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The processor module contains the high-speed components:
The devices on the processor module communicate with the main logic board by way of the PCI bus.
This section includes a description of the microprocessor, the backside cache, and the memory controller IC. For a description of the SO-DIMMs that contain the main memory, please see Chapter 4, "Expansion."
The latest family of PowerPC microprocessor designs is called "G3," for "generation three." The G3 microprocessors have several features that contribute to improved performance, including:
The G3 microprocessor in the iMac runs at a clock speed of 233 MHz.
The controller and tag storage for the backside cache are built into the microprocessor chip. The cache controller includes bus management and control hardware that allows the cache to run at a sub-multiple of the processor's clock speed, rather than at the clock speed of the main system bus. In the iMac, the clock speed of the backside cache is half that of the microprocessor.
The data storage for the backside L2 cache consists of 512 KB of fast static RAM on the processor module.
The memory controller and PCI bus bridge IC is a Motorola MPC106, also called Grackle. The Grackle IC provides the bus bridge between the processor bus used on the processor module and the PCI bus used for the ICs on the main logic board. The Grackle IC also contains the memory controller for the main memory.
The main memory bus runs at a clock speed of 66.67 MHz. The internal PCI bus runs at 33.33 MHz. To enhance performance, the Grackle IC supports concurrent transactions on the main memory bus and the PCI bus.
Information about the Grackle IC is available on the World Wide Web at
http://www.mot.com/SPS/PowerPC/products/semiconductor/support_chips/106.html