Macintosh Hardware - Processor Architecture

Processor Architecture

The original Macintosh used a Motorola 68000, a 16/32-bit (32-bit internal) CISC processor that ran at 8 MHz. The Macintosh Portable and PowerBook 100 both used a 16 MHz version. The Macintosh II featured a full 32-bit Motorola 68020 processor, but the Mac ROMs at the time contained software that only supported 24-bit memory addressing, therefore using only a fraction of the chip's memory addressing capabilities unless a software patch was applied. Macs with this limitation were referred to as not being “32-bit clean.” The successor Macintosh IIx introduced the Motorola 68030 processor, which added a memory management unit. The 68030 did not have a built-in floating point unit (FPU); thus, '030-based Macintoshes incorporated a separate unit—either the 68881 or 68882. Lower-cost models did without, although they incorporated an FPU socket, should the user decide to add one as an option. The first “32-bit clean” Macintosh that could use 32-bit memory addressing without a software patch was the IIci. In 1991, Apple released the first computers containing the Motorola 68040 processor, which contained the floating point unit in the main processor. Again, lower-cost models did not have FPUs, being based on the cut-down Motorola 68LC040 instead.

After 1994 Apple used the PowerPC line of processors, starting with the PowerPC 601, which were later upgraded to the 603 and 603e and 604, 604e, and 604ev. In 1997, Apple introduced its first computer based on the significantly upgraded PowerPC G3 processor; this was followed in 1999 with the PowerPC G4. The last generation of PowerPC processor to be introduced was the 64-bit PowerPC 970FX ("G5"), introduced in 2003. During the transition to the PowerPC, Apple’s “Cognac” team wrote a 68030-to-PowerPC emulator that booted very early in OS loading. Initially the emulation was very slow, but later versions used a dynamic recompilation emulator which boosted performance by caching frequently used sections of translated code. The first version of the OS to ship with the earliest PowerPC systems was estimated to run 95% emulated. Later versions of the operating system increased the percentage of PowerPC native code until OS X brought it to 100% native.

The PowerPC 604 processor introduced symmetric multiprocessing (SMP) to the Macintosh platform, with dual PowerPC 604e-equipped Power Macintosh 9500 and 9600 models. The G3 processor was not SMP-capable, but the G4 and G5 were, and Apple introduced many dual-CPU G4 and G5 Power Macs. The top of the range Power Macintosh G5 uses up to two dual core processors, for a total of four cores.

On June 6, 2005, Steve Jobs announced that the company would begin transitioning the Macintosh line from PowerPC to Intel microprocessors (the transition was completed on August 7, 2006) and demonstrated a version of Mac OS X running on a computer powered by an Intel Pentium 4 CPU. Intel-powered Macs are able to run Macintosh software compiled for PowerPC processors using a dynamic translation system known as “Rosetta.”

The first Macs with Intel processors were the iMac and the 15-inch MacBook Pro, both announced at the Macworld Conference and Expo in January 2006. Throughout the year the Mac mini was transitioned to the Intel architecture, with users having choice of either Core Solo or Core Duo CPUs. The iBook product line was phased out by the MacBook and on August 7, 2006, the Power Mac G5 was discontinued in favor of the Mac Pro, based on the new Intel Xeon "Woodcrest". The Xserve was also transitioned to an Intel Xeon "Woodcrest". In the second half of 2006 Apple launched new iMac and MacBook lines using the Core 2 Duo processor.