Apple M9592LL Technology Overview - Page 7

In Power Mac G5 Quad systems, each dual-core PowerPC G5 processor has its own bidirectional frontside bus. Technology Overview 7 Power Mac G5 Bidirectional Frontside Bus Leveraging the dual frontside bus architecture pioneered in the original Power Mac G5, each dual-core processor has an independent data path to the system controller running at up to 1.25GHz. Unlike conventional processor interfaces, which carry data in only one direction at a time, this dual-channel frontside bus has two 32-bit pointto-point links (64 bits total): One link travels into the processor and another travels from the processor, which means no wait time while the processor and the system controller negotiate which will use the bus or while the bus switches direction. This enables data to move in opposite directions simultaneously-a dramatic improvement over previous processor interfaces. In Power Mac G5 Quad systems, each processor has its own bidirectional interface to the system controller, unlike traditional dual-processor systems, which constrain throughput by placing all processor resources on one bus. Each G5 processor has a dedicated interface to main memory for total bandwidth of up to 10 GBps per processor, or a total of 20 GBps for a quad system. This high-performance frontside bus architecture also enables each core to discover and access data in the other cores' caches-further increasing performance on quad-core systems. 64-Bit Processor Architecture The dual-core PowerPC G5 joins forces with Mac OS X v10.4 Tiger to enable 64-bit computation, including the ability to address vast amounts of main memory and to perform double-precision floating-point calculations. Support for massive amounts of memory The move to 64-bit processors results in a dramatic leap in the amount of memory supported. In practice, memory addressing is defined by the physical address space of the processor. The PowerPC G5, with 42 bits of physical address space, supports a colossal 242 bytes, or 4 terabytes (4TB), of system memory. Although it's not currently feasible to purchase 4TB of RAM, the advanced architecture of this processor allows for plenty of growth in the future. More practical and still far more than a typical PC, the Power Mac G5 can be configured with 16GB of addressable memory. Such large quantities of memory enable the system to contain a complex 3D model, massive digital images, a scientific simulation, or a sequence of video entirely in RAM. When data is stored in memory, the processor can access it 40 times faster than from the hard drive, drastically reducing the time to manipulate, modify, and render the data and making it feasible to tackle gigantic projects on a desktop system. 64-bit computation power The other advantage provided by the 64-bit PowerPC G5 is the ability to perform multiple simultaneous 64-bit floating-point and integer calculations. The PowerPC G5 features full 64-bit data paths and data registers, allowing it to express the extreme precision needed for floating-point mathematics and to express integers up to 18 billion billion. By contrast, a 32-bit processor must break these types of computations into multiple pieces-requiring multiple passes through the processor and slowing down application performance.