Exec is the multitasking kernel of AmigaOS. It enabled pre-emptive multitasking in as little as 256 KB of memory (as supplied with the first Amiga 1000s). Exec provided functionality for multitasking, memory allocation, interrupt handling and handling of dynamic shared libraries.
It acts as a scheduler for tasks running on the system, providing pre-emptive multitasking with prioritized round-robin scheduling. Exec also provides access to other libraries and high-level inter-process communication via message passing. 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.
Unlike newer modern operating systems, the exec kernel does not run "privileged". Contemporary operating systems for the 68000 such as Atari TOS and SunOS used trap 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.
One limitation of the Exec kernel was that an uncooperative program could disable multitasking for a long period (or indefinitely) by invoking Amiga_Exec's Forbid() or Disable() with no later invocation of corresponding Permit() or Enable() calls, causing the environment to run as a single task. Multitasking could also be disabled by programs which – by software bug or intent – modify Exec's data structures or the code itself stored in RAM, possible due to lack of MMU support.
Even with such limitations, Exec satisfies the definition of preemptive scheduling algorithm, using a preemptive scheduling routine and basing its interrupt intervals on a clock.
Linux kernel developer Linus Torvalds once described the Amiga design as cooperative even though it uses a preemptive scheduling policy. The reason for that, he argued, was because the lack of [memory] protection between tasks, meant a task could hinder the system from operating preemptively. As tasks would need to choose not to stop the preemptive mechanism this would reduce to a kind of inverted cooperative system. This kind of conflation between protection and scheduler policy is nonstandard.
ExecSG (Second Generation) is the kernel used by AmigaOS 4. This kernel was licensed by Hyperion Entertainment from Thomas Frieden and Hans-Jörg Frieden. It exposes the same functions and structures as the Commodore Amiga Exec kernel but only runs on PowerPC CPUs. The ultimate origin of ExecSG is unspecified.
- Sassenrath, Carl (1986). Amiga ROM Kernel Reference Manual. Exec.
- Holloway, Tim (January 1991). "The Object-Oriented Amiga Exec: The design of the Amiga operating-system kernel follows the rules of object-oriented programming". Byte (January 1991) (McGraw-Hill). pp. 329–332, 234. ISSN 0360-5280.
- Stallings, William (2009). Operating Systems - Internals and Design Principles (6th ed.). Upper Saddle River, NJ: Pearson Prentice Hall. ISBN 9788120336292.
- Linus Torvalds (October 17, 1996). "Jalleen mahtava uusi keksinto m$oftilta" (in Finnish). Newsgroup: sfnet.atk.sodat. Usenet: firstname.lastname@example.org.Helsinki.FI. Retrieved February 25, 2011.
... due to lack of memory protection. Any program can take possession of the machine completely and thus prevent multitasking. Multitasking in Amiga is co-operative because it requires approval and help from running applications [... muistinsuojauksen puutteen vuoksi. Mikä tahansa ohjelma voi ottaa koneen haltuunsa täysin ja estää siten moniajo. Eli Amigan moniajo on kooperatiivinen, koska se vaatii sen että ajettavat ohjelmat hyväksyvät sen moniajon ja auttavat sitä]
- "Justia: Hyperion Entertainment vs. Amiga Inc.".