Hardware-Software Interface

Most computer software is not written at the processor instruction level in assembly language. Almost all the applications we work with daily are written in one high-level programming language or another, relying on pre-existing libraries of capabilities that the application programmers built upon during the software development process. Practical programming environments, consisting of high-level languages and their associated libraries, offer many services, including disk input/output (I/O), network communication, and interactions with users, all easily accessible from program code.

This chapter describes the software layers that implement these features, beginning with processor instructions within device drivers. Several key aspects of operating systems will be covered in this chapter, including booting, process scheduling, multithreading, and multiprocessing.

After completing this chapter, you will understand the services provided by operating...

Technical requirements

Files for this chapter, including answers to the exercises, are available at https://github.com/PacktPublishing/Modern-Computer-Architecture-and-Organization-Second-Edition.

Device drivers

A device driver provides a standardized interface for software applications to interact with a category of peripheral devices such as disk storage. This avoids the need for the application developer to understand and implement all of the technical details required for the proper operation of each device type. Device drivers also manage the coordination needed when applications written by different developers attempt to access the same device at the same time. Most device drivers allow multiple simultaneously executing applications to interact with multiple instances of associated peripherals in a secure and efficient manner.

At the lowest level, the device driver code provides software instructions that manage communication interactions with the peripheral, including handling interrupts generated by device service requests. A device driver controls the operation of hardware resources in the processor, in the peripheral device, and in other system components such...

BIOS

A computer’s BIOS contains code that first executes at system startup. In the early days of personal computers, the BIOS provided a set of programming interfaces that abstracted the details of peripheral interfaces such as keyboards and video displays.

In modern PCs, the BIOS performs system testing and peripheral device configuration during startup. After that process has been completed, the processor (under software control) interacts with peripheral devices directly without further intervention by the BIOS.

Early PCs stored the BIOS code in a read-only memory (ROM) chip on the motherboard. This code was permanently programmed and could not be altered. Modern motherboards generally store the motherboard BIOS in a reprogrammable flash memory device. This allows BIOS updates to be installed to add new features or to fix problems found in earlier firmware versions. The process of updating the BIOS is commonly known as flashing the BIOS.

One downside of BIOS...

The boot process

The procedure for booting a system image varies depending on the partition style of the mass storage device containing the image and the security features enforced during boot. The goal of the boot process is to bring up the system following power application and initialize the operating system, leaving the computer in a known state and ready to perform useful work.

Beginning in the early 1980s, the standard disk partition format was called the master boot record (MBR). An MBR partition has a boot sector located at the logical beginning of its storage space. The MBR boot sector contains information describing the device’s logical partitions. Each partition contains a filesystem organized as a tree structure of directories and the files within them.

Due to the fixed format of MBR data structures, an MBR storage device can contain a maximum of four logical partitions and can be no larger than 2 TB in size, equal to 2³² 512-byte data sectors. These limits...

Operating systems

An operating system is a multilayer suite of software that provides an environment in which applications perform useful functions such as word processing, placing telephone calls, or managing the operation of a car engine. Applications running under control of the operating system execute algorithms implemented as processor instruction sequences and perform I/O interactions with peripheral devices as required to complete their tasks.

The operating system provides standardized programming interfaces that application developers use to access system resources such as processor execution threads, disk files, input from a keyboard or other peripherals, and output to devices such as a computer screen or instruments on an automotive dashboard.

Operating systems can be broadly categorized as real-time or non-real-time:

• A real-time operating system (RTOS) provides features to ensure that responses to inputs occur within a defined time limit.

  Processors...

Processes and threads

Many, but not all, operating systems support the concept of multithreaded execution. A thread is a sequence of program instructions that logically executes in isolation from other threads. An operating system running on a single-core processor creates the illusion of multiple simultaneously running threads by performing time-slicing.

In time-slicing, an operating system scheduler grants each ready-to-run thread a period of time in which to execute. As a thread’s execution interval ends, the scheduler interrupts the running thread and continues executing the next thread in its queue. In this manner, the scheduler gives each thread a bit of time to run before going back to the beginning of the list and starting over again.

In operating systems capable of supporting multiple runnable programs simultaneously, the term process refers to a running instance of a computer program. The system allocates resources, such as memory and membership in the scheduler...

Multiprocessing

A multiprocessing computer contains two or more processors that simultaneously execute sequences of instructions. The processors in such a system typically share access to system resources, such as main memory and peripheral devices. The processors in a multiprocessing system may be of the same architecture, or individual processors may be of different architectures to support unique system requirements. Systems in which all processors are treated as equal are referred to as symmetric multiprocessing systems. Devices that contain multiple processors within a single integrated circuit package are called multi-core processors.

At the level of the operating system scheduler, a symmetric multiprocessing environment simply provides more processors for use in thread scheduling. In such systems, the scheduler treats additional processors as resources when assigning threads for execution.

In a well-designed symmetric multiprocessing system, throughput can approach...

Summary

This chapter began with an overview of device drivers, including details on the instruction sequences used by driver code to read from and write to a simple I/O device: the PC parallel port. We continued with a discussion of the legacy BIOS and the newer UEFI, which provide the code that first executes on PC power-up, performs device testing and initialization, and initiates loading of the operating system. We saw how the trusted boot process can ensure that only authorized and unmodified code can be permitted to execute during system startup.

We continued with a description of some of the fundamental elements of operating systems, including processes, threads, and the scheduler. Various scheduling algorithms used in past computers and the systems of today were introduced. We examined the output of tools available in Linux and Windows that present information about running processes.

The chapter concluded with a discussion of multiprocessing and its performance impact...

Exercises

1. Restart your computer and enter the BIOS or UEFI settings. Examine each of the menus available in this environment. Does your computer have BIOS or does it use UEFI? Does your motherboard support overclocking? When you are finished, be sure to select the option to quit without saving changes unless you are absolutely certain you want to make changes.
2. Run the appropriate command on your computer to display the currently running processes. What is the PID of the process you are using to run this command?

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