The PowerPC G4: From 350 MHz to 2.0 GHz
Dan Knight - 2007.05.24
The Macintosh made a big leap forward when Apple introduced the Power Mac G4 on August 31, 1999. Available in two models, one with PCI graphics and a faster one with AGP graphics, these new Power Macs not only looked more professional with their graphite finish, they also had a lot more processing power for audio and video content thanks to the AltiVec vector processing unit Motorola had created.
Where the PowerPC G3 had been optimized to run existing PowerPC code and had been the most efficient CPU used in a Mac to date, AltiVec added a whole new range of commands that would be especially useful for working with images and video. Applications and OS components such as iTunes, Photoshop, QuickTime, and Quartz were designed to take advantage of the vector processing capabilities in the G4 family of processors.
AltiVec, which Motorola developed from 1996 through 1998, is similar to the SSE (Streaming SIMD Extensions) extensions Intel began using in 1999 with the Pentium III. Both technologies use 128-bit vector registers and can represent sixteen 8-bit numbers, eight 16-bit numbers, or four 32-bit numbers. What sets AltiVec apart from SSE is that it supports an RGB pixel data type and doesn't handle 64-bit numbers.
The first G4 CPU, released in late summer 1999, supported a 100 MHz system bus and ran at speeds up to 500 MHz, although it was several months before Motorola was able to provide the 500 MHz chip in quantity. The 500 MHz "Sawtooth" Power Mac G4 officially shipped in February 2000.
In addition to adding AltiVec to technologies found in the PowerPC G3, the G4 supported an improved bus technology for faster memory access and included far better multiprocessor support, paving the way for the Summer 2000 "Mystic" Power Mac G4 with dual 450 MHz and 500 MHz CPUs.
Like early G3 designs, the PPC 7400 uses an external level 2 (L2) cache, which Apple ran at half CPU speed on the G4 Power Macs.
The PowerPC 7410 (identified as "CPU Type: PowerPC G4 (11.3)" by System Profiler), is a low power version of the G4 found in early G4 PowerBooks and the 466-533 MHz Digital Audio Power Macs.
PowerPC 7450 (G4e or G4+)
The first major update to the G4 was introduced in 2001. The PPC 7450 includes a longer pipeline, an on-chip 256 KB L2 cache, support for an external level 3 (L3) cache, and the ability to use a 133 MHz system bus. This CPU was first used in the Power Mac G4 "Digital Audio" in January 2001.
Motorola used a wider data path to the cache in the PPC 7455 (identified as "CPU Type: PowerPC G4 (2.1)" by System Profiler), which was the first G4 CPU to hit the 1 GHz mark. The 7445 is the 7455 without support for a L3 cache. The 7455 was released in January 2002.
This was the CPU that made it into Apple's last generation PowerBooks, where it ran at up to 1.67 GHz. It includes a 512 KB onboard cache and supports a 167 MHz system bus. The PPC 7447/57 came to market in September 2003.
The next (and possibly final) revision of the G4 came out in early 2005 and includes a 1 MB L2 cache, a 200 MHz system bus, and runs at speeds up to 2.0 GHz. Apple never used it, but it has been used in some CPU upgrades.
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