Exec is the object-oriented multi-tasking

Computer multitasking

In computing, multitasking is a method where multiple tasks, also known as processes, share common processing resources such as a CPU. In the case of a computer with a single CPU, only one task is said to be running at any point in time, meaning that the CPU is actively executing instructions for...

 kernel

Kernel (computing)

In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...

 of AmigaOS

AmigaOS

AmigaOS is the default native operating system of the Amiga personal computer. It was developed first by Commodore International, and initially introduced in 1985 with the Amiga 1000...

. It enabled pre-emptive multitasking in as little as 256k of memory (as supplied with the first Amiga 1000

Amiga 1000

The A1000, or Commodore Amiga 1000, was Commodore's initial Amiga personal computer, introduced on July 23, 1985 at the Lincoln Center in New York City....

s).
Exec provided functionality for multi-tasking, memory allocation, interrupt

Interrupt

In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

 handling and handling of dynamic shared libraries

Dynamic linker

In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries for an executable when it is executed. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented...

. "Exec's heavy reliance on inheritance is what makes it so compact".

It acts as a scheduler

Scheduling (computing)

In computer science, a scheduling is the method by which threads, processes or data flows are given access to system resources . This is usually done to load balance a system effectively or achieve a target quality of service...

 for tasks running on the system, providing pre-emptive multitasking with prioritised round-robin scheduling

Round-robin scheduling

Round-robin is one of the simplest scheduling algorithms for processes in an operating system. As the term is generally used, time slices are assigned to each process in equal portions and in circular order, handling all processes without priority . Round-robin scheduling is simple, easy to...

. Exec also provides access to other libraries and high-level inter-process communication

Inter-process communication

In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...

 via message passing

Message passing

Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...

. Other comparable microkernels have had performance problems because of the need to copy messages between address spaces. Since the Amiga has only one address space, Exec message passing is quite efficient. The only fixed memory address in the Amiga software (address 4) is a pointer to exec.library, which can then be used to access other libraries. Exec was designed and implemented by Carl Sassenrath

Carl Sassenrath

Carl Sassenrath is an architect of operating systems and computer languages. He brought multitasking to personal computers in 1985 with the creation of the Amiga Computer operating system kernel, and he is currently the designer of the REBOL computer language as well as the CTO of REBOL...

.

Unlike traditional operating systems, the exec kernel does not run "privileged". Contemporary operating systems for the 68000

Motorola 68000

The Motorola 68000 is a 16/32-bit CISC microprocessor core designed and marketed by Freescale Semiconductor...

 such as Atari TOS

Atari TOS

TOS is the operating system of the Atari ST range of computers. This range includes the 520 and 1040ST, their STF/M/FM and STE variants and the Mega ST/STE. Later, 32-bit machines were developed using a new version of TOS, called MultiTOS, which allowed multitasking...

 and SunOS

SunOS

SunOS is a version of the Unix operating system developed by Sun Microsystems for their workstation and server computer systems. The SunOS name is usually only used to refer to versions 1.0 to 4.1.4 of SunOS...

 used trap

Exception handling

Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....

 instructions for invoking kernel functions. This made the kernel functions run in the 68000's supervisor mode, while user software ran in the unprivileged user mode. By contrast, exec function calls are made with the library jump table, and the kernel code normally executes in user mode. Whenever supervisor mode is needed, either by the kernel or user programs, the library functions Supervisor or SuperState are used.

Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 kernel developer Linus Torvalds

Linus Torvalds

Linus Benedict Torvalds is a Finnish software engineer and hacker, best known for having initiated the development of the open source Linux kernel. He later became the chief architect of the Linux kernel, and now acts as the project's coordinator...

disputes the term of pre-emptive multitasking in favor of cooperative, due to lack of memory protection.