Preemptive multitasking

Preemptive multitasking (or PMT for short) is a multitasking model in which the operating system takes control away from a program and gives it to another program, in order to ensure that all applications get their fair share of time. Most current operating systems use preemptive multitasking, although RISC OS does not currently use this model, instead opting for a model known as cooperative multitasking, where applications decide when to cede control to the operating system. However, there has been much discussion about the issue for many years, and there have been attempts to bring it to RISC OS.

Benefits of PMT

• A frozen program should not freeze the system, the OS should be able to take control once that program's "time slice" is over, and then you should be able to kill the frozen program, without losing anything you were working on.
• When running software that heavily loads the CPU, the rest of the system doesn't become unresponsive while the program is doing something.
• Programs don't have to be written to stop in the middle of what they're doing to preserve system responsiveness, as the OS does this for them. This makes development simpler.

Cons of PMT

• Developing a PMT OS is usually trickier than a CMT OS.
• Software in a PMT OS doesn't automatically get the whole CPU to itself for as long as it wants. That's the primary benefit of PMT, but some people consider it a disadvantage. However, with use of interrupts and priority levels, a PMT OS can usually do the tasks just as effectively, while keeping the system responsive.

Previous attempts to bring PMT to RISC OS

Gold

Gold was a project by Acorn to move RISC OS over to the Mach microkernel, which is used today by Apple as the basis of the Darwin kernel used by OS X.

Galileo

Galileo was another project by Acorn to move to a PMT OS, this time from the ground up. Galileo was cancelled with the closure of the workstation division.

Wimp2

Wimp2, by Niall Douglas, used a different approach from previous attempts to build a PMT OS. Rather than build a PMT OS from the ground up, Wimp2 essentially ran inside of the existing OS, as a PMT environment for applications to run in. Programs could either be patched to run in the Wimp2 environment (with varying success,) or they could be recompiled for Wimp2. Incompatible programs ran outside of the Wimp2 environment. This approach has some serious issues, however.

More info

• Wikipedia article on preemption
• Wimp2 page
• Microsoft knowledge base article on how multitasking works in Windows 95 (relevant due to similar issues existing between the Windows 3.1 to Windows 95 transition, and a transition from current versions of RISC OS to a PMT version. However, this may not be the best approach.)

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