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Exec is the object-oriented multi-tasking kernel of AmigaOS. It enabled true pre-emptive multitasking in as little as 256 kByte of free memory (as supplied with the first Amiga 1000s).Exec provided functionality for multi-tasking, memory allocation, interrupt handling and handling of dynamic shared libraries. "Exec's heavy reliance on inheritance is what makes it so compact".
It acts as a scheduler for tasks running on the system, providing pre-emptive multitasking with prioritised 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 traditional 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.
- ^ a b Holloway, Tim. "Byte Magazine on the Amiga Exec". http://www.cunningham-lee.com/misc/amiga_exec.html. Retrieved 2008-04-12.