PowerPC G4

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.

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 was also used in the eMac, first-generation Xserves, first-generation Mac minis as well as their flat-panel iMac before the introduction of the G5 processor. It was also used in the now-discontinued Power Mac G4 Cube.

PowerPC 7400

The 7400 (Codename: Max) 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.

Design

Much of the 7400 design was done by Motorola in close co-operation with Apple and IBM. IBM, the third member of the AIM alliance, did design the chip together with Motorola in its Somerset design center, but chose not to manufacture it, because it did not see the need back then for the Vector Processing Unit. 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".

The PPC970 aka G5 was the first IBM-manufactured CPU to implement VMX/Altivec, for which IBM reused the old 7400-Design they still had from the design they did with Motorola in Somerset. The Waternoose-CPU in the Xbox 360 also features VMX, with added proprietary extensions made especially for Microsoft. It is expected that the Nintendo Revolution CPU will also feature VMX. Power6, expected to be introduced in 2006, will be IBMs first "big iron" CPU to also implement VMX.

With the AltiVec unit, the 7400 microprocessor can do four-way single precision (32-bit) floating point math, or 16-way 8-bit, 8-way 16-bit or four-way 32-bit integer 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) thanks to an improved Cache coherency protocol (MERSI) 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 PowerPC 750 CPU.

PowerPC 7410 "Nitro"

The PowerPC 7410 uses the same design as the 7400 but it was manufactured at 180 nm instead of 200 nm. Like the 7400 it has 10.5 Million transistors. It debuted in the first-generation PowerBook G4, introduced on January 9 2001.

This chip added the ability to lock all or half of the cache as high-speed non-cache memory, mapping it into the processor's physical address space as desired. The feature was used by embedded systems vendors such as Mercury Computer Systems.

PowerPC 7450 "Voyager"

The PowerPC 7450 was the first (and, as of April 2005, only) major redesign of the G4 processor. The 33 million transistor chip added a longer pipeline, 256 KB on-chip L2 cache, and introduced external L3 Cache (up to 2 MB). Altivec got improved with the 7450, instead of dispatching one permute- and one VALU-Instruction per cycle like its predecessors the 7450 and its Motorola/Freescale-followers can dispatch two arbitrary Altivec-Instructions at the same time. It was introduced with the 733 MHz Power Mac G4 on January 9 2001. Motorola followed with an interim release, the 7451, codenamed "Apollo 6", just like the 7455.

PowerPC 7445/7455 "Apollo 6"

The PowerPC 7455 came with a wider, 256-bit on-chip cache path, and was made on a 180 nm, SOI process. It was the first processor in an Apple computer to break the 1 GHz barrier. The 7445 is the same chip without the L3-Cache Interface. The 7455 is used in the AmigaOne XE G4.

PowerPC 7447/7457 "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 58-million-transistor 7447 is slightly improved from the 7450/55, it has 512 KB on-chip L2-Cache and was manufactured at 130 nm, hence drawing less power. With the 7447A, which introduced an integrated thermal diode as well as DFS (dynamic frequency scaling) Freescale was able to reach a slightly higher clock. The 7457 has an additional L3 Cache interface. However, its frequency scaling stagnated when Apple chose to use the 7447 instead of the L3 cache-enabled 7455 they used before. This decision was most likely taken because of cost concerns, since fast SRAM used for L3-Cache is fairly expensive. The only companies that offer the 7457 in the form of upgrades for the Power Mac G4 and Power Mac G4 Cube are Giga Designs, Daystar Computers (now OWC-owned, they use the 7457 only for iMac G4 upgrades) and Powerlogix. The Pegasos computer platform also uses 7447 in its currently sold Pegasos-II/G4.

PowerPC 7448 "Apollo 8"

The 7448 is an evolution of the 7447A and is essentially a faster and more power-efficient version of the 7447A manufactured in 90 nm with 1 MB L2-Cache and up to 200 MHz front side bus and it features Freescale's new standard core, the e600. Daystar ships a 7448 upgrade for Aluminium Power Book G4s, running at up to 2 GHz.

Future

Apple completely phased out the G4 series for desktop models after it selected the 64-bit IBM-produced PowerPC 970 processor as the basis for its PowerPC G5 series. The last desktop model that used the G4 was the Mac Mini which now comes with either an Intel Core Solo or Core Duo processor . The last portable to use the G4 was the iBook G4 but now has been replaced by the MacBook. The PowerBook G4 has been replaced by the Intel based MacBook Pro.

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, which will debut in Freescale's MPC8641 and MPC8641D processor models, is still in pre-production as of early 2006.

e700

In 2004 Freescale announced a new high performace core, e700. Since then they have disclosed but a few details but it will be similar to e600 with the addition of being a 64-bit processor, running at speeds up to 3 GHz and beyond, and be named MPC87xx.

G4 and Apple

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. The first Intel Macs were released on January 10, 2006 and Apple has indicated that they will continue to replace current PowerPC-based models with Intel-based models. It has been announced that Apple will phase out use of the PowerPC architecture by the end of 2006 and as of spring 2006, Apple no longer sells any machines using G4 processors.

PowerPC microprocessors

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