11.7.2 Motorola’s State-of-the-art Microprocessors
As part of their plans to carry the PowerPC architecture into the future, Motorola /IBM/ Apple already announced AltiVec extensions for the PowerPC family. The result is the MPC7400 PowerPC microprocessor. This microprocessor is available in 400 MHz, 450 MHz and 500 MHz clock speeds. Motorola’s AltiVec technology is the foundation for the Velocity Engine of Apple Computer’s next generation desktop computers. For example, Apple rececently announced Power Mac G5 which uses Motorola’s 64-bit microprocessor, G5. AltiVec extensions are somewhat comparable to the MMX extensions in Intel’s Pentium family. AltiVec has independent processing units while Intel tied MMX to the floating-point unit. Both utilize SIMD (Chapter 8). A comparison of some of the features
of AltiVec vs. MMX is provided below:
In AltiVec, each processing unit can work independent of the others. This provides more parallelism by separate units. Since Intel tied MMX to floating-point unit, Pentiums can perform either floating-point math or switch over to MMX, but not both simultaneously. The switch requires a mode change that can cost hundreds of cycles, both going into and coming out ofMMX mode. It may be very tricky with Pentiums to write good and efficient codes when mixing of modes are required in some computing algorithms.
AltiVec can vetorize the floating-point operations. This means that one can use AltiVec to work on some data in the Floating-point Unit, then load the data in the AltiVec side (Vector Unit) without any significant mode switch. This may save hundreds of cycles . Also, this allows programmers to do more with the Vector Unit since they can go back and forth to mix and match.
The biggest drawback with MMX or AltiVec is getting programmers to use them. Programmers are required to use assembly language for MMX. Therefore, a few programmers used MMX for dedicated applications. For example, Intel hand tuned some photoshop filters for Adobe. Programmers can use C language with AltiVec. Therefore, it is highly likely that more programmers will use AltiVec than MMX.
In the future, Motorola and IBM plan to introduce the PowerPC series 2K. It is expected that the chip will contain 100 million transistors and have clock speeds greater than 1 GHz.