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Architecture
This chapter describes the architecture of the eMac. It includes information about the major 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.
Block Diagram and Buses
This section is an overview of the major ICs and buses on the computer’s main logic board.
Block Diagram
Figure 2-1 is a simplified block diagram of the main logic board. The diagram shows the main ICs and the buses that connect them together.
Main ICs and Buses
The architecture of eMac is designed around the PowerPC G4 microprocessor and the Intrepid memory and I/O device controller. The Intrepid occupies the center of the block diagram.
The MaxBus connects the PowerPC G4 microprocessor to the Intrepid ASIC. The MaxBus has 64 data lines, 32 address lines, and a bus clock speed of 167 MHz. The Intrepid ASIC has other buses that connect with the boot ROM, the hard disk drive, and the optical drive, the power controller IC, the sound IC, the internal modem module, and the optional wireless LAN module.
The Intrepid I/O controller has a 32-bit PCI bus with a bus clock speed of 33 MHz.
Each of the components listed here is described in one of the following sections.
Microprocessor and Cache
The microprocessor is a 1.25 GHz or 1.42 GHz PowerPC G4 with a built-in level 2 (L2) cache.
PowerPC G4 Microprocessor
The PowerPC G4 microprocessor used in the eMac has many powerful features, including.
32-bit PowerPC implementation
superscalar PowerPC core
Velocity Engine (AltiVec technology): 128-bit wide vector execution unit
pipelined, high bandwidth system bus, called MaxBus
dual 32 KB instruction and data caches (level 1)
built-in 512 KB backside L2 cache
To find more information, see the reference at PowerPC G4 Microprocessor.
Level 2 Cache
The data storage for the L2 cache consists of 512 KB of fast static RAM that is built into the microprocessor chip along with the cache controller. The built-in L2 cache runs at the same clock speed as the microprocessor.
Memory and I/O Device Controller
The Intrepid ASIC combines several functions into a single IC. The IC contains the memory controller, the PCI bus bridge, the Ethernet and FireWire 400 interfaces, the USB 1.1 interface, and the AGP interface.
In addition to the buses listed in Table 2-1, the Intrepid ASIC also has separate interfaces to the physical layer (PHY) ICs for Ethernet and FireWire 400 and an I2C interface that is used for configuring the memory subsystem.
The Intrepid ASIC provides DB-DMA (descriptor-based direct memory access) support for the I/O channels. The DBDMA system provides a scatter-gather process based on memory resident data structures that describe the data transfers. The DMA engine is enhanced to allow bursting of data files for improved performance.
The following sections describe the subsystems that are connected to the Intrepid ASIC.
System RAM
The memory subsystem in the eMac consists of 256 MB or 512 MB of DDR333 (PC2700) SDRAM in one of the two DIMM slots. The data bus to the RAM is 64 bits wide, and the memory interface is synchronized to the MaxBus interface at 167 MHz. The system supports a maximum of 1 GB.
For more information on system RAM, see RAM Expansion.
Video Display Subsystem
The display subsystem consists of a graphics controller IC and 32 MB or 64 MB of DDR on the main logic board. The graphics controller IC is either an ATI Radeon 9200 with 32 MB 190 MHz DDR memory or an ATI Radeon 9600 with 64 MB 200 MHz DDR memory. It contains 2D and 3D acceleration engines, front-end and back-end scalers, a CRT controller, and an AGP bus interface with bus master capability.
The interface between the graphics IC and the rest of the system is a 4x AGP (accelerated graphics port) bus on the Intrepid IC. To give the graphics IC fast access to system memory, the AGP bus has separate address and data lines and supports deeply pipelined read and write operations. The AGP bus has 32 data lines and a clock speed of 66 MHz.
The graphics IC uses a graphics address remapping table (GART) to translate AGP logical addresses into physical addresses. The graphics driver software can allocate memory in both the dedicated SDRAM and the main memory.
For information about the display and supported resolutions, see Video Display.
Boot ROM
The boot ROM is a 1 M by 8 bit device and is connected to the Intrepid ASIC by way of the high byte of the PCI bus plus three additional control signals: chip select, write enable, and output enable.
Ethernet Controller
The eMac includes an ethernet media access controller (MAC) that implements the Link layer. The Intrepid ASIC provides DB-DMA support for the Ethernet interface.
The controller is connected by a PCI bus to a PHY interface IC that is capable of operating in either 10-BaseT or 100-BaseT mode. The actual speed of the link is automatically negotiated by the PHY and the bridge or router to which it is connected. For information about the port, see Ethernet Port.
FireWire 400 Controller
The eMac includes an IEEE 1394a FireWire 400 controller with a maximum data rate of 400 Mbps (50MBps). The Intrepid IC provides DMA (direct memory access) support for the FireWire interface. The FireWire 400 controller complies with the Open Host Controller Interface (OHCI) specification.
The controller IC implements the FireWire link layer. A physical layer IC, called a PHY, implements the electrical signalling protocol of the FireWire interface. The PHY supports two FireWire 400 ports by way of external connectors in the I/O bay.
The PHY is powered as long as the computer is connected to AC power. While the PHY is operating, it acts as a repeater from one port to another so that the FireWire bus remains connected. For more information, see FireWire 400 Ports.
Ultra ATA/100 Interface
The Ultra ATA/100 interface complies with and supports a subset of industry standard ATA/ATAPI-6 and transfer protocols up to ultra DMA mode 5. The internal hard disk drive is configured as cable select. For more information, see Hard Disk Drive.
EIDE Interface
The EIDE interface complies with and supports a subset of industry standard ATA/ATAPI-5 and transfer protocols up to Multiword DMA Mode 2. The Combo drive and SuperDrive are device-selected as cable select in an ATA device configuration. For more information, see CD-ROM Drive, Combo Drive, and SuperDrive.
Modem Support
The internal modem is connected to an internal I2S interface. The modem provides digital call progress signals to the sound circuitry. The analog side of the connection gets power from the phone line; the digital side gets power from the eMac.
The modem is a separate module that contains the datapump and the interface to the telephone line (DAA). For more information about the modem, see Internal Modem.
Bluetooth Technology
Bluetooth is available as an internal build-to-order option in the eMac. Bluetooth is an open specification that enables short-range wireless connections between desktop and laptop computers and a host of other peripheral devices. For more information on Bluetooth technology, refer to Bluetooth Technology.
Power Controller
The power management controller in the eMac is a custom IC called the PMU99. In addition to turning the machine on and off, the power controller supports several power-saving modes of operation.
Sound Circuitry
The sound circuitry is connected to the Intrepid IC by standard IIC and IIS buses. The IIC bus provides configuration access to the audio circuitry ICs and the IIS bus is used for data transfers. The Intrepid IC provides DB-DMA (descriptor-based direct memory access) support for the IIS port.
The audio circuitry performs digital audio processing and codec functions. The audio processing functions include output equalization and volume control. The codec functions include A-to-D and D-to-A conversion.
Stereo signals from the audio line-in jack drive the audio circuitry’s A-to-D converter. Audio data from the Intrepid IC drives the audio circuitry’s D-to-A converter. Analog output from the D-to-A converter is routed to the headphone jack and the audio power amplifier.
The power amplifier drives the internal speaker. When headphones are connected to the headphone jack, the internal speaker is muted.
For a description of the features of the sound system, see Audio System.
Wireless LAN Support
Apple’s internal wireless LAN module, the 54 Mbps AirPort Extreme Card, is available as a build-to-order option or as a user-installable upgrade. The proprietary connector for the AirPort Extreme Card uses the PCI bus. A second connector attaches to the AirPort antenna. The AirPort antenna is built into the system. For information about operation, see and AirPort Extreme Card.
The build-to-order Bluetooth connectivity is connected to the Intrepid IC via a USB 1.1 interface. See Bluetooth Technology for more information.
USB Interface
The Intrepid ASIC has three USB 1.1 controllers, numbered 0, 1, and 2. Port 2 is used by Bluetooth and ports 0 and 1 are not available for use. The two ports on the keyboard support USB 1.1 devices.
The eMac uses a PCI USB 2.0 controller to support three external ports on the side of the computer. These ports support USB 2.0 and 1.1 devices with data transfer rates of 480 Mbps, 12 Mbps, or 1.5 Mbps.
USB devices connected to the eMac are required to support USB-suspend mode as defined in the USB specification.
The USB ports on the eMac comply with the Universal Serial Bus Specification 2.0. The USB 2.0 controllers comply with the Enhanced Host Controller Interface (EHCI) specification.
For more information about USB, see USB Ports. For USB reference information, see USB Interface.
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