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iMac G5 Developer Note
This note describes the iMac G5 introduced in October 2005. It includes information about distinguishing features of the computer, including components on the main logic board: the microprocessor, the other main ICs, and the buses that connect them to each other and to the I/O interfaces.
The iMac G5 comes with Mac OS X version 10.4.2 installed. The Classic environment can be installed from the included system software optical disk and used to run Mac OS 9 applications.
The value of the iMac G5 model identifier string is PowerMac12,1.
Architecture
The architecture of the iMac G5 is based on the PowerPC G5 microprocessor and two custom ICs, the U4 memory controller and the Shasta I/O controller, connected by a HyperTransport bus. The U4 provides the bridging functionality among the processor, memory system, HyperTransport bus, and PCI Express bus. The Shasta controller supports these components:
Ultra ATA/133 bus for the optical drive
Serial ATA (SATA) bus for the disk drive
Vesta IC, comprising the FireWire 400 (1394a) PHY and the Ethernet PHY
I2S channel to the audio subsystem
33 MHz, 32-bit internal PCI bus, which in turn supports the USB2 controller, the AirPort Extreme module, and the boot ROM.
Figure 1 provides a simplified block diagram of the U4 and Shasta ICs and other major components, and the buses that connect them together.
Features
The iMac G5 computer includes a programmable Apple Mighty Mouse, a built-in iSight video camera, an integrated IR receiver, and the Apple Remote. For a complete list of user-visible features, see the iMac G5 specification sheet at Apple's Specifications site. Other features are described in this section.
PowerPC G5 Processor
The microprocessor in the iMac G5 is a PowerPC G5 with a clock speed of 1.9 GHz in the 17-inch configuration and 2.1 GHz in the 20-inch configuration. See the support site at IBM for detailed microprocessor documentation. The processor connects to the U4 IC through a bus run at one-third the speed of the processor (up to 700 MHz) comprising two unidirectional 32-bit data buses.
Memory
One of two 64-bit 533MHz buses connects the U4 IC to the on-board DDR2 (PC2-4200) SDRAM memory, and the other connects it to the DIMM expansion slot. For additional information, refer to RAM Expansion Developer Note.
PCI Express Bus and Graphics Subsystem
In the iMac G5, the graphics subsystem is connected to the U4 IC by a x16 link (16 lane), dual simplex, 2.5 GHz PCI Express bus. For more information on PCI Express, refer to PCI Developer Note. For more information on the graphics IC, refer to Video Developer Note.
HyperTransport Technology
The HyperTransport bus between the U4 IC and the Shasta IC is 400 MHz DDR, 8 bits wide in both directions, supporting a total of 800 MBps bidirectional throughput. For more information on HyperTransport, see the HyperTransport Consortium website.
Serial ATA Drive Interface
The iMac G5 supports a 7200 rpm disk drive on an independent drive bus based on the Serial ATA (SATA) 1.0 specification. For more information on SATA, see the Serial ATA International Organization (SATA-IO) website.
PCI USB Controller
The iMac G5 uses a PCI USB controller ASIC with a total of five ports available to support three external USB ports, the Bluetooth module, and a USB2 hub for internal connections. The five USB ports comply with the Universal Serial Bus Specification 2.0. For more information, see Universal Serial Bus Developer Note.
AirPort Extreme and Bluetooth 2.0 + EDR Module
The iMac G5 has a combined internal AirPort Extreme and Bluetooth 2.0 + EDR (enhanced data rate) module. AirPort Extreme and Bluetooth share two built-in antennas. For more information, see AirPort Developer Note and Bluetooth Developer Note.
Ethernet Controller
The Vesta Ethernet PHY provides 10BASE-T/UTP, 100BASE-TX, or 1000BASE-T operation over a standard twisted-pair interface. For more information, see Ethernet Developer Note.
FireWire Controller
The Shasta IC provides the FireWire functions and includes a FireWire controller that supports IEEE 1394a (FireWire 400) with a maximum data rate of 400 Mbps (50 MBps). The Shasta IC provides DMA (direct memory access) support for the FireWire interface. Vesta provides the FireWire PHY.
For more information, see FireWire Developer Note.
Ultra ATA/133 Interface and Optical Drive
In the iMac G5 computer, the Shasta IC provides an Ultra ATA/133 interface to the slot-loading, 8x SuperDrive with double layer burning capability. The drive can read and write DVD media and CD media, as shown below.
Media type | Reading speed | Writing speed |
|---|---|---|
DVD +/- R | 6x (CAV) | 8x ZCLV |
DVD+R DL | 6x (CAV) | 2.4x CLV |
DVD-ROM | 8x (CAV) | – |
DVD-ROM DL | 6x (CAV) | – |
DVD +/- RW | 6x (CAV) | 4x ZCLV |
CD-R | 24x (CAV) | 24x ZCLV |
CD-RW | 24x (CAV) | 16x ZCLV |
CD-ROM | 24x (CAV) | – |
The SuperDrive is cable-select as device 0 (master) and complies with the ATA/ATAPI-5 industry standard. For information on parallel ATA interfaces, see the International Committee on Information Technology Standards (INCITS) Technical Committee 13 AT Attachment website.
Interrupt Support
The interrupt controller for the iMac G5 system is an MPIC cell in the U4 IC. In addition to accepting internal interrupt sources from the I/O, the MPIC controller accepts internal interrupts from the Shasta IC and dedicated interrupt pins.
Audio System
The iMac G5 has a built-in microphone (located at the top of the display), and both an analog audio line-in jack and a combined analog and S/PDIF audio line-out jack on the rear panel. For more information, see Audio Developer Note.
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