G5, Power4, Altivec, Etc.
In comparing the G4, Speculated G5, Power4, Athlon, and Pentium 4, processors, there are three factors which determine speed of the CPU: 1) SIMD/Altivec, 2) Pipeline length, and 3) Bus speed. The G4, Athlon, and Pentium 4 all have implementations of SIMD (which is Altivec on the G4). The Pentium 4 has the weakest floating point unit, the Athlon the strongest floating point unit. The Pentium 4 has the longest pipelines, the G4 the shortest. Pipelines allow you to work very fast serially with data, while SIMD units allow you to work very fast with data in parallel fashion. Pipelines allow you to speed up the clockrate of the CPU. The Power4 is the only CPU with a super speedy bus speed. It is capable of transferring 25 gigabytes of data a second. The other CPUs are limited to single gigs of data a second.
The limitation of SIMD units is that your data must be able to be chunked up in pieces to be useable and worked on in parallel. Data which are great for SIMD units include video, sound, data to be encrypted, genetics data. For these, an SIMD unit is like having several CPUs working in parallel. If the data cannot be chunked, then you have to use the CPU's integer or floating point units. The Pentium 4 is slower than a Pentium 3 at the same clockspeed when it's SIMD unit is not used.
The strength of a long pipeline is that as you crank up the speed of the CPU, the speed of processing data through the integer and floating point units increases as well. The limitation of a pipeline is that if the program you are working with makes frequent decision changes, such as with random data, you have to reload the pipeline from memory, wasting a lot of time.
The weakness of using the G4 is that many programs rely on serial processing of data (i.e. work using the integer or floating point unit), not parallel processing. This includes database work, word processing, games, etc. These programs work faster with a higher speed CPU, such as the Pentium 4. In games, the Graphics Processing Unit does the heavy SIMD work, relying mainly on the integer unit on the CPU to get things done.
The speculated G5 (or G4++ CPU) has a longer pipeline. This allows it to have both SIMD and a higher clockspeed. The implementation of DDR and faster bus (such as Rapid-IO or Hypertransport) will further improve on the three factors in CPU design, giving the G5 a greater ability to surpass the Pentium 4.
If you are waiting for the Pentium 4 to hit a wall - you will be waiting a long time. People have made this prediction for years. What Intel has done is to improve on the *86 architecture so that it has RISC features, has SIMD, longer pipelines, and greater bus speed. As it rose past 2.5 GHz in speed, it outstripped the Athlon (which has a slow bus speed and shorter pipelines, though better SIMD and Floating point unit).
If more pipelines and a better bus can be added to the Athlon, it would be a monster chip. Already, it is so much faster in Photoshop than the G4 (because the Athlon has a much stronger floating point unit, and a nice SIMD unit), that Steve does not compare a dual G4 to a single Athlon. The single 1.7 GHz Athlon beats a dual 1 GHz G4 in Photoshop, and is cheap to boot.
Once more pipelines (to allow higher clockrates) and a better bus and a better floating point unit are added to the G4, it will then beat the Athlon.
This leaves one matter: multiple CPUs. With Mac OS X capable of supporting multiple CPUs, the two easiest ways to improve Mac speeds are to have Quad G4 Mac with faster busses.
