The Road Ahead: 64-Bit Computing

2009 – Apple is billing Mac OS X 10.6 Snow Leopard as its first fully 64-bit operating system, but this isn’t the first time the Mac OS has changed it bitness.

8-bit Computing

The world of personal computing began with 8-bit CPUs that could support up to 64 KB of memory. That seemed like a lot at the time, as many early PCs (back when that stood for Personal Computers) only shipped with 4 KB, 8 KB, or perhaps 16 KB of RAM.

The biggest technological breakthrough in the 8-bit era was bank switching, which allowed a CPU to switch between two or more 64 KB banks of memory. The Apple, Apple II, and Apple III were 8-bit computers, and the III did use bank switching. The most popular 8-bit operating system was CP/M.

16-bit Computing

Although the IBM PC was the machine that marked the shift from 8-bit to 16-bit computing, it wasn’t the first 16-bit PC. That honor goes to the Texas Instruments TI-99/4, which was released in June 1979. It was overpriced for the home computer market, as it shipped with a 13″ Zenith color monitor. It used a chiclet keyboard, had a cartridge slot for games and programs, and used a costly expansion chassis if you wanted to add drives. Like early Apple II computers, it didn’t even have lower case.

The 3 MHz 16-bit CPU only had access to 256 Bytes of 16-bit memory – everything else was on an 8-bit bus and handled by a multiplexer. This is because TI had intended to ship an 8-bit PC, but the 8-bit CPU it was designing for the machine was abandoned. This was far from the last time the PC world would see such an architecture.

The most popular 16-bit CPU on an 8-bit bus was the Intel 8088 used in the original IBM PC, its early successors, and a glut of IBM clones. Intel had designed the 8086 as a 16-bit CPU with a 16-bit bus, and it could address up to 1 MB of memory in 64 KB banks. The 8088 allowed computer makers to use a less expensive 8-bit memory bus. Lower costs mean lower prices – so what if performance suffered a bit.

MS-DOS, Microsoft’s 16-bit adaptation of CP/M, was the most popular 16-bit operating system.

The 24-bit Mac OS

Although the Macintosh Team had originally hoped to use the Motorola 6809 8-bit CPU to keep costs down, the decision to go with a relatively high resolution graphical user interface (GUI) required more power. Apple chose the Motorola 68000 CPU as the heart of the original Macintosh, the same chip the Lisa team had settled on for its $10,000 machine.

The 68000 was a 32-bit CPU on a 16-bit bus with a 23-bit address bus that could handle up to 16 MB of RAM. Although it had a 32-bit architecture, it ignored the high-order byte and functioned as a 24-bit CPU. The Mac’s ROMs and operating system were all created for this 24-bit environment with a maximum 16 MB address space.

That wasn’t a limitation in the early Mac era – 512 KB was viewed as a lot of memory after the 128 KB Mac, and the Mac Plus could be expanded from a mind-boggling (for 1986) 1 MB of RAM to 4 MB – if you could afford it. It wasn’t until the 1987 introduction of the Mac II that any Mac had a true 32-bit CPU or could handle more than 4 MB of RAM. As high capacity memory chips weren’t yet available, the 128 MB theoretical maximum sounded very future oriented, but the early Mac OS couldn’t address more than 8 MB (later versions could use up to 10 MB).

The 32-bit Mac OS

Apple introduced System 7 in 1991, the first version of the Mac OS with 32-bit support. Multitasking (optional with MultiFinder in System 6) was a standard feature for the first time, 32-bit QuickDraw meant 24-bit color, and those Macs that supported more than 8-10 MB  of RAM (ranging from 16 MB to 128 MB) could finally use it.

The problem was that 68000-based Macs couldn’t use 32-bit mode at all, as they were designed to function only as 24-bit CPUs. Worse, some early Macs with 32-bit CPUs had ROMs designed only for a 24-bit environment – ROMs that were not compatible with 32-bit operation. These were called “32-bit dirty” Macs, and Connectix released a software patch (Mode32) to give them full 32-bit support.

From System 7 in 1991 through Mac OS X 10.5 Leopard, through architectural changes from Motorola 680×0 CPUs to PowerPC CPUs to Intel CPUs, the Mac OS has been 32-bit.

The 64-bit Future

IBM designed the PowerPC 970 to handle both 32-bit and 64-bit instructions natively. The chip became known as the G5 when Apple introduced the Power Mac G5 in June 2003 as the first 64-bit personal computer. Just as Apple had sold Macs with 32-bit CPUs in the era of the 24-bit Mac OS, the G5 and most Intel-based Macs had 64-bit CPUs coupled with a 32-bit operating system.

Apple has slowly added a bit of 64-bit support – first with OS X 10.4.7 Tiger to allow more than 4 GB of RAM (although individual apps were still limited to the old maximum), and adding additional 64-bit support in OS X 10.5 Leopard. With OS X 10.6 Snow Leopard, the entire operating system will have 64-bit capabilities – although it appears that most Macs will default to 32-bit mode, at least initially.

Benefits of a 64-bit Operating System

One of the primary arguments for 64-bit computing is breaking the 4 GB memory barrier. A 64-bit OS can theoretically support 16 exabytes (16 million gigabytes) of RAM, although none of today’s personal computers can come anywhere near that. A few support 128 GB or more – which is horribly expensive – and most of today’s Macs top out at “just” 8 GB.

As a low-end user, I’ve never had a Mac with more than 2 GB of RAM, and that has always seemed like plenty. Frankly, going beyond 4 GB isn’t a real issue for most computer users.

However, there are places where you can benefit from 64-bit operation regardless of how much RAM your Mac has:

  • Files over 4 GB in size are a reality – DVDs, disk images, video, etc. 32-bit operating systems struggle with them; 64-bit operating systems take them in stride.
  • Data encryption/decryption can run 3-5 times faster in a 64-bit environment. This may apply to stuffing and unstuffing files as well. Think email attachments.

Complex numerical analysis is another place where 64-bit operating systems doing 128-bit math will be more accurate, but that’s probably not significant to most computer users.

Drawbacks of a 64-bit Operating System

The biggest drawback of a 64-bit OS is that data occupies more space in memory – which can be offset by installing more RAM. One way of addressing this is to allow 32-bit operating within the 64-bit operating system. Snow Leopard fully supports 32-bit software when running in 64-bit mode, and both the G5 and Core 2 Duo CPUs support 32-bit apps when running a 64-bit OS.

The Road Ahead

The 8-bit era was relatively short, from the mid 1970s to the mid 1980s. The 16-bit era took off with the introduction of the IBM PC in August 1981 and started fading out with the introduction of Windows 95. The Mac’s 24-bit era ran from 1984 through 1996, when Mac OS 7.6 was introduced with no 24-bit support. The 32-bit era began in 1995 on PCs and 1991 on Macs, and it remains dominant to this day. [Editor’s note: This article was written in 2009.]

64-bit Windows

The first 64-bit version of Windows for PCs was introduced in April 2005 (previous 64-bit versions of Windows supported PowerPC, Alpha, MIPS, and Itanium), and Apple began adding 64-bit support to Mac OS X in 2006. To this day, 32-bit Windows dominates. Microsoft estimated that over 25% of Vista users were 64-bit at the end of 2008, and some dealers are now seeing upwards of 50% 64-bit sales. Unfortunately for Microsoft, only about 20% of Windows users are running Vista, so perhaps 6-7% of Windows users have gone 64-bit at this point.

Windows 7 is expected to be the last version of Windows available with 32-bit mode, and already projections are that over half of all users will choose the 64-bit version. Windows 7 could be attractive to Windows XP users (75% of all Widows users!) with relatively up-to-date hardware.

64-bit Macs

Mac users are blessed: We don’t have to choose whether to buy a 32-bit or 64-bit OS, because Snow Leopard installs both – much as Leopard installs PowerPC and Intel support. At this point, about 75% of Mac users are using Intel-based Macs, 25% PowerPC hardware. The adoption rate for Mac OS X 10.5 has been very high, outnumbering 10.4 users about 3.5:1 – and the ratio is probably higher on Intel Macs than PowerPC ones.

Looking at a $29 upgrade fee for Snow Leopard and assuming Apple is going to actively market it to the installed base, I’m guessing that two-thirds of current Intel Mac users will make the upgrade and over half of all Mac users will be on Snow Leopard by the end of the year.

The road ahead is 64-bit, and it appears to be a long road. There are no 128-bit CPUs available for personal computers, and there doesn’t appear to be any reason for them in the foreseeable future.

The road ahead is multicore computing with 64-bit CPUs and 64-bit operating systems. While many of us may continue to work and play productively with 32-bit operating systems for years, 64-bit computing will be the norm by the end of 2010.

We’re fortunate that Apple is making the migration easy.

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