PowerPC G4 is a designation used by Apple Computer to describe a fourth generation of PowerPC microprocessors. Apple has applied this name to various different (though closely related) processor models from Freescale, a former part of Motorola.
Apple Macintosh computers such as the PowerBook G4 and iBook G4 laptops and the Power Mac G4 desktop all take their name from the processor. A PowerPC G4 is also used in the eMac, iMac (flat panel), Xserve and Mac mini.
The 7400 debuted in late summer of 1999 and was the first processor to carry the G4 moniker. The chip operates at speeds ranging from 400 to 500 MHz and contains 10.5 million transistors, manufactured using Motorola's 0.20 μm HiPerMOS6 process. The chip die measures 83 mm² and features 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 Mac G4 tower series of computers. The Power Mac series was downgraded abruptly from 400, 450, and 500 MHz processor speeds to 350, 400, and 450 MHz. The incident generated 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 500 MHz model was reintroduced on February 16 2000.
Much of the 7400 design was done by Motorola in close co-operation with Apple. IBM, the third member of the AIM alliance, chose 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. Some examples are Adobe Photoshop which utilises the AltiVec unit for faster rendering of effects and transitions, and Apple's iLife suite which takes advantage of the unit for importing and converting files on the fly.
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.
PowerPC 7410 "Nitro"
PowerPC 7450 "Voyager"
The PowerPC 7450 was the first (and, as of April 2005, only) major redesign of the G4 processor. It added a longer pipeline and on-chip L2 cache. It was introduced with the 733 MHz Power Mac G4 on January 9 2004.
PowerPC 7455 "Apollo"
The PowerPC 7455 came with a wider, 256-bit on-chip cache path, and was made on a 180nm, SOI process. It was the first processor in an Apple computer to break the 1 GHz barrier.
PowerPC 7447 "Apollo 7"
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. The 7447 uses a similar design to the 7450.
The 7448 is an evolution of the 7447A and was, as of early 2005, shipping in engineering quantities. It is essentially a faster and more power-efficient version of the 7447A and features Freescale's new standard core, the e600.
The problems associated with the bandwidth constrained MPX bus interface found on the 745x series are believed to be relieved with Freescale's proposed line of SoC devices, sporting a single or dual e600 core and an option for a faster system interface via a RapidIO or PCI Express, and an onboard DDR memory controller. This architecure is still in pre-production as of early 2005.
On June 7 2005, during the World Wide Developers Conference 2005 (WWDC2005), Steve Jobs announced that Apple will be dropping the PowerPC line of processors and will slowly move over to Intel processors starting in 2006, phasing out their use of the PowerPC architecture by 2007. This leaves Apple's use of future generation G4 processors in doubt.