of solutions for different environments. For example, the requirements for Wind River VxWorks, a real-time operating system for embedded systems, must have been substantially different from those for Windows Server, a large multiaccess operating system designed for enterprise applications.

The first problem in designing a system is to define goals and specifications. At the highest level, the design of the system will be affected by the choice of hardware and the type of system: traditional desktop/laptop, mobile, distributed, or real time.

In sum, all of these differences mean that unless an interpreter, RTE, or binary executable file is written for and compiled on a specific operating system on a specific CPU type (such as Intel x86 or ARMv8), the application will fail to run. Imagine the amount of work that is required for a program such as the Firefox browser to run on Windows, macOS, various Linux releases, iOS, and Android, sometimes on various CPU architectures.

Each operating system has a binary format for applications that dictates the layout of the header, instructions, and variables. Those components need to be at certain locations in specified structures within an executable file so the operating system can open the file and load the application for proper execution.

In theory, these three approaches seemingly provide simple solutions for developing applications that can run across different operating systems. However, the general lack of application mobility has several causes, all of which still make developing cross-platform applications a challenging task. At the application level, the libraries provided with the operating system contain APIs to provide features like GUI interfaces, and an application designed to call one set of APIs (say, those available from IOS on the Apple iPhone) will not work on an operating system that does not provide those APIs (such as Android). Other challenges exist at lower levels in the system, including the following.

In theory, these three approaches seemingly provide simple solutions for developing applications that can run across different operating systems. However, the general lack of application mobility has several causes, all of which still make developing cross-platform applications a challenging task. At the application level, the libraries provided with the operating system contain APIs to provide features like GUI interfaces, and an application designed to call one set of APIs (say, those available from IOS on the Apple iPhone) will not work on an operating system that does not provide those APIs (such as Android). Other challenges exist at lower levels in the system, including the following.

1. The application can be written in an interpreted language (such as Python or Ruby) that has an interpreter available for multiple operating systems. The interpreter reads each line of the source program, executes equivalent instructions on the native instruction set, and calls native operating system calls. Performance suffers relative to that for native applications, and the interpreter provides only a subset of each operating system's features, possibly limiting the feature sets of the associated applications.

Based on our earlier discussion, we can now see part of the problem—each operating system provides a unique set of system calls. System calls are part of the set of services provided by operating systems for use by applications. Even if system calls were somehow uniform, other barriers would make it difficult for us to execute application programs on different operating systems. But if you have used multiple operating systems, you may have used some of the same applications on them. How is that possible?

Object files and executable files typically have standard formats that include the compiled machine code and a symbol table containing metadata about functions and variables that are referenced in the program. For UNIX and Linux systems, this standard format is known as ELF (for Executable and Linkable Format). There are separate ELF formats for relocatable and executable files. One piece of information in the ELF file for executable files is the program's entry point, which contains the address of the first instruction to be executed when the program runs. Windows systems use the Portable Executable (PE) format, and macOS uses the Mach-O format.

Source files are compiled into object files that are designed to be loaded into any physical memory location, a format known as an relocatable object file. Next, the linker combines these relocatable object files into a single binary executable file. During the linking phase, other object files or libraries may be included as well, such as the standard C or math library (specified with the flag -lm).

The view of the operating system seen by most users is defined by the application and system programs, rather than by the actual system calls. Consider a user's PC. When a user's computer is running the macOS operating system, the user might see the GUI, featuring a mouse-and-windows interface. Alternatively, or even in one of the windows, the user might have a command-line UNIX shell. Both use the same set of system calls, but the system calls look different and act in different ways. Further confusing the user view, consider the user dual-booting from macOS into Windows. Now the same user on the same hardware has two entirely different interfaces and two sets of applications using the same physical resources. On the same hardware, then, a user can be exposed to multiple user interfaces sequentially or concurrently.

Program loading and execution. Once a program is assembled or compiled, it must be loaded into memory to be executed. The system may provide absolute loaders, relocatable loaders, linkage editors, and overlay loaders. Debugging systems for either higher-level languages or machine language are needed as well.

File management. These programs create, delete, copy, rename, print, list, and generally access and manipulate files and directories.

Another aspect of a modern system is its collection of system services. Recall Figure 1.1, which depicted the logical computer hierarchy. At the lowest level is hardware. Next is the operating system, then the system services, and finally the application programs. System services, also known as system utilities, provide a convenient environment for program development and execution. Some of them are simply user interfaces to system calls. Others are considerably more complex. They can be divided into these categories:

Typically, system calls providing protection include set_permission() and get_permission(), which manipulate the permission settings of resources such as files and disks. The allow_user() and deny_user() system calls specify whether particular users can—or cannot—be allowed access to certain resources. We cover protection in Chapter 17 and the much larger issue of security—which involves using protection against external threats—in Chapter 16.

Protection provides a mechanism for controlling access to the resources provided by a computer system. Historically, protection was a concern only on multiprogrammed computer systems with several users. However, with the advent of networking and the Internet, all computer systems, from servers to mobile handheld devices, must be concerned with protection.

Both of the models just discussed are common in operating systems, and most systems implement both. Message passing is useful for exchanging smaller amounts of data, because no conflicts need be avoided. It is also easier to implement than is shared memory for intercomputer communication. Shared memory allows maximum speed and convenience of communication, since it can be done at memory transfer speeds when it takes place within a computer. Problems exist, however, in the areas of protection and synchronization between the processes sharing memory.

There are two common models of interprocess communication: the message-passing model and the shared-memory model. In the message-passing model, the communicating processes exchange messages with one another to transfer information. Messages can be exchanged between the processes either directly or indirectly through a common mailbox. Before communication can take place, a connection must be opened. The name of the other communicator must be known, be it another process on the same system or a process on another computer connected by a communications network. Each computer in a network has a host name by which it is commonly known. A host also has a network identifier, such as an IP address. Similarly, each process has a process name, and this name is translated into an identifier by which the operating system can refer to the process. The get_hostid() and get_processid() system calls do this translation. The identifiers are then passed to the general-purpose open() and close() calls provided by the file system or to specific open_connection() and close_connection() system calls, depending on the system's model of communication. The recipient process usually must give its permission for communication to take place with an accept_connection() call. Most processes that will be receiving connections are special-purpose daemons, which are system programs provided for that purpose. They execute a wait_for_connection() call and are awakened when a connection is made. The source of the communication, known as the client, and the receiving daemon, known as a server, then exchange messages by using read_message() and write_message() system calls. The close_connection() call terminates the communication.

Many operating systems provide a time profile of a program to indicate the amount of time that the program executes at a particular location or set of locations. A time profile requires either a tracing facility or regular timer interrupts. At every occurrence of the timer interrupt, the value of the program counter is recorded. With sufficiently frequent timer interrupts, a statistical picture of the time spent on various parts of the program can be obtained.

Many system calls exist simply for the purpose of transferring information between the user program and the operating system. For example, most systems have a system call to return the current time() and date(). Other system calls may return information about the system, such as the version number of the operating system, the amount of free memory or disk space, and so on.

Once the device has been requested (and allocated to us), we can read(), write(), and (possibly) reposition() the device, just as we can with files. In fact, the similarity between I/O devices and files is so great that many operating systems, including UNIX, merge the two into a combined file–device structure. In this case, a set of system calls is used on both files and devices. Sometimes, I/O devices are identified by special file names, directory placement, or file attributes.

The various resources controlled by the operating system can be thought of as devices. Some of these devices are physical devices (for example, disk drives), while others can be thought of as abstract or virtual devices (for example, files). A system with multiple users may require us to first request() a device, to ensure exclusive use of it. After we are finished with the device, we release() it. These functions are similar to the open() and close() system calls for files. Other operating systems allow unmanaged access to devices. The hazard then is the potential for device contention and perhaps deadlock, which are described in Chapter 8.

We may need these same sets of operations for directories if we have a directory structure for organizing files in the file system. In addition, for either files or directories, we need to be able to determine the values of various attributes and perhaps to set them if necessary. File attributes include the file name, file type, protection codes, accounting information, and so on. At least two system calls, get_file_attributes() and set_file_attributes(), are required for this function. Some operating systems provide many more calls, such as calls for file move() and copy(). Others might provide an API that performs those operations using code and other system calls, and others might provide system programs to perform the tasks. If the system programs are callable by other programs, then each can be considered an API by other system programs.

The file system is discussed in more detail in Chapter 13 through Chapter 15. Here, we identify several common system calls dealing with files. We first need to be able to create() and delete() files. Either system call requires the name of the file and perhaps some of the file's attributes. Once the file is created, we need to open() it and to use it. We may also read(), write(), or reposition() (rewind or skip to the end of the file, for example). Finally, we need to close() the file, indicating that we are no longer using it.

There are so many facets of and variations in process control that we next use two examples—one involving a single-tasking system and the other a multitasking system—to clarify these concepts. The Arduino is a simple hardware platform consisting of a microcontroller along with input sensors that respond to a variety of events, such as changes to light, temperature, and barometric pressure, to just name a few. To write a program for the Arduino, we first write the program on a PC and then upload the compiled program (known as a sketch) from the PC to the Arduino's flash memory via a USB connection. The standard Arduino platform does not provide an operating system; instead, a small piece of software known as a boot loader loads the sketch into a specific region in the Arduino's memory

Quite often, two or more processes may share data. To ensure the integrity of the data being shared, operating systems often provide system calls allowing a process to lock shared data. Then, no other process can access the data until the lock is released. Typically, such system calls include acquire_lock() and release_lock().

A process executing one program may want to load() and execute() another program. This feature allows the command interpreter to execute a program as directed by, for example, a user command or the click of a mouse. An interesting question is where to return control when the loaded program terminates. This question is related to whether the existing program is lost, saved, or allowed to continue execution concurrently with the new program. If control returns to the existing program when the new program terminates, we must save the memory image of the existing program; thus, we have effectively created a mechanism for one program to call another program. If both programs continue concurrently, we have created a new process to be multiprogrammed. Often, there is a system call specifically for this purpose (create_process()).

A running program needs to be able to halt its execution either normally (end()) or abnormally (abort()). If a system call is made to terminate the currently running program abnormally, or if the program runs into a problem and causes an error trap, a dump of memory is sometimes taken and an error message generated. The dump is written to a special log file on disk and may be examined by a debugger—a system program designed to aid the programmer in finding and correcting errors, or bugs—to determine the cause of the problem. Under either normal or abnormal circumstances, the operating system must transfer control to the invoking command interpreter. The command interpreter then reads the next command. In an interactive system, the command interpreter simply continues with the next command; it is assumed that the user will issue an appropriate command to respond to any error. In a GUI system, a pop-up window might alert the user to the error and ask for guidance. Some systems may allow for special recovery actions in case an error occurs. If the program discovers an error in its input and wants to terminate abnormally, it may also want to define an error level. More severe errors can be indicated by a higher-level error parameter. It is then possible to combine normal and abnormal termination by defining a normal termination as an error at level 0. The command interpreter or a following program can use this error level to determine the next action automatically.

System calls can be grouped roughly into six major categories: process control, file management, device management, information maintenance, communications, and protection. Below, we briefly discuss the types of system calls that may be provided by an operating system. Most of these system calls support, or are supported by, concepts and functions that are discussed in later chapters. Figure 2.8 summarizes the types of system calls normally provided by an operating system. As mentioned, in this text, we normally refer to the system calls by generic names. Throughout the text, however, we provide examples of the actual counterparts to the system calls for UNIX, Linux, and Windows systems.

hree general methods are used to pass parameters to the operating system. The simplest approach is to pass the parameters in registers. In some cases, however, there may be more parameters than registers. In these cases, the parameters are generally stored in a block, or table, in memory, and the address of the block is passed as a parameter in a register (Figure 2.7). Linux uses a combination of these approaches.

System calls occur in different ways, depending on the computer in use. Often, more information is required than simply the identity of the desired system call. The exact type and amount of information vary according to the particular operating system and call. For example, to get input, we may need to specify the file or device to use as the source, as well as the address and length of the memory buffer into which the input should be read. Of course, the device or file and length may be implicit in the call.

The caller need know nothing about how the system call is implemented or what it does during execution. Rather, the caller need only obey the API and understand what the operating system will do as a result of the execution of that system call. Thus, most of the details of the operating-system interface are hidden from the programmer by the API and are managed by the RTE.

Another important factor in handling system calls is the run-time environment (RTE)—the full suite of software needed to execute applications written in a given programming language, including its compilers or interpreters as well as other software, such as libraries and loaders. The RTE provides a system-call interface that serves as the link to system calls made available by the operating system. The system-call interface intercepts function calls in the API and invokes the necessary system calls within the operating system. Typically, a number is associated with each system call, and the system-call interface maintains a table indexed according to these numbers

Why would an application programmer prefer programming according to an API rather than invoking actual system calls? There are several reasons for doing so. One benefit concerns program portability. An application programmer designing a program using an API can expect her program to compile and run on any system that supports the same API (although, in reality, architectural differences often make this more difficult than it may appear). Furthermore, actual system calls can often be more detailed and difficult to work with than the API available to an application programmer. Nevertheless, there often exists a strong correlation between a function in the API and its associated system call within the kernel. In fact, many of the POSIX and Windows APIs are similar to the native system calls provided by the UNIX, Linux, and Windows operating systems.

As you can see, even simple programs may make heavy use of the operating system. Frequently, systems execute thousands of system calls per second. Most programmers never see this level of detail, however. Typically, application developers design programs according to an application programming interface (API). The API specifies a set of functions that are available to an application programmer, including the parameters that are passed to each function and the return values the programmer can expect. Three of the most common APIs available to application programmers are the Windows API for Windows systems, the POSIX API for POSIX-based systems (which include virtually all versions of UNIX, Linux, and macOS), and the Java API for programs that run on the Java virtual machine

When both files are set up, we enter a loop that reads from the input file (a system call) and writes to the output file (another system call). Each read and write must return status information regarding various possible error conditions. On input, the program may find that the end of the file has been reached or that there was a hardware failure in the read (such as a parity error). The write operation may encounter various errors, depending on the output device (for example, no more available disk space).

Once the two file names have been obtained, the program must open the input file and create and open the output file. Each of these operations requires another system call. Possible error conditions for each system call must be handled. For example, when the program tries to open the input file, it may find that there is no file of that name or that the file is protected against access. In these cases, the program should output an error message (another sequence of system calls) and then terminate abnormally (another system call).

Before we discuss how an operating system makes system calls available, let's first use an example to illustrate how system calls are used: writing a simple program to read data from one file and copy them to another file. The first input that the program will need is the names of the two files: the input file and the output file. These names can be specified in many ways, depending on the operating-system design

System calls provide an interface to the services made available by an operating system. These calls are generally available as functions written in C and C++, although certain low-level tasks (for example, tasks where hardware must be accessed directly) may have to be written using assembly-language instructions.

Although there are apps that provide a command-line interface for iOS and Android mobile systems, they are rarely used. Instead, almost all users of mobile systems interact with their devices using the touch-screen interface. The user interface can vary from system to system and even from user to user within a system; however, it typically is substantially removed from the actual system structure. The design of a useful and intuitive user interface is therefore not a direct function of the operating system. In this book, we concentrate on the fundamental problems of providing adequate service to user programs. From the point of view of the operating system, we do not distinguish between user programs and system programs.

In contrast, most Windows users are happy to use the Windows GUI environment and almost never use the shell interface. Recent versions of the Windows operating system provide both a standard GUI for desktop and traditional laptops and a touch screen for tablets. The various changes undergone by the Macintosh operating systems also provide a nice study in contrast.

The choice of whether to use a command-line or GUI interface is mostly one of personal preference. System administrators who manage computers and power users who have deep knowledge of a system frequently use the command-line interface. For them, it is more efficient, giving them faster access to the activities they need to perform. Indeed, on some systems, only a subset of system functions is available via the GUI, leaving the less common tasks to those who are command-line knowledgeable

Because a either a command-line interface or a mouse-and-keyboard system is impractical for most mobile systems, smartphones and handheld tablet computers typically use a touch-screen interface. Here, users interact by making gestures on the touch screen—for example, pressing and swiping fingers across the screen. Although earlier smartphones included a physical keyboard, most smartphones and tablets now simulate a keyboard on the touch screen

Graphical user interfaces first appeared due in part to research taking place in the early 1970s at Xerox PARC research facility. The first GUI appeared on the Xerox Alto computer in 1973. However, graphical interfaces became more widespread with the advent of Apple Macintosh computers in the 1980s. The user interface for the Macintosh operating system has undergone various changes over the years, the most significant being the adoption of the Aqua interface that appeared with macOS. Microsoft's first version of Windows—Version 1.0—was based on the addition of a GUI interface to the MS-DOS operating system

In one approach, the command interpreter itself contains the code to execute the command. For example, a command to delete a file may cause the command interpreter to jump to a section of its code that sets up the parameters and makes the appropriate system call. In this case, the number of commands that can be given determines the size of the command interpreter, since each command requires its own implementing code.

The main function of the command interpreter is to get and execute the next user-specified command. Many of the commands given at this level manipulate files: create, delete, list, print, copy, execute, and so on. The various shells available on UNIX systems operate in this way. These commands can be implemented in two general ways.

Most operating systems, including Linux, UNIX, and Windows, treat the command interpreter as a special program that is running when a process is initiated or when a user first logs on (on interactive systems). On systems with multiple command interpreters to choose from, the interpreters are known as shells. For example, on UNIX and Linux systems, a user may choose among several different shells, including the C shell, Bourne-Again shell, Korn shell, and others

Protection and security. The owners of information stored in a multiuser or networked computer system may want to control use of that information. When several separate processes execute concurrently, it should not be possible for one process to interfere with the others or with the operating system itself. Protection involves ensuring that all access to system resources is controlled. Security of the system from outsiders is also important.

Logging. We want to keep track of which programs use how much and what kinds of computer resources. This record keeping may be used for accounting (so that users can be billed) or simply for accumulating usage statistics. Usage statistics may be a valuable tool for system administrators who wish to reconfigure the system to improve computing services.

Resource allocation. When there are multiple processes running at the same time, resources must be allocated to each of them. The operating system manages many different types of resources. Some (such as CPU cycles, main memory, and file storage) may have special allocation code, whereas others (such as I/O devices) may have much more general request and release code.

Error detection. The operating system needs to be detecting and correcting errors constantly. Errors may occur in the CPU and memory hardware (such as a memory error or a power failure), in I/O devices (such as a parity error on disk, a connection failure on a network, or lack of paper in the printer), and in the user program (such as an arithmetic overflow or an attempt to access an illegal memory location).

Speaking: Explain how igneous, sedimentary, and metamorphic rocks differ to a partner using compare and contrast language

If we don’t challenge the status quo regarding the improvement of educational opportunities for multilingual learners through equitable practices and policies, who will?

Additionally, multilingual learners may face challenges when teachers use “tricky” language, such as idiomatic expressions (e.g., “learn this by heart” or “the assignment is a walk in the park”

acknowledging the implicit and explicit ideologies and power structures inherent in language, (2) understanding that the use of such language, even unintentionally, can and does legitimate and reproduce social inequalities, and (3) striving to become agents of long-term change in society

Getting to know your students should begin on the first day—or even earlier, if possible, by meeting them (and their families) before the academic year starts—and should continue throughout the year

“I’m not sure what the problem is. These kids can’t speak well in English or Spanish. Rather than teaching them both languages, we should just focus on English

So the economy is about work: organizing it, doing it, and dividing up andmaking use of its final output. And in our work, one way or another, we alwayswork (directly or indirectly) with other people.The link between the economy and society goes two ways. The economy is afundamentally social arena. But society as a whole depends strongly on the stateof the economy. Politics, culture, religion, and international affairs are all deeplyinfluenced by the progress of the economy. Governments are re-elected or turfedfrom office depending on the state of the economy. Family life is organized aroundthe demands of work (both inside and outside the home). Being able to comfortablysupport oneself and one’s family is a central determinant of happiness.

land and driven into cities, where they suffered horrendous exploitation andconditions that would be considered intolerable today: seven-day working weeks,twelve-hour working days, child labour, frequent injury, early death. Vast profitswere earned by the new class of capitalists, most of which they ploughed backinto new investment, technology, and growth – but some of which they used tofinance their own luxurious consumption. The early capitalist societies were not atall democratic: the right to vote was limited to property owners, and basic rights tospeak out and organize (including to organize unions) were routinely (and oftenviolently) trampled.

Instead, economists refer simply to “the economy” – as if there is only one kindof economy, and hence no need to name or define it. This is wrong. As we havealready seen, “the economy” is simply where people work to produce the things weneed and want. There are different ways to organize that work. Capitalism is justone of them.

Instead, economists refer simply to “the economy” – as if there is only one kindof economy, and hence no need to name or define it. This is wrong. As we havealready seen, “the economy” is simply where people work to produce the things weneed and want. There are different ways to organize that work. Capitalism is justone of them

An economy in which private, profit-seeking companies undertake mostproduction, and in which wage-earning employees do most of the work, is acapitalist economy. We will see that these twin features (profit-driven productionand wage labour) create particular patterns and relationships, which in turn shapethe overall functioning of capitalism as a system.Any economy driven by these two features – production for profit and wagelabour – tends to replicate the following trends and patterns, over and over again:• Fierce competition between private companies for markets, sales, and profit.• Innovation, as companies constantly experiment with new technologies,new products, and new forms of organization – in order to succeed in thatcompetition.• An inherent tendency to growth, resulting from the desire of each individualcompany to make more profit.• Deep inequality between those who own successful companies, and the restof society who do not own companies.• A general conflict of interest between those who work for wages, and theemployers who hire them.• Economic cycles or “rollercoasters,” with periods of strong growth followedby periods of stagnation or depression; sometimes these cycles even producedramatic economic and social crises

Conventionally trained economists take it as a proven fact that free tradebetween two countries always makes both sides better off. People who questionor oppose free trade – trade unionists, social activists, nationalists – must eitherbe acting from ignorance, or else are pursuing some narrow vested interest thatconflicts with the broader good. These troublesome people should be lectured to(and economists love nothing better than expounding their beautiful theory ofcomparative advantage*), or simply ignored. And that’s exactly what mostgovernments do. (Ironically, even some conventional economists now recognizethat traditional free trade theory is wrong, for many reasons – some of whichwe’ll discuss in Chapter 22 of this book. But that hasn’t affected the profession’snear-religious devotion to free trade policies.)

At its simplest, the “economy” simply consists of all the work that human beingsperform, in order to produce the things we need and use in our lives. (By work,we mean all productive human activity, not just employment; we’ll discuss thatdistinction later.) We need to organize and perform our work (economists call thatproduction). And then we need to divide up the fruits of our work (economistscall that distribution), and use it

Economics is the study of human economic behaviour: the production anddistribution of the goods and services we need and want. Hence, economicsis a social science, not a physical science.

Never trust an economist with your job

But quite apart from whether you think capitalism is good or bad, capitalism issomething we must study. It’s the economy we live in, the economy we know. Andthe more ordinary people understand about capitalism

I alsobelieve that it is ultimately possible to build an alternative economic system guideddirectly by our desire to improve the human condition, rather than by a hunger forprivate profit. (Exactly what that alternative system would look like, however, isnot at all clear today.) We’ll consider these criticisms of capitalism, and alternativevisions, in the last chapters of this book

Unfortunately, most professional economists don’t think about economics inthis common-sense, grass-roots context. To the contrary, they tend to adopt arather superior attitude in their dealings with the untrained masses. They invokecomplicated technical mumbo-jumbo – usually utterly unnecessary to theirarguments – to make their case. They claim to know what’s good for the people

Most production of goods and services is undertaken by privately-ownedcompanies, which produce and sell their output in hopes of making a profit.This is called production for profit.2. Most work in the economy is performed by people who do not own theircompany or their output, but are hired by someone else to work in return for amoney wage or salary. This is called wage labour.

economics is inherently a social subject. It’s not just technical forces liketechnology and productivity that matter. It’s also the interactions and relationshipsbetween people that make the economy go around.

Although the concept of self-fulfilling prophecies was originally developed to be applied to social inequality and discrimination, it has since been applied in many other contexts, including interpersonal communication. This research has found that some people are chronically insecure, meaning they are very concerned about being accepted by others but constantly feel that other people will dislike them.

Americans "Cinderella" is not a story of rags to riches, but rather riches recovered; not poor girl into princess but rather rich girl (or princess) rescued from improper or wicked enslavement; not suffering Griselda enduring but shrewd and practical girl persevering and winning a share of the power. It is really a story that is about "the stripping away of the disguise that conceals the soul from the eyes of others "

Since then, America's Cinderella has been a coy, helpless dreamer, a "nice" girl who awaits her rescue with patience and a song.

The idea in the West is to make a product which will sell well.

But each person’s self-concept is also influenced by context, meaning we think differently about ourselves depending on the situation we are in. In some situations, personal characteristics, such as our abilities, personality, and other distinguishing features, will best describe who we are.

I’m sure you have a family member, friend, or coworker with whom you have ideological or political differences.

In the conflict thus far, success has been on our side

our social fabric is firmly planted; and I cannot permit myself to doubt the ultimate success of a full recognition of this principle throughout the civilized and enlightened world.

The architect, in the construction of buildings, lays the foundation with the proper material-the granite-then comes the brick or the marble. The substratum of our society is made of the material fitted by nature for it, and by experience we know that it is the best, not only for the superior but for the inferior race, that it should be so. It is, indeed, in conformity with the Creator.

We also organize information that we take in based on difference. In this case, we assume that the item that looks or acts different from the rest doesn’t belong with the group. Perceptual errors involving people and assumptions of difference can be especially awkward, if not offensive.

In answering this letter, please state if there would be any safety for my Milly and Jane, who are now grown up, and both good-looking girls. You know how it was with poor Matilda and Catherine. I would rather stay here and starve—and die, if it come to that—than have my girls brought to shame by the violence and wickedness of their young masters. You will also please state if there has been any schools opened for the colored children in your neighborhood. The great desire of my life now is to give my children an education, and have them form virtuous habits.

He knows that his breakfast depends upon workerson the coffee plantations of Brazil, the citrus groves ofFlorida, the sugar fields of Cuba, the wheat farms ofthe Dakotas, the dairies of New York; that it has beenassembled by ships, railroads, and trucks, has beencooked with coal from Pennsylvania in utensils madeof aluminum, china, steel, and glass.

yourself mine the metals, process them, combine them, andshape them. To mine the metals, you would have to be ableto locate them. You would need machinery, which you wouldhave to build yourself

“Filling in Frameworks” wrestles with the misconceptionthat economics is a science. This section looks at the difficul-ties that economists face in trying to adopt scientific methods. Isuggest that economics differs from the natural sciences in thatwe have to rely much less on verifiable hypotheses and muchmore on hard-to-verify interpretative frameworks. Economicanalysis is a challenge, because judging interpretive frame-works is actually harder than verifying scientific hypotheses.

“Instructions and Incentives” deals with the misconceptionthat economic activity is directed by planners. This sectionexplains that although people within a firm are guided totasks through instruction from managers, the economy as awhole is not coordinated that way. Instead, the price systemfunctions as the coordination mechanism.

Scarcity and choice are certainly important concepts, butmaking them the central focus can lead to economic analysisthat is simplistic and mechanistic. In fact, the approach to eco-nomics that took hold after World War II treats the economyas a machine governed by equations. Textbooks using thatapproach purport to offer a repair manual, with policy toolsto fix the economic machine when something goes wrong.The mechanistic metaphor is inappropriate and even dan-gerous. A better metaphor would be that of a rainforest. Theeconomy is a complex, evolving system

Even more strikingis the fact that almost everything you consume is somethingyou could not possibly produce. Your daily life depends on thecooperation of hundreds of millions of other people

If trade entails trust among strangers, then financial inter-mediation entails trust over time. If people lose trust infinancial intermediaries, then financial intermediation candecline precipitously. That sharp decline can have a broadeffect on the structure of production in the economy

Instead of headlines, the “crawl” on the TV lists all of thetasks and people needed to produce your breakfast. Your cerealwas manufactured in a factory that had a variety of workersand many machines. People had to manage the factory. Orga-nization of the firm required many functions in finance andadministration. First, however, people had to build the factory

When patterns of specialization become unsustainable, theindividuals affected can face periods of unemployment. Theyare like soldiers waiting for new orders, except that the orderscome not from a commanding general but from the decen-tralized actions of many entrepreneurs testing ideas in searchof profit.

Look at the list of ingredients in the cereal. Those ingre-dients had to be refined and shipped to the cereal manu-facturer. Again, those processes required many machines,which in turn had to be manufactured. The cereal grainsand other ingredients had to be grown, harvested, and pro-cessed. Machines were involved in those processes, and thosemachines had to be manufactured.

But in the last analysis, it is the people themselves who are filed away through the lack of creativity, transformation, and knowledge in this (at best) misguided system

This is the "banking" concept of education, in which the scope of action allowed to the students extends only as far as receiving, filing, and storing the deposits.

Words are emptied of their concreteness and become a hollow, alienated, and alienating verbosity.

The contents, whether values or empirical dimensions of reality, tend in the process of being narrated to become lifeless and petrified.

Acceptable Use of AI in this Course

1 final reflection worth 10 points

Define important concepts such as: authority, peer review, bias, point of view, editorial process, purpose, audience, information privilege and more.

Generative AI models are trained on vast amounts of internet data. This data, while rich in information, contains both accurate and inaccurate content, as well as societal and cultural biases. Since these models mimic patterns in their training data without discerning truth, they can reproduce any

. These generative AI biases can have real-world consequences. For instance, adding biased generative AI to “virtual sketch artist” software used by police departments could “put already over-targeted populations at an even increased risk of harm ranging from physical injury to unlawful imprisonment”

trial and error

herefore of making a provi-sional determination of the absolute values ofthe charges carried by the drop

supported by evidence from many sourcesthat all electrical charges,

The error in a single ob-servation need not exceed one third of oneper cent.

mean free path

Others have been tempted to argue that implicit bias is overrated (maybe even justified) and that minorities simply need to toughen up.

Take the example of the Müller-Lyer illusion. Your task is to decide whether line A or line B is the longer one.

The act of study demands a sense of modesty.

In fact, a book reflects its author’s confrontation with the world. It expresses thisconfrontation.

This critical attitude is precisely what“banking education” does not engender.

f the reader is transformed into a “vessel” filled by extracts from an internalizedtext

In our study, developers accepted around 30% of GitHub Copilot’s suggestions

In our survey, respondents most commonly reported using the time they save with AI coding tools to design systems, collaborate, and learn.

nearly all of the survey participants reported using AI coding tools both outside of work or at work at some point

Easier to work with new programming languages, and understand existing codebases.

It is not often that educators are permitted to strike because they are employed by the state and are considered vital to public service.

Therefore, some states have begun to change tenure laws to adhere to the accountability requirements stipulated by the U.S. Department of Education as it relates to teacher evaluation and student achievement.

(CCSSO, InTASC Standard #9, 2013).

“There has never been a more important time for children to become storytellers, and there have never been so many ways for them to share their stories” (p. 3). Our students and their stories should be an essential part of our teaching. As educators, we need to encourage students to tell their stories and help build community. Each shared story has the potential of teaching us.

When students’ lives are taken off the margins and placed in the curriculum, they don’t feel the same need to put down someone else” (p. 7). Students need to feel that their voices matter, that they have a story to contribute or share and that their stories are a rich part of the curriculum

Students who search their memories for details about an event as they are telling it orally will later find those details easier to capture in writing

“there has probably never been a human society in which people did not tell stories”

one critical problem

Having said that, always remember that artificial intelligence is only an assistant; an executive’s value comes from his or her own intelligence.

That which diverges from the run-of-the-mill is not only valuable; it is indeed becoming invaluable in the age of AI.

Our priority should be to discover and innovate, not imitate neural networks.

We can use AI for unengaging and repetitive tasks, but we should also remember that humaneness is the key to creativity.

has almost innumerable advantages over the human mind

The Egyptian empires lasted for nearly 2300 years before being conquered, in succession, by the Assyrians, Persians, and Greeks between about 700 BCE and 332 BCE.

Farming developed in a number of different parts of the ancient world, before the beginning of recorded history. That means it’s very difficult for historians to describe early agricultural societies in as much detail as we’d like. Also, because there are none of the written records historians typically use to understand the past, we rely to a much greater extent on archaeologists, anthropologists, and other specialists for the data that informs our histories. And because the science supporting these fields has advanced rapidly in recent years, our understanding of this prehistoric period has also changed – sometimes abruptly.

Temperature also affected the behavioural preferences of the infauna associated with mussels. Polychaetes, crustaceans, and molluscs altered their behaviour to colonise the habitat created by one species of mussel to another. This altered behavioural preference of infauna can be driven by habitat-specific cues and the ability of infauna to make habitat choices

After the 4-week acclimation period, the mussels were defaunated by carefully removing all infauna and separating adult mussels (>1 cm) into 10-cm-diameter clumps (Cole, 2010).

The outdoor experiment was performed in a purpose-built facility (Pereira et al., 2019) at the Sydney Institute of Marine Science (SIMS), Chowder Bay, Sydney Harbour, New South Wales, Australia. The experiment was performed during the summer peak recruitment period of marine invertebrates in Sydney Harbour.

Previous studies have shown that the loss of a biogenic habitat in an ecosystem can be functionally replaced (or the loss of function is slowed to some extent) by another habitat-forming organism (Nagelkerken et al., 2016; Sunday et al., 2017).

For example, under acidification, fleshy seaweeds outcompete calcareous species

Molluscs actively chose to colonise T. hirsuta and actively avoided M. galloprovincialis, regardless of warming or pCO2 levels (Table 1).

The native mussel T. hirsuta grew more under warming (Fig. 1; ANOVA Species × Temperature F1,32 = 6.13, P < 0.05; Supplementary Table 2). In contrast, M. galloprovincialis grew the same at ambient and elevated temperatures (Fig. 1; Supplementary Table 2). There was no effect of elevated pCO2 on growth in either of the mussel species (ANOVA CO2 F1,32 = 0.53, P > 0.05; Supplementary Table 2).

All work turned in must adhere to the following format.

All assignments for this course must be written and submitted directly in Google Docs

It places too high of aburden on me to investigate and evaluate AI possible AI usage instead of focusingon the important educational aspects of the course.

The speculative bubble created by railroad financing burst in the Panic of 1873, which began a period called the Long Depression that lasted until nearly the end of the century and was so bad that before the Great Depression of the 1930s the period was known simply as “The Depression”.

Nearly 100 Americans died in “The Great Upheaval.” Workers destroyed nearly $40 million worth of property. The strike galvanized the country. It convinced laborers of the need for institutionalized unions, persuaded businesses of the need for even greater political influence and government aid, and foretold a half century of labor conflict in the United States.

not only can such freedom be granted without prejudice to the public peace, but also, that without such freedom, piety cannot flourish nor the public peace be secure.

How many evils spring from luxury, envy, avarice, drunkenness, and the like, yet these are tolerated

but what question was ever settled so wisely that no abuses could possibly spring therefrom?

men would daily be thinking one thing and saying another”—a practice that will weave deceit and hypocrisy into the social fabric, thereby permitting “the avaricious, the flatterers, and other numskulls” to rise to the top.

disastrous results

he “justly cedes the right of free action, though not of free reason.”

Unlike many earlier defenders of toleration, he did not exclude atheists, Jews, Catholics, and the like.

The sovereign’s obligation to respect the liberty of his subjects is solely a matter of self-​interest; to mistreat subjects is bound to generate resentment and possibly seditious tendencies, and those sentiments, in turn, will render the sovereign’s authority less secure than it would otherwise be

I mean the pace of the finished film, how the edits speed up or slow down to serve the story, producing a kind of rhythm to the edit.

ther ways cinema manipulates time include sequences like flashbacks and flashforwards. Filmmakers use these when they want to show events from a character’s past, or foreshadow what’s coming in the future.

The most obvious example of this is the ellipsis, an edit that slices out time or events we don’t need to see to follow the story. Imagine a scene where a car pulls up in front of a house, then cuts to a woman at the door ringing the doorbell. We don’t need to spend the screen time watching her shut off the car, climb out, shut and lock the door, and walk all the way up to the house.

He wants you to feel the terror of those peasants being massacred by the troops, even if you don’t completely understand the geography or linear sequence of events. That’s the power of the montage as Eisenstein used it: A collage of moving images designed to create an emotional effect rather than a logical narrative sequence.

he audience was projecting their own emotion and meaning onto the actor’s expression because of the juxtaposition of the other images. This phenomenon – how we derive more meaning from the juxtaposition of two shots than from any single shot in isolation – became known as The Kuleshov Effect.

ilm editing and how it worked on an audience. He had a hunch that the power of cinema was not found in any one shot, but in the juxtaposition of shots. So, he performed an experiment. He cut together a short film and showed it to audiences in 1918. Here’s the film:

but it is the juxtaposition of that word (or shot) in a sentence (or scene) that gives it its full power to communicate. As such, editing is fundamental to how cinema communicates with an audience.

The filmmakers behind Deadpool (2016), for example, shot 555 hours of raw footage for a final film of just 108 minutes. That’s a shooting ratio of 308:1. It would take 40 hours a week for 14 weeks just to watch all of the raw footage, much less select and arrange it all into an edited film![2]

When the screenwriter hands the script off to the director, it is no longer a literary document, it’s a blueprint for a much larger, more complex creation. The production process is essentially an act of translation, taking all of those words on the page and turning them into shots, scenes and sequences.

While navigating through the text, you’ll notice that the major part of the text you’re working within is identified at the top of the page

The special effects make-up for the gory bits of your favorite horror films can sometimes take center stage.

The dataset was normalized to 10000 counts per cell, Log1p transformed and filtered to contain2000 highly variable genes. The first important observation is that state-of-the-art approaches,except CPM

you will not benefit fully from this class.

flatten your writing

risking your academic integrity.

drought and global warming

don’t care about your education