Okay, here's part three
(I edited the first part (Akira: Yes, OS4 will run on current Amigas with PPC cards) and the bit about emulators (added a note that these are "dead ends").
Amiga and the PowerPC processor
In 1995 (when Amiga was a subsidiary of the German PC distributor Escom), it became clear that Motorola's 68k processors were a dead end: Motorola were part of the PPC consortium and were not interested in further developing their 68k processor line.
Amiga Technologies decided that new Amigas should be equipped with PowerPC (PPC) processors. There were several reasons for this decision:
1. PPC processors are the superior architecture and at that time were quite a lot faster than their x86 counterparts
2. Motorala wanted to boost the PPC platform, so Amiga expected them to support the move towards PPC (as they did when Apple moved to PPC)
3. Perfect Mac emulation (which was a major selling point)
4. Most of the commercial Amiga developers regard a "move to x86" as the ultimate death blow for the Amiga.
But it seemed to be quite a big task to port the AmigaOS to PPC (and to force all software developers to adopt their applications immediately), therefore Amiga choosed a step by step approach:
The next Amiga computers should have been equipped with two processors: Both a standard 68k processor (as used in previous Amiga models) and a PPC processor. This would allow both Amiga and 3rd party software developers to migrate their software slowly to the PowerPC without the need to rewrite complete projects immediately.
The "Walker" prototype that was demonstrated at several shows back then already was prepared to accept a PPC processor next to the 68030 that was already fitted to the motherboard. Unfortunately, Escom went bancrupt in 1996 and the Walker never made it close to production stage.
But the well known 3rd party hardware manufacturer Phase5 adopted the idea and in 1997 presented his PPC accellerator boards for Amiga1200, 3000 and 4000 models. These were dual processor boards (as proposed by Amiga earlier) with the 68k processor running the OS and all applications. The PPC processor acts as a "co-processor": Only applications that are specifically adopted will use the PPC processor for CPU intensive tasks.
Unfortunately, the design Phase5 used (had to use) suffered from several limitations: First of all, the PPC processor has no L2 cache at all, which results in very slow memory access. Additionally, the fact that both processors share the same ressources (e.g. memory) makes switching between the two quite time consuming. So the PPC cards turned out to be much less of a performance wonder than the users had hoped for. Basically, they only make sense for real time consuming tasks that do not use the OS very much (e.g. games or calculating complicated picture effects in a paint program).
What is PowerUp, what is WarpUp/WarpOS? What's the "kernel war"?
In dual processor systems, the PPC processor is completely independent from the 68k processor and the AmigaOS. That means there has to be a seperate kernel running on the PPC processor that is starting and managing the PPC tasks. Phase5's PPC kernel is called "PowerUp".
PowerUp wass not just an unimportant, intermediate solution: Phase5 planned to offer PPC only systems in the future, running a PPC native version of AmigaOS. The PowerUp kernel was a very important part of this strategy.
Haage-Partner, one of the most important software developers at that time, didn't like the PowerUp kernel and developed a replacement for it, called "WarpUp". They never admitted it officially, but they probably had two main reasons for this:
1. They wanted to boost their own compiler system "StormC" (which was - of course - the first compiler to support WarpUp and the one with the best support for it)
2. They had their own plans for the Amiga's future and didn't want to give Phase5 the advantage of controlling the PPC kernel used in PPC Amigas
Only one kernel could be active at the same time, so Amiga users wanting to use both of them usually had to stop all PowerUp applications before starting a WarpUp program (and vice versa of course).
In the following months, the socalled "kernel war" broke loose. Phase5 started to implement certain "Anti-WarpUp" measures in later models of their PPC cards while H&P developed hacks to circumvent these. Both parties accused each other of using dirty tricks to make the opponent's kernel look bad and praised the superior design of their own kernel. None of the solutions was backed officially and support by 3rd party developers was split equally among the two.
The kernel war further hampered the sale of PPC accelerator cards. All in all, only ten thousand PPC cards were sold.
In 1999, Phase5 went bankrupt. WarpOS became the official PPC kernel delivered with AmigaOS 3.5/3.9 (see below). A PowerUp emulation layer for WarpOS was released. It looked like H&P had won the kernel war.
In 2002, the whole thing looks just hilarious: Now that a PPC native AmigaOS finally is developed, neither WarpOS nor PowerUp are used. Instead, it is based on a new (and superior) kernel, a reimplementation of AmigaOS' own kernel Exec.library. AmigaOS4 is backwards compatible with WarpOS though.