Here is how the Operating System Works
When the power to a computer is turned on, the first program that runs is usually a set of instructions kept in the computer’s Read-Only Memory (ROM) that examines the system hardware to make sure everything is functioning properly. This Power-On Self-Test (POST) checks the CPU, memory, and basic input-output systems for errors and stores the result in a special memory location. Once the POST has successfully completed, the software loaded in ROM (sometimes called firmware) will begin to activate the computer’s disk drives. In most modern computers, when the computer activates the hard disk drive, it finds the first piece of the operating system, the bootstrap loader.
The bootstrap loader is a small program that has a single function: It loads the operating system into memory and allows it to begin operation. In the most basic form, the bootstrap loader sets up the small driver programs that interface with and control the various hardware sub-systems of the computer. It sets up the divisions of memory that hold the operating system, user information and applications. It establishes the data structures that will hold the myriad signals, flags and semaphores that are used to communicate within and between the sub-systems and applications of the computer. Finally it turns control of the computer over to the operating system.
The operating system’s tasks, in the most general sense, fall into six categories:
- Processor management
- Memory management
- Device management
- Storage management
- Application Interface
- User Interface
While there are some who argue that an operating system should do more than these six tasks, and some operating system vendors that build many more utility programs and auxiliary functions into their operating systems, these six tasks define the core of essentially all operating systems. Let’s look at the tools the operating system uses to perform each of these functions.
The heart of managing the processor comes down to two related issues: First, ensuring that each process and application receives enough of the processor’s time to function properly and, second, using as many processor cycles for real work as is possible. The basic unit of software that the operating system deals with in scheduling the work done by the processor is either a process or a thread, depending on the operating system.
It’s tempting to think of a process as an application, but that gives an incomplete picture of how processes relate to the operating system and hardware. The application you see (word processor or spreadsheet or game) is, indeed, a process, but that application may cause several other processes to begin, for tasks like communications with other devices or other computers. There are also numerous processes that run without giving you direct evidence that they ever exist. A process, then, is software that performs some action and can be controlled — by a user, by other applications or by the operating system.
It is processes, rather than applications, that the operating system controls and schedules for execution by the CPU. In a single-tasking system, the schedule is straightforward. The operating system allows the application to begin running, suspending the execution only long enough to deal with interrupts and user input. Interrupts are special signals sent by hardware or software to the CPU. It’s as if some part of the computer suddenly raised its hand to ask for the CPU’s attention in a lively meeting. Sometimes the operating system will schedule the priority of processes so that interrupts are masked, that is, the operating system will ignore the interrupts from some sources so that a particular job can be finished as quickly as possible. There are some interrupts (such as those from error conditions or problems with memory) that are so important that they can’t be ignored. These non-maskable interrupts (NMIs) must be dealt with immediately, regardless of the other tasks at hand.
While interrupts add some complication to the execution of processes in a single-tasking system, the job of the operating system becomes much more complicated in a multi-tasking system. Now, the operating system must arrange the execution of applications so that you believe that there are several things happening at once. This is complicated because the CPU can only do one thing at a time. In order to give the appearance of lots of things happening at the same time, the operating system has to switch between different processes thousands of times a second. Here’s how it happens. A process occupies a certain amount of RAM. In addition, the process will make use of registers, stacks and queues within the CPU and operating system memory space. When two processes are multi-tasking, the operating system will allow a certain number of CPU execution cycles to one program. After that number of cycles, the operating system will make copies of all the registers, stacks and queues used by the processes, and note the point at which the process paused in its execution. It will then load all the registers, stacks and queues used by the second process and allow it a certain number of CPU cycles. When those are complete, it makes copies of all the registers, stacks and queues used by the second program, and loads the first program.
All of the information needed to keep track of a process when switching is kept in a data package called a process control block. The process control block typically contains an ID number that identifies the process, pointers to the locations in the program and its data where processing last occurred, register contents, states of various flags and switches, pointers to the upper and lower bounds of the memory required for the process, a list of files opened by the process, the priority of the process, and the status of all I/O devices needed by the process. When the status of the process changes, from pending to active, for example, or from suspended to running, the information in the process control block must be used like the data in any other program to direct execution of the task-switching portion of the operating system.
This process swapping happens without direct user interference, and each process will get enough CPU time to accomplish its task in a reasonable amount of time. Trouble can come, though, if the user tries to have too many processes functioning at the same time. The operating system itself requires some CPU cycles to perform the saving and swapping of all the registers, queues and stacks of the application processes. If enough processes are started, and if the operating system hasn’t been carefully designed, the system can begin to use the vast majority of its available CPU cycles to swap between processes rather than run processes. When this happens, it’s called thrashing, and it usually requires some sort of direct user intervention to stop processes and bring order back to the system.
One way that operating system designers reduce the chance of thrashing is to reduce the need for new processes to perform various tasks. Some operating systems allow for a “process-lite,” called a thread that can deal with all the CPU-intensive work of a normal process, but generally does not deal with the various types of I/O, and does not establish structures requiring the extensive process control block of a regular process. Finally, a process may start many threads or other processes, but a thread cannot start a process.
So far, all the scheduling we’ve discussed has concerned a single CPU. In a system with two or more CPUs, the operating system must divide the workload among the CPUs, trying to balance the demands of the required processes with the available cycles on the different CPUs. Some operating systems (called asymmetric) will use one CPU for their own needs, dividing application processes among the remaining CPUs. Other operating systems (called symmetric) will divide themselves among the various CPUs, balancing demand versus CPU availability even when the operating system itself is all that’s running. Even if the operating system is the only software with execution needs, the CPU is not the only resource to be scheduled. Memory management is the next crucial step in making sure that all processes run smoothly.
Memory and Storage Management
When an operating system manages the computer’s memory, there are two broad tasks to be accomplished. First, each process must have enough memory in which to execute, and it can neither run into the memory space of another process, nor be run into by another process. Next, the different types of memory in the system must be used properly, so that each process can run most effectively. The first task requires the operating system to set up memory boundaries for types of software, and for individual applications.
As an example, let’s look at an imaginary system with 1 megabyte of RAM. During the boot process, the operating system of our imaginary computer is designed to go to the top of available memory and then “back up” far enough to meet the needs of the operating system itself. Let’s say that the operating system needs 300 kilobytes to run. Now, the operating system goes to the bottom of the pool of RAM, and starts building up with the various driver software required to control the hardware subsystems of the computer. In our imaginary computer, the drivers take up 200 kilobytes. Now, after getting the operating system completely loaded, there are 500 kilobytes remaining for application processes.
When applications begin to be loaded into memory, they are loaded in block sizes determined by the operating system. If the block size is 2 kilobytes, then every process that is loaded will be given a chunk of memory that is a multiple of 2 kilobytes in size. Applications will be loaded in these fixed block sizes, with the blocks starting and ending on boundaries established by words of 4 or 8 bytes. These blocks and boundaries help to ensure that applications won’t be loaded on top of one another’s space by a poorly calculated bit or two. With that ensured, the larger question is what to do when the 500 kilobyte application space is filled.
In most computers it’s possible to add memory beyond the original capacity. For example, you might expand RAM from 1 to 2 megabytes. This works fine, but tends to be relatively expensive. It also ignores a fundamental fact of life — most of the information that an application stores in memory is not being used at any given moment. A processor can only access memory one location at a time, so the vast majority of RAM is unused at any moment. Since disk space is cheap compared to RAM, then moving information in RAM to hard disk intelligently can greatly expand RAM space at no cost. This technique is called Virtual Memory Management.
Disk storage is only one of the memory types that must be managed by the operating system, and is the slowest. Ranked in order of speed, the memory in a computer system is:
- High-speed cache — This is fast, relatively small amounts of memory that are available to the CPU through the fastest connections. Cache controllers predict which pieces of data the CPU will need next and pull it from main memory into high-speed cache to speed system performance.
- Main memory –The RAM that you see measured in megabytes when you buy a computer.
- Secondary memory –This is most often some sort of rotating magnetic storage that keeps applications and data available to be used, and serves as virtual RAM under the control of the operating system.
The operating system must balance the needs of the various processes with the availability of the different types of memory, moving data in blocks called pages between available memory as the schedule of processes dictates.
The path between the operating system and virtually all hardware not on the computer’s motherboard goes through a special program called a driver. Much of a driver’s function is as translator between the electrical signals of the hardware sub-systems and the high-level programming languages of the operating system and application programs. Drivers take data that the operating system has defined as a file and translate them into streams of bits placed in specific locations on storage devices, or a series of laser pulses in a printer.
Because there are such wide differences in the hardware controlled through drivers, there are differences in the way that the driver programs function, but most are run when the device is required, and function much the same as any other process. The operating system will frequently assign high priorities blocks to drivers so that the hardware resource can be released and readied for further use as quickly as possible.
One reason that drivers are separate from the operating system is so that new functions can be added to the driver-and thus to the hardware subsystems-without requiring the operating system itself to be modified, recompiled and redistributed. Through the development of new hardware device drivers, development often performed or paid for by the manufacturer of the subsystems rather than the publisher of the operating system, input/output capabilities of the overall system can be greatly enhanced.
Managing input and output is largely a matter of managing queues and buffers, special storage facilities that take a stream of bits from a device, from keyboards to serial communications ports, holding the bits, and releasing them to the CPU at a rate slow enough for the CPU to cope with. This function is especially important when a number of processes are running and taking up processor time. The operating system will instruct a buffer to continue taking input from the device, but to stop sending data to the CPU while the process using the input is suspended. Then, when the process needing input is made active once again, the operating system will command the buffer to send data. This process allows a keyboard or a modem to deal with external users or computers at a high speed even though there are times when the CPU can’t use input from those sources.
Managing all the resource of the computer system is a large part of the operating system’s function and, in the case of real-time operating systems, may be virtually all the functionality required. For other operating systems, though, providing a relatively simple, consistent way for applications and humans to use the power of the hardware is a crucial part of their reason for existing.