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49-513: The GAS executable is named as , the standard name for a Unix assembler. GAS is cross-platform , and both runs on and assembles for a number of different computer architectures . GAS is free software released under the GNU General Public License v3. The first version of GAS was released in 1986–1987. It was written by Dean Elsner and supported the VAX architecture. GAS supports

98-466: A Workload Manager feature to the scheduler, which schedules processor resources according to an elaborate scheme defined by the installation. Very early MS-DOS and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. Windows 3.1x used a non-preemptive scheduler, meaning that it did not interrupt programs. It relied on the program to end or tell the OS that it didn't need

147-520: A multilevel feedback queue with priority levels ranging from 0 to 140 was used; 0–99 are reserved for real-time tasks and 100–140 are considered nice task levels. For real-time tasks, the time quantum for switching processes was approximately 200 ms, and for nice tasks approximately 10 ms. The scheduler ran through the run queue of all ready processes, letting the highest priority processes go first and run through their time slices, after which they will be placed in an expired queue. When

196-412: A runtime system , which implements runtime language features (such as task scheduling , exception handling , calling static constructors and destructors, etc.) and interactions with the operating system, notably passing arguments, environment, and returning an exit status , together with other startup and shutdown features such as releasing resources like file handles . For C, this is done by linking in

245-432: A common division is made between foreground (interactive) processes and background (batch) processes. These two types of processes have different response-time requirements and so may have different scheduling needs. It is very useful for shared memory problems. A work-conserving scheduler is a scheduler that always tries to keep the scheduled resources busy, if there are submitted jobs ready to be scheduled. In contrast,

294-512: A computer "to perform indicated tasks according to encoded instructions ", as opposed to a data file that must be interpreted ( parsed ) by an interpreter to be functional. The exact interpretation depends upon the use. "Instructions" is traditionally taken to mean machine code instructions for a physical CPU . In some contexts, a file containing scripting instructions (such as bytecode ) may also be considered executable. Executable files can be hand-coded in machine language, although it

343-542: A file is executed by loading it into memory and jumping to the start of the address space and executing from there. In more complicated interfaces, executable files have additional metadata specifying a separate entry point . For example, in ELF, the entry point is defined in the header's e_entry field, which specifies the (virtual) memory address at which to start execution. In the GNU Compiler Collection , this field

392-446: A general syntax that works for all of the supported architectures. The general syntax includes assembler directives and a method for commenting. The default syntax is AT&T syntax . GAS uses assembler directives (also known as pseudo ops), which are keywords beginning with a period that behave similarly to preprocessor directives in the C programming language . While most of the available assembler directives are valid regardless of

441-575: A lighter load on the system if converted to a multithreaded structure. AIX 5 implements the following scheduling policies: FIFO, round robin, and a fair round robin. The FIFO policy has three different implementations: FIFO, FIFO2, and FIFO3. The round robin policy is named SCHED_RR in AIX, and the fair round robin is called SCHED_OTHER. Linux 1.2 used a round-robin scheduling policy. Linux 2.2 added scheduling classes and support for symmetric multiprocessing (SMP). In Linux 2.4, an O(n) scheduler with

490-435: A network and managed through an administrative back end. The scheduler is an operating system module that selects the next jobs to be admitted into the system and the next process to run. Operating systems may feature up to three distinct scheduler types: a long-term scheduler (also known as an admission scheduler or high-level scheduler), a mid-term or medium-term scheduler , and a short-term scheduler . The names suggest

539-425: A non-work conserving scheduler is a scheduler that, in some cases, may leave the scheduled resources idle despite the presence of jobs ready to be scheduled. There are several scheduling problems in which the goal is to decide which job goes to which station at what time, such that the total makespan is minimized: A very common method in embedded systems is to schedule jobs manually. This can for example be done in

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588-515: A preemptive scheduling algorithm. All Process Manager processes run within a special multiprocessing task, called the blue task . Those processes are scheduled cooperatively, using a round-robin scheduling algorithm; a process yields control of the processor to another process by explicitly calling a blocking function such as WaitNextEvent . Each process has its own copy of the Thread Manager that schedules that process's threads cooperatively;

637-441: A process that has a low priority, a process that is page faulting frequently, or a process that is taking up a large amount of memory in order to free up main memory for other processes, swapping the process back in later when more memory is available, or when the process has been unblocked and is no longer waiting for a resource. In many systems today (those that support mapping virtual address space to secondary storage other than

686-465: A scheduler will implement a suitable compromise. Preference is measured by any one of the concerns mentioned above, depending upon the user's needs and objectives. In real-time environments, such as embedded systems for automatic control in industry (for example robotics ), the scheduler also must ensure that processes can meet deadlines ; this is crucial for keeping the system stable. Scheduled tasks can also be distributed to remote devices across

735-411: A target quality-of-service . Scheduling is fundamental to computation itself, and an intrinsic part of the execution model of a computer system; the concept of scheduling makes it possible to have computer multitasking with a single central processing unit (CPU). A scheduler may aim at one or more goals, for example: In practice, these goals often conflict (e.g. throughput versus latency), thus

784-748: A thread yields control of the processor to another thread by calling YieldToAnyThread or YieldToThread . macOS uses a multilevel feedback queue, with four priority bands for threads – normal, system high priority, kernel mode only, and real-time. Threads are scheduled preemptively; macOS also supports cooperatively scheduled threads in its implementation of the Thread Manager in Carbon . In AIX Version 4 there are three possible values for thread scheduling policy: Threads are primarily of interest for applications that currently consist of several asynchronous processes. These applications might impose

833-451: A time-multiplexed fashion. Sometimes the kernel is divided in three or more parts: Manual scheduling, preemptive and interrupt level. Exact methods for scheduling jobs are often proprietary. When designing an operating system, a programmer must consider which scheduling algorithm will perform best for the use the system is going to see. There is no universal best scheduling algorithm, and many operating systems use extended or combinations of

882-490: Is capable of forcibly removing processes from a CPU when it decides to allocate that CPU to another process, or non-preemptive (also known as voluntary or co-operative ), in which case the scheduler is unable to force processes off the CPU. A preemptive scheduler relies upon a programmable interval timer which invokes an interrupt handler that runs in kernel mode and implements the scheduling function. Another component that

931-444: Is far more convenient to develop software as source code in a high-level language that can be easily understood by humans. In some cases, source code might be specified in assembly language instead, which remains human-readable while being closely associated with machine code instructions. The high-level language is compiled into either an executable machine code file or a non-executable machine code – object file of some sort;

980-447: Is given equal time (for instance 1 ms, usually between 1 ms and 100 ms) in a cycling list. So, process A executes for 1 ms, then process B, then process C, then back to process A. More advanced algorithms take into account process priority, or the importance of the process. This allows some processes to use more time than other processes. The kernel always uses whatever resources it needs to ensure proper functioning of

1029-463: Is important that a long-term scheduler selects a good process mix of I/O-bound and CPU-bound processes. If all processes are I/O-bound, the ready queue will almost always be empty, and the short-term scheduler will have little to do. On the other hand, if all processes are CPU-bound, the I/O waiting queue will almost always be empty, devices will go unused, and again the system will be unbalanced. The system with

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1078-458: Is involved in the CPU-scheduling function is the dispatcher, which is the module that gives control of the CPU to the process selected by the short-term scheduler. It receives control in kernel mode as the result of an interrupt or system call. The functions of a dispatcher involve the following: The dispatcher should be as fast as possible since it is invoked during every process switch. During

1127-462: Is offered, as opposed to best-effort communication, weighted fair queuing may be utilized. In advanced packet radio wireless networks such as HSDPA (High-Speed Downlink Packet Access) 3.5G cellular system, channel-dependent scheduling may be used to take advantage of channel state information . If the channel conditions are favourable, the throughput and system spectral efficiency may be increased. In even more advanced systems such as LTE ,

1176-468: Is often required to prevent them from blocking due to waiting on each other. In these cases, special-purpose job scheduler software is typically used to assist these functions, in addition to any underlying admission scheduling support in the operating system. Some operating systems only allow new tasks to be added if it is sure all real-time deadlines can still be met. The specific heuristic algorithm used by an operating system to accept or reject new tasks

1225-554: Is reserved for the Operating System. User interfaces and APIs work with priority classes for the process and the threads in the process, which are then combined by the system into the absolute priority level. The kernel may change the priority level of a thread depending on its I/O and CPU usage and whether it is interactive (i.e. accepts and responds to input from humans), raising the priority of interactive and I/O bounded processes and lowering that of CPU bound processes, to increase

1274-552: Is set by the linker based on the _start symbol. Task scheduling In computing , scheduling is the action of assigning resources to perform tasks . The resources may be processors , network links or expansion cards . The tasks may be threads , processes or data flows . The scheduling activity is carried out by a mechanism called a scheduler . Schedulers are often designed so as to keep all computer resources busy (as in load balancing ), allow multiple users to share system resources effectively, or to achieve

1323-414: Is the admission control mechanism . The medium-term scheduler temporarily removes processes from main memory and places them in secondary memory (such as a hard disk drive ) or vice versa, which is commonly referred to as swapping out or swapping in (also incorrectly as paging out or paging in ). The medium-term scheduler may decide to swap out a process that has not been active for some time,

1372-435: Is to be executed (allocated a CPU) after a clock interrupt , an I/O interrupt, an operating system call or another form of signal . Thus the short-term scheduler makes scheduling decisions much more frequently than the long-term or mid-term schedulers – A scheduling decision will at a minimum have to be made after every time slice, and these are very short. This scheduler can be preemptive , implying that it

1421-452: The crt0 object, which contains the actual entry point and does setup and shutdown by calling the runtime library . Executable files thus normally contain significant additional machine code beyond that directly generated from the specific source code. In some cases, it is desirable to omit this, for example for embedded systems development, or simply to understand how compilation, linking, and loading work. In C, this can be done by omitting

1470-636: The back-end for a popular compiler suite, namely GCC, the GNU Assembler is very widely used in compiling modern free and open source software . GAS is often used as the assembler on Linux operating systems in conjunction with other GNU software. A modified version of GAS can also be found in the macOS development tools package. A standard "Hello, world!" program for Linux on IA-32 : Executable In computer science , executable code , an executable file , or an executable program , sometimes simply referred to as an executable or binary , causes

1519-448: The best performance will thus have a combination of CPU-bound and I/O-bound processes. In modern operating systems, this is used to make sure that real-time processes get enough CPU time to finish their tasks. Long-term scheduling is also important in large-scale systems such as batch processing systems, computer clusters , supercomputers , and render farms . For example, in concurrent systems , coscheduling of interacting processes

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1568-815: The context switches, the processor is virtually idle for a fraction of time, thus unnecessary context switches should be avoided. The time it takes for the dispatcher to stop one process and start another is known as the dispatch latency . A scheduling discipline (also called scheduling policy or scheduling algorithm ) is an algorithm used for distributing resources among parties which simultaneously and asynchronously request them. Scheduling disciplines are used in routers (to handle packet traffic) as well as in operating systems (to share CPU time among both threads and processes ), disk drives ( I/O scheduling ), printers ( print spooler ), most embedded systems, etc. The main purposes of scheduling algorithms are to minimize resource starvation and to ensure fairness amongst

1617-617: The equivalent process on assembly language source code is called assembly . Several object files are linked to create the executable. Object files -- executable or not -- are typically stored in a container format , such as Executable and Linkable Format (ELF) or Portable Executable (PE) which is operating system -specific. This gives structure to the generated machine code, for example dividing it into sections such as .text (executable code), .data (initialized global and static variables), and .rodata (read-only data, such as constants and strings). Executable files typically also include

1666-419: The high-priority threads and FIFO among the lower-priority ones. In this sense, response time is short for most threads, and short but critical system threads get completed very quickly. Since threads can only use one time unit of the round-robin in the highest-priority queue, starvation can be a problem for longer high-priority threads. The algorithm used may be as simple as round-robin in which each process

1715-405: The next to be scheduled for execution. Similar to shortest job first (SJF). With this strategy the scheduler arranges processes with the least estimated processing time remaining to be next in the queue. This requires advanced knowledge or estimations about the time required for a process to complete. The operating system assigns a fixed-priority rank to every process, and the scheduler arranges

1764-674: The parties utilizing the resources. Scheduling deals with the problem of deciding which of the outstanding requests is to be allocated resources. There are many different scheduling algorithms. In this section, we introduce several of them. In packet-switched computer networks and other statistical multiplexing , the notion of a scheduling algorithm is used as an alternative to first-come first-served queuing of data packets. The simplest best-effort scheduling algorithms are round-robin , fair queuing (a max-min fair scheduling algorithm), proportional-fair scheduling and maximum throughput . If differentiated or guaranteed quality of service

1813-462: The processes in the ready queue in order of their priority. Lower-priority processes get interrupted by incoming higher-priority processes. The scheduler assigns a fixed time unit per process, and cycles through them. If process completes within that time-slice it gets terminated otherwise it is rescheduled after giving a chance to all other processes. This is used for situations in which processes are easily divided into different groups. For example,

1862-523: The processor so that it could move on to another process. This is usually called cooperative multitasking. Windows 95 introduced a rudimentary preemptive scheduler; however, for legacy support opted to let 16-bit applications run without preemption. Windows NT -based operating systems use a multilevel feedback queue. 32 priority levels are defined, 0 through to 31, with priorities 0 through 15 being normal priorities and priorities 16 through 31 being soft real-time priorities, requiring privileges to assign. 0

1911-422: The ready queue (in main memory); that is, when an attempt is made to execute a program, its admission to the set of currently executing processes is either authorized or delayed by the long-term scheduler. Thus, this scheduler dictates what processes are to run on a system, the degree of concurrency to be supported at any one time – whether many or few processes are to be executed concurrently, and how

1960-434: The ready queue. This is commonly used for a task queue , for example as illustrated in this section. Earliest deadline first (EDF) or least time to go is a dynamic scheduling algorithm used in real-time operating systems to place processes in a priority queue. Whenever a scheduling event occurs (a task finishes, new task is released, etc.), the queue will be searched for the process closest to its deadline, which will be

2009-662: The relative frequency with which their functions are performed. The process scheduler is a part of the operating system that decides which process runs at a certain point in time. It usually has the ability to pause a running process, move it to the back of the running queue and start a new process; such a scheduler is known as a preemptive scheduler , otherwise it is a cooperative scheduler . We distinguish between long-term scheduling , medium-term scheduling , and short-term scheduling based on how often decisions must be made. The long-term scheduler , or admission scheduler , decides which jobs or processes are to be admitted to

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2058-712: The responsiveness of interactive applications. The scheduler was modified in Windows Vista to use the cycle counter register of modern processors to keep track of exactly how many CPU cycles a thread has executed, rather than just using an interval-timer interrupt routine. Vista also uses a priority scheduler for the I/O queue so that disk defragmenters and other such programs do not interfere with foreground operations. Mac OS 9 uses cooperative scheduling for threads, where one process controls multiple cooperative threads, and also provides preemptive scheduling for multiprocessing tasks. The kernel schedules multiprocessing tasks using

2107-457: The scheduling algorithms above. For example, Windows NT /XP/Vista uses a multilevel feedback queue , a combination of fixed-priority preemptive scheduling, round-robin, and first in, first out algorithms. In this system, threads can dynamically increase or decrease in priority depending on if it has been serviced already, or if it has been waiting extensively. Every priority level is represented by its own queue, with round-robin scheduling among

2156-410: The scheduling is combined by channel-dependent packet-by-packet dynamic channel allocation , or by assigning OFDMA multi-carriers or other frequency-domain equalization components to the users that best can utilize them. First in, first out ( FIFO ), also known as first come, first served (FCFS), is the simplest scheduling algorithm. FIFO simply queues processes in the order that they arrive in

2205-467: The split between I/O-intensive and CPU-intensive processes is to be handled. The long-term scheduler is responsible for controlling the degree of multiprogramming. In general, most processes can be described as either I/O-bound or CPU-bound . An I/O-bound process is one that spends more of its time doing I/O than it spends doing computations. A CPU-bound process, in contrast, generates I/O requests infrequently, using more of its time doing computations. It

2254-461: The swap file), the medium-term scheduler may actually perform the role of the long-term scheduler, by treating binaries as swapped-out processes upon their execution. In this way, when a segment of the binary is required it can be swapped in on demand, or lazy loaded , also called demand paging . The short-term scheduler (also known as the CPU scheduler ) decides which of the ready, in-memory processes

2303-486: The system, and so can be said to have infinite priority. In SMP systems, processor affinity is considered to increase overall system performance, even if it may cause a process itself to run more slowly. This generally improves performance by reducing cache thrashing . IBM OS/360 was available with three different schedulers. The differences were such that the variants were often considered three different operating systems: Later virtual storage versions of MVS added

2352-420: The target architecture, some directives are machine dependent. Since version 2.10, Intel syntax can be used through use of the .intel_syntax directive. GAS supports two comment styles. Multi-line As in C, multi-line comments start and end with mirroring slash-asterisk pairs: Single-line Single line comments have a few different formats varying on which architecture is being assembled for. Being

2401-436: The usual runtime, and instead explicitly specifying a linker script, which generates the entry point and handles startup and shutdown, such as calling main to start and returning exit status to the kernel at the end. In order to be executed by the system (such as an operating system , firmware , or boot loader ), an executable file must conform to the system's application binary interface (ABI). In simple interfaces,

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