G5 Energizes 64-Bit Race

Players must still win over 32-bit world.

With the arrival two months from now of Apple Computer Inc.s new desktop machines and their 64-bit CPUs, the race to the next generation of mass-market processing power suddenly has a third horse running hard. Intel Corp.s Itanium, Advanced Micro Devices Inc.s Opteron and Athlon 64, and now the IBM PowerPC 970 (or G5) processor in forthcoming Apple systems offer distinctly different options to users of memory-intensive applications.


  • PowerPC 970 chip shares key strengths with IBMs superserver Power4 CPU; Apples G5 version largely lives up to the promise of G4 compatibility, but its not an across-the-board improvement over G4
  • Deeper pipelines, as with Pentium 4s, handicap unpredictable code
  • Fewer on-board processing units than G4 for simple integer operations; system design emphasizes high throughput, efficient use of hardware
  • Fast front-side bus, HyperTransport I/O interface
  • Low power consumption, especially at reduced clock rates
All three 64-bit architectures must compete against the frugal enterprise buyers base case of continuing to buy commodity-priced workstations and servers using variants of Intels 32-bit Pentium 4. Intels gamble on a long instruction pipeline in the P4, although it slows performance of unpredictable operations, has enabled a rapid climb to clock speeds once considered unthinkable for an x86 machine.

Indeed, the P4 has been almost too successful: Intel introduced the architecture now called Itanium by arguing that buyers needed to abandon x86 software and skills to attain the next level of performance, but the P4s progress has weakened that claim.

Intel celebrated the graduation season by shipping a 3.2GHz P4 that should enjoy a short but hot summer as leader of the pack. At the end of next month, however, the P4 will return from the holiday weekend to labor against the Athlon 64s offer of x86 compatibility combined with 64-bit headroom.

PC users who remember the debut of the 32-bit 386 chip will understand the immediate value proposition of AMD64 architecture, even in a world of 32-bit packaged software. Just as "DOS extenders" provided independent execution environments to 16-bit applications on 32-bit machines, so AMD promises that 32-bit applications will benefit from having the elbowroom of separate 4GB neighborhoods within their 16-exabyte universe.



  • Larger address space for memory-intensive applications
  • Separate 4GB spaces for 32-bit applications
  • Early buy-in to leading-edge processor speed improvements

Why not

  • More bits being moved around invites bandwidth bottlenecks
  • Every 32-to-64-bit migration has some degree of software incompatibility
  • Pentium 4 still has headroom for faster clocks and other enhancements
Applications that want to go out and play in the AMD chips 64-bit yard can be recompiled to run in native mode. Theyll enjoy the support of mainstream operating systems including Windows Server 2003 as well as Linux.

Applications for the Itanium, which uses a completely different architecture rather than an x86 superset, must also be recompiled to run in anything but a sadly underperforming x86-compatibility mode. But Itanium compilers must do more: The Itanium achieves relative simplicity of hardware by requiring that compilers support Explicitly Parallel Instruction Computing (hence the Intel acronym EPIC) in application binaries. A mature Itanium software base is still a distant goal.

Intel proposes to remedy the bandwidth bottlenecks of loading all those explicit instructions by providing enormous cache memories. The dual-core Itanium planned for 2005 is expected to have 18MB of on-chip cache.

The PowerPC 970 chip in the new Apple machines, by contrast, is the baby brother of the IBM Power4 processor, which is among the titans of bit-stream bandwidth; the 970s potential is well-supported with a fast front-side bus. Like the AMD64 architecture, Apples 64-bit machines use the multivendor HyperTransport protocol for improved throughput among other subsystems as well.

Also like the AMD64-family processors, the PowerPC 970 has substantial compatibility with the 32-bit G4 software base (with a few exceptions and key performance gotchas). Overall, it appears to deserve the "G5" appellation connoting continuity with Apples G4 machines.

Power consumption of the G5 looks to be about three-fourths that of a P4 when clocked for comparable performance—the G5s efficiency and speed may quickly spread its use from Apples top tier throughout the rest of its line and may also make Apple a top-tier option for even the most performance-conscious users.

Technology Editor Peter Coffee can be reached at peter_coffee@ziffdavis.com.