PowerPC G4 is a designation used by Apple Computer to describe a fourth generation of PowerPC microprocessors. The term is often, incorrectly, imagined to be a physical model of processor when in fact a number of processors from Freescale (was Motorola) have been used.

Such designations were applied to Apple Macintosh computers such as the PowerBook G4 laptop, the iMac G4 and the Power Macintosh G4 desktop. This generation of processor was also used in later models of the eMac, the Mac mini and the third generation iBook laptop.

The first model of processor to be used in G4 branded systems was the Motorola PowerPC 7400, debuting in the late summer of 1999 as speeds ranging from 350 to 500 Mhz.

Much of the 7400 design was done by Motorola in close co-operation with Apple and IBM, the third member of the AIM alliance, choosing not to participate citing disagreements concerning a Vector Processing Unit on the chip. Ultimately, the G4 architecture design contained a 128-bit vector processing unit labelled AltiVec by Motorola while Apple marketing referred to it as the "Velocity Engine".

With the AltiVec unit, the 7400 microprocessor can do four-way single precision floating point math, or 16-way byte math in a single cycle. Furthermore, the vector processing unit is superscalar, and can do two vector operations at the same time. Compared to Intel's x86 microprocessors at the time, this feature offered a substantial performance boost to applications designed to take advantage of the AltiVec unit.

Additionally, the 7400 has enhanced support for symmetric multiprocessing (SMP) and a 64-bit ALU, derived in part from the 604 series ALU. The 603 series had 32-bit ALUs, which took two clock cycles to accomplish 64-bit floating point arithmetic.

The floating point unit (FPU) in the 7400 was also taken from the earlier 604 CPU, because it was roughly 25% faster per clock than the FPU in the 750 CPU. While the MPX bus utilised in the 7400 series is 167MHz FSB, the width of the bus is such that the vector units (which process Altivec) are never starved for data as some have erroneously claimed.

Variants

The 7400 debuted in late summer of 1999 at speeds ranging from 350 to 500 MHz. The chip contained 10.5 million transistors and was manufactured using Motorola's 0.20 μm HiPerMOS6 process. The chip die measured 83 mm² and featured copper interconnects.

Motorola's inability in 1999 to obtain yields of the 7400 series at Apple's desired clock speed caused Apple to do an abrupt about-face on sales of its Power Macintosh G4 tower series of computers. The PowerMac series was downgraded abruptly from 400, 450, and 500 MHz processor speeds, to 350, 400, and 450 MHz. The incident caused a rift in the Apple-Motorola relationship, and reportedly caused Apple to ask IBM for assistance to get the production yields up on the Motorola 7400 series line.

The 1999 problems foreshadowed difficulities Motorola and Apple faced in competing with Wintel-x86 system clock speed increases, and the "Megahertz Myth". It also perhaps ultimately caused Apple to release SMP versions of the Power Mac G4 series (with the ad campaign "Two brains are better than one") to make up for a perceived gap in performance between the Power Mac line, and competing x86-based systems running at higher microprocessor clock speeds.

As of early 2005 the fastest processor shipping in Apple's G4 lineup is the MPC 7447B, running at 1.67 GHz and found in the January 2005 revision PowerBooks.

Future

Apple is phasing out the G4 series for desktop models after it selected the 64-bit IBM-produced PowerPC 970 processor as the basis for its G5 series.

The problems associated with the bandwidth constrained MPX bus interface found on the 7400 series are believed to be relieved with Freescale's proposed line of SoC devices, sporting an option for a faster system interface via a RapidIO or PCI Express, and an onboard DDR memory controller. This architecure, however, is still in pre-production as of Q1 2005.