CompuServe Thread

Dom, I'm not a techy but my understanding of Windows multitasking is that each application has to be programmed to release the processor for other programs. If the program isn't written correctly it will hog the system. Also, while one program is running, the other effectively halts until the first one is finished. I believe this is the way the Mac also works. Also, each program has to be writen to allow m/ting instead of the OS taking care of that. This partly results in longer development cycles and large applications. The Amiga, OS/2, WindowsNT & Unix (me thinks) all use premptive m/t. What happens here is that each program is given a priority as to where it is postioned on a task list. The processor runs through the task list with each clock cycle and any program that is waiting for processing is done for a certain amount of time. Once it's time limit is up the processor goes to the next application and so on. Although theoretically only one program is being run at a time the effect is that all programs are more or less equally sharing the processor, as required. The difference between AmigaDOS & both NT & OS/2 is that the PC operating systems support multithreaded m/t. What happens here is that an application can start a new thread, or procedure, within itself. So, for example, a word processor can do a seach and replace while you continue to edit the file. Therefore, each program is internally multitasking. I hope this is more fact than fiction. I'm sure someone else will correct me if I'm wrong. 8^) Andrew

The Amiga uses Pre-emptive Multitasking… basically the same as OS/2.. thing is where the Amiga is special is that it can do it with so little overhead… try getting OS/2 to run in 512K let alone multitask ! I'll let some of the more knowledgable guys in here give you a longer answer 🙂 Tim On AutoPilot Over Hong Kong

The Amiga uses pre-emptive multitasking, as does OS2. Windows use co-operative. they are not the same, or even similar. They can look similar on the surface, IF all the developers doing co-operative work VERY hard (they sure haven't under the windows operating environment!), but generally you'll see all kinds of performance degradations with co-operative that you do not with pre-emptive. Anyone who tells you that there isn't much difference between Windows and OS2 hasn't looked, or looked and didn't know what they were seeing. Windows, as always, has the software baase – OS2 kicks its fanny from here to Orion as a base operating system, and not only from the multitasking viewpoint (where it is vastly superior to Windows 3.1). The Amiga's multitasking has absolutely nothing to do with it's "chips". You can write pre-emptive multitasking for an 8-bit microprocessor in about 2k of rom; the Amiga's chipset shares some of the work of keeping the machine going, it's true, but then again a machine like a windows machines doesn't NEED to have a processor maintaining the display the way the Amiga needs the copper… because display cards on a PC "just do it" without help, or knowledge from the OS. Technically, the only hardware you "need" to build a pre-emptive multitasking system is (a) a "timer" that can signal the CPU through (b) a reasonably fast "interrupt" to stop what it is doing, look at the situation around it, and based on that, do something else for a while, coming back later to the original job. The Amiga's OS is probably most comparable to OS2, however, OS2 has many basic OS-level advantages such as intertask protection, virtual memory management, a standard graphics layer, and threading.

Andrew has it right, although there are a few hardware particulars involved. In some systems, the chips involved manage the multitasking management as well as protecting one task from manipulating another's memory. That isn't available on the Amiga, nor is is available on many systems, although Unix does have it, usually. As for the difference between cooperative multitasking and preemptive multitasking, it is a difference between night and day. A cooperative multitasking system, such as Windows, depends on the program relinquishing resources that it does not need at any given time. Most programmers do not bother to "cooperate" with the system in this respect. With a preemptive multitasking system, even if the program decides that it is going to get ugly and hog resources, it gets locked out every once in a while and other programs can run. That is the significant difference between cooperative and preemptive multitasking. Does the programmer have to allow other tasks their due? In a preemptive system, generally, he doesn't need to worry about it as the OS takes care of that for him. In a cooperative system, it is only as good as the least cooperative program. BTW, on the Amiga, even tasks with negative priorities will run once in a while as the other tasks are waiting for input from either the disk or keyboard, or such. A definite plus in the preemptive column over the usual approach ala Mac and PC DOS. One last item, it is possible for AmigaDOS to spin off tasks and processes so that you can be editing a document while it is doing a print or search and replace. Unfortunately, it is not much used as far as I can tell. Brian bjbart@watnxt01.ucr.edu

Multi-tasking, a fun subject. When multi-tasking the CPU can perform processing on only one task at any given time. So each program is given a "slice" of time inwhich to be processed by the CPU. This is a time-slicing. The two main forms of multi-tasking differ in their method of deciding on how the timeslice is allocated per task. With co-operative multi-tasking, a program (task) is allowed to take its time-slice, runs, then has the responsiblity of returning control of the system — so the next program can take a slice of the CPU's processing time. The programs have to "co-operate" in the real sense of the word. If a program doesn't offer to give the system back, no other programs can take it. Pre-emptive multi-tasking, pre-sets the maximum amount of time all tasks can allocate. When a program's time is up, the system switches to the next task. Also some OS's allow the tasks to pre-empt themselves before their time-slice is over. With a pre-emptive system, this allows a tasks to forfeit its "leftover" time for use by other tasks. The other tasks would still get the same maximum amount of time for a time-slice, but their time would come sooner. I believe Windows (for MS-DOS) uses co-operative multitasking, but the new Windows-NT (full OS) uses pre-emptive. The Amiga and OS/2 uses a pre-emptive multi-tasking. — The Amiga's multi-tasking is handled by "Exec". Exec, is the core of the Amiga's operating system. The Layers & Graphic libraries are run on top of Exec. Intuition (GUI) & AmigaDOS run on top of Layers. The Workbench, shells, ARexx, and applications are then run on top of the two interfaces (Intuition & AmigaDOS). It could be said that the OS is already multi-tasking, before you even load any programs. When a task (or process) runs, it gives itself a task-priority. Exec, runs the task with the highest priority first, then the next task with same level of priorty, then drops to lower priority tasks. You might think of several 33" vinyl records stacked on one turntable. Each record is a priority level, and each song on that record is a task sharing that level. The needle (exec) plays the first song on the top record (highest), then the next song on that record (switching when the time-slice is up or finished.). The needle then drops down to the next record, playing all songs at that level. Some tasks pre-empt themselves using almost no CPU time, allowing the next task to run earlier. While other tasks use all their timeslice, and are pre-empted by Exec. Some tasks, are more important than others. For example Trackdisk, SCSI, Console, & Input are tasks which need higher priorities than programs that use the CON, keyboard, or drives. So, its not a simple top to bottom running of tasks, a task with a lower priority is run only after all higher tasks are finished. Finally, there is a programing option that allows a program to freeze the multi-tasking, giving itself all the CPU time. Ron. – ap'ing with AP. (Eagles may fly, but weasels aren't sucked into jet engines.)

Dom, It's a pleasure to see that so many Amiga user's have these concepts of multi-tasking down. It's an indication to me of how literate Amiga users are of all platforms. Yet, we still are using Amigas… I'm from the IBM mainframe world and saw the usefullness of pre-emptive multitasking when I bought my 1000 long ago (what 10 years?). Although, the Amiga OS doesn't protect processes from one another memory-wise it's still a big advantage over windoze (except for the volume of units factor). It still baffles me how any corporation could blow a ten year lead… Robb