Block Diagram

Figure 2-1 provides a simplified block diagram of the main logic board and the buses that connect them together.

Main IC and Buses

The architecture of iMac computer 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.

PowerPC G4 Microprocessor

The PowerPC G4 microprocessor used in the iMac computer 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 one)

• 256 KB on-chip L2 cache with a 1:1 ratio of L2 clock speed to processor clock speed

To find more information, see the reference at PowerPC G4 Microprocessor.

Level 2 Cache

The data storage for the L2 cache consists of 256 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 cache.

System RAM

The memory subsystem in the iMac consists of 256 MB or 512 MB of DDR333 (PC2700) SDRAM in the internal slot. The data bus to the RAM is 64 bits wide, and the memory interface is synchronized to the MaxBus interface at 167 MHz. The expansion slot capacity is an additional 512 MB, supporting a maximum of 1 GB.

For more information on system RAM, see RAM Expansion Slots.

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.

FireWire 400 Controller

The iMac computer includes an IEEE 1394a FireWire 400 controller with a data rate of 400 Mbits (50 MBytes) per second and provides DMA (direct memory access) support for the FireWire 400 interface.

The controller IC implements the FireWire 400 link layer. A physical layer IC, called a PHY, implements the electrical signalling protocol of the FireWire 400 interface. The PHY supports two FireWire 400 ports by way of external connectors on the back of the enclosure.

Ethernet Controller

The iMac computer 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-BaseTX mode. The actual speed of the link is automatically negotiated by the PHY and the bridge or router to which it is connected. For more information, see Ethernet Port.

Video Display Subsystem

The display subsystem for the iMac computer consists of an nVidia graphics controller ASIC on the main logic board. In the 15” iMac, the graphics controller is an nVidia GeForce4 MX with 32 MB DDR RAM. In the 17” iMac, the graphics controller is an nVidia GeForce FX 5200 Ultra with 64 MB DDR RAM. Both graphics controllers contain 2D and 3D acceleration engines, front-end and back-end scalers, a display controller, and an AGP 4x bus interface with bus master capability.

The nVidia GeForce4 MX supports a display size of 1024x768 pixels, with lesser resolutions scaled accordingly. And, the nVidia GeForce FX 5200 Ultra supports a display size of 1440x900 pixels, with lesser resolutions scaled accordingly.

The display generated for the flat panel display is simultaneously available for an external monitor in mirror mode; see Video Monitor Port. Composite video and S-video signals can be displayed on either an NTSC display or a PAL display; see Video Display Adapter.

The interface between the graphics IC and the rest of the system is an AGP (accelerated graphics port) 4x bus on the Intrepid ASIC. 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 running in 4x mode.

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 DDR RAM and the main memory.

For information about the display and supported resolutions, see Flat Panel Display and Video Monitor Port.

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. The Ultra ATA-100 conforms to a subset of ATA/ATAPI-6 and -5 protocols. For more information, see Hard Disk Drive.

ATA Interface

The ATA Interface complies with and supports a subset of industry standard ATA/ATAPI 6 and transfer protocols up to DMA mode 2. The Combo drive and SuperDrive are device-selected as Cable Select in an ATA device configuration. For more information, see SuperDrive (CD-RW/DVD-R) and Combo (CD-RW/DVD-ROM) Drive.

Modem Support

The internal modem is connected to an internal USB 1.1 port. The Intrepid ASIC provides DB-DMA support for the modem interface. The modem provides digital call progress signals to the sound circuitry.

The internal hardware 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 iMac. 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.

Sound Circuitry

The audio codec circuitry exchanges audio data with the main IC over a standard I2S bus and receives commands from the main IC over an I2C bus. The main IC provides DB-DMA (descriptor-based direct memory access) support for the I2S bus.

The sound circuitry includes a signal processing IC for equalization and volume control functions and a codec IC for A/D and D/A conversion.

The sound circuitry performs analog-to-digital conversion for the internal microphone and digital-to-analog conversion for the audio signals it sends to the internal speaker and the headphone jack. A switch-mode power amplifier drives the internal speaker and the Apple Pro Speaker minijack.

For a description of the features of the sound system, see Sound System.

Power Controller

The power management controller in the iMac 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.