The G5 is a 64-bit member of the PowerPC processor family that is fully compatible with 32-bit code. It was first used when the Power Mac G5 was introduced in June 2003. Only three different versions of the chip were produced before Apple made the move to Intel CPUs in 2006. IBM was the only manufacturer of G5 CPUs.

Where the PowerPC G4 used much slower memory on a 100, 133, or 167 MHz data bus, the G5 accesses system memory at one-half or, at worst, one-third of CPU speed. This gives the G5 much faster access to system memory. The G5 has separate 32-bit read and write double data rate (DDR) data buses.

The G5 CPU has two arithmetic logic units (ALUs), two double-precision floating point units, two load/store units, and two AltiVec units. Or, more precisely, two partial AltiVec units. One works on integer and floating point operations, the second handles permute operations.

PowerPC 970

The first G5 CPU, the PowerPC 970, built on everything IBM had learned in producing its POWER4 CPU for servers. As CPUs designed for servers, low power consumption was not one of IBM’s goals, so the G5 Power Macs all needed very sophisticated cooling systems to deal with the heat the PowerPC 970 created. The computer had a total of nine cooling fans to control temperatures.

IBM announced the PowerPC 970 in October 2003, and it was used in the first Power Mac G5 models (1.6 and 1.8 GHz single-processor and 2.0 GHz dual-processor), which were introduced in June 2003.

The PPC 970 has a 512 KB Level 2 (L2) cache and uses a system bus running at half of the CPU’s core speed in the Power Mac G5. That meant the new Power Macs had 800 MHz to 1.0 GHz data buses, a far cry from the 133 and 167 MHz buses used in later G4 Power Macs.

When he announced the Power Mac G5, Steve Jobs promised 3.0 GHz within a year, based on promises from IBM that were never fulfilled.

PowerPC 970FX

In 2004, Apple moved to IBM’s improved CPU, the 970FX with a 64 KB instruction cache and 32 KB data cache – a step forward from using 32 KB for each throughout the G3 and G4 eras. The 970FX also had a much longer data pipeline than the original 970 to help boost clock speed, much like the Pentium 4.

Using liquid cooling to help dissipate the heat created by overclocking IBM’s CPUs, Apple was able to offer 2.5 GHz and later 2.7 GHz Power Mac G5 models. The 970FX was also used in the 2004 Xserve G5 and the iMac G5. The 970FX powered all Xserve G5 models and all iMac G5 models. (Running the data bus in the iMac G5 at one-third of clock speed helped reduce power draw and heat.)

Although the 970FX was far more power efficient than the 970, it didn’t have the energy saving features required for a notebook CPU, so no PowerBook G5 was ever produced.

PowerPC 970MP

The final CPU in the G5 line was the dual-core PowerPC 970MP, which has two cores on a single die, each core with its own 1 MB Level 2 (L2) cache, making it a bit more powerful than the 970FX. For the power hungry, Apple even made a Power Mac G5 Quad with two dual-core 970FX CPUs running at 2.5 GHz.

Apple only used the 970MP in the last generation of Power Mac G5 models, which were introduced in October 2005 and discontinued in late 2006.

With one exception, all future Macs would have multi-core processors.

PowerPC Family Overview

CPU         speed*    instructions  L1 cache  L2 cache
601       60-120 MHz   3 per cycle     32 KB  external to 1 MB
603       75-160 MHz   2 per cycle    2x8 KB
603e     100-300 MHz   2 per cycle   2x16 KB
604      100-180 MHz   4 per cycle   2x16 KB  external to 1 MB
604e     166-233 MHz   6 per cycle   2x32 KB  external to 1 MB
604ev    250-350 MHz   6 per cycle   2x32 KB  external to 1 MB
G3/750   200-450 MHz   3 per cycle   2x32 KB  external to 1 MB
750CX    366-466 MHz   3 per cycle   2x32 KB  256 MB onboard
750CXe   400-700 MHz   3 per cycle   2x32 KB  256 MB onboard
750FX    600-900 MHz   3 per cycle   2x32 KB  512 MB onboard
750GX   733-1100 MHz   3 per cycle   2x32 KB  1024 MB onboard
G4/7400  350-600 MHz  19 per cycle+  2x32 KB  supports 2 MB L2 cache
7410     466-533 MHz  20 per cycle+  2x32 KB  supports 1 MB L2 cache
7450     667-733 MHz  20 per cycle+  2x32 KB  256 KB onboard, up to 2 MB L3 
7455    600-1420 MHz  20 per cycle+  2x32 KB  256 KB onboard, up to 2 MB L3
7447A   600-1500 MHz  20 per cycle+  2x32 KB  512 KB onboard, no L3 cache
7457    867-1267 MHz  20 per cycle+  2x32 KB  512 KB onboard, up to 4 MB L3
        7457 used in some third-party Mac upgrades, never by Apple 
7448     1.0-1.7 GHz  20 per cycle+  2x32 KB  1024 KB onboard, no L3 cache
G5/970   1.6-2.0 GHz  38 per cycle+  2x32 KB  512 KB onboard 
970FX    1.8-2.7 GHz  38 per cycle+  64+32 KB 512 KB onboard
970MP    1.8-2.5 GHz  38 per cycle+  64+32 KB 1 MB per core 
__________ 
* as used in Apple or Maclone 
+ each AltiVec unit can perform up to 16 simultaneous calculations

PowerPC family: 601, 603/603e, 604/604e, G3, G4, G5

Other Resources

• Great CPUs, past and present, John Bayko. See especially sections on 8080/85, Z-80, 6502, 6809, 680×0, 80×86, ARM (used in Newton), PA-RISC, Sparc, Alpha, PowerPC, and Itanium.
• MacTips, RISC, CISC, and Your Mac
• PC Magazine, Motorola and PowerPC (also covers 680×0 series)
• PowerPC 970, Wikipedia
• PowerPC 601 v. 603, MacKiDo
• How the G4 Compares to the G3, Scott Barber
• AltiVec, MacKiDo
• G4, MacKiDo
• Pipelines, MHz, latency, caches, and more, MacKiDo
• AltiVec Performance Comparison, Apple
• MPC7400 PowerPC Microprocessors, Motorola

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