Take a Look Inside the G5-Based Dual-Processor Power Mac

Oct 13, 2006

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Take a look inside the book that's topped best-seller lists since its debut. In this sample chapter, author Amit Singh looks at the system architecture of a specific type of Apple computer: a G5-based dual-processor Power Mac. Moreover, he discusses a specific PowerPC processor used in these systems: the 970FX. He focuses on a G5-based system because the 970FX is more advanced, more powerful, and more interesting in general than its predecessors. It is also the basis for the first 64-bit dual-core PowerPC processor: the 970MP.

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Mac OS X Internals: A Systems Approach

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Apple initiated its transition from the 68K hardware platform to the PowerPC in 1994. Within the next two years, Apple's entire line of computers moved to the PowerPC. The various PowerPC-based Apple computer families available at any given time have often differed in system architecture,^[1] the specific processor used, and the processor vendor. For example, before the G4 iBook was introduced in October 2003, Apple's then current systems included three generations of the PowerPC: the G3, the G4, and the G5. Whereas the G4 processor line is supplied by Motorola, the G3 and the G5 are from IBM. Table 3–1 lists the various PowerPC processors^[2] used by Apple.

Table 3–1. Processors Used in PowerPC-Based Apple Systems

Processor	Introduced	Discontinued
PowerPC 601	March 1994	June 1996
PowerPC 603	April 1995	May 1996
PowerPC 603e	April 1996	August 1998
PowerPC 604	August 1995	April 1998
PowerPC 604e	August 1996	September 1998
PowerPC G3	November 1997	October 2003
PowerPC G4	October 1999	—
PowerPC G5	June 2003	—
PowerPC G5 (dual-core)	October 2005	—

On June 6, 2005, at the Worldwide Developers Conference in San Francisco, Apple announced its plans to base future models of Macintosh computers on Intel processors. The move was presented as a two-year transition: Apple stated that although x86-based Macintosh models would become available by mid-2006, all Apple computers would transition to the x86 platform only by the end of 2007. The transition was faster than expected, with the first x86 Macintosh computers appearing in January 2006. These systems—the iMac and the MacBook Pro—were based on the Intel Core Duo^[3] dual-core processor line, which is built on 65 nm process technology.

In this chapter, we will look at the system architecture of a specific type of Apple computer: a G5-based dual-processor Power Mac. Moreover, we will discuss a specific PowerPC processor used in these systems: the 970FX. We focus on a G5-based system because the 970FX is more advanced, more powerful, and more interesting in general than its predecessors. It is also the basis for the first 64-bit dual-core PowerPC processor: the 970MP.

3.1 The Power Mac G5

Apple announced the Power Mac G5—its first 64-bit desktop system—in June 2003. Initial G5-based Apple computers used IBM's PowerPC 970 processors. These were followed by systems based on the 970FX processor. In late 2005, Apple revamped the Power Mac line by moving to the dual-core 970MP processor. The 970, 970FX, and 970MP are all derived from the execution core of the POWER4 processor family, which was designed for IBM's high-end servers. G5 is Apple's marketing term for the 970 and its variants.

Before we examine the architecture of any particular Power Mac G5, note that various Power Mac G5 models may have slightly different system architectures. In the following discussion, we will refer to the system shown in Figure 3–1.

Figure 3–1 ArchitecturePower Mac G5Dual-processor architecturePower Mac G5dual-processor architectureProcessor interconnectArchitecture of a dual-processor Power Mac G5 system

3.1.1 The U3H System Controller

The U3H system controller combines the functionality of a memory controller^[5] and a PCI bus bridge.^[6] It is a custom integrated chip (IC) that is the meeting point of key system components: processors, the Double Data Rate (DDR) memory system, the Accelerated Graphics Port (AGP)^[7] slot, and the HyperTransport bus that runs into a PCI-X bridge. The U3H provides bridging functionality by performing point-to-point routing between these components. It supports a Graphics Address Remapping Table (GART) that allows the AGP bridge to translate linear addresses used in AGP transactions into physical addresses. This improves the performance of direct memory access (DMA) transactions involving multiple pages that would typically be noncontiguous in virtual memory. Another table supported by the U3H is the Device Address Resolution Table (DART),^[8] which translates linear addresses to physical addresses for devices attached to the HyperTransport bus. We will come across the DART in Chapter 10, when we discuss the I/O Kit.

3.1.2 The K2 I/O Device Controller

The U3H is connected to a PCI-X bridge via a 16-bit HyperTransport bus. The PCI-X bridge is further connected to the K2 custom IC via an 8-bit HyperTransport bus. The K2 is a custom integrated I/O device controller. In particular, it provides disk and multiprocessor interrupt controller (MPIC) functionality.

3.1.3 PCI-X and PCI Express

The Power Mac system shown in Figure 3–1 provides three PCI-X 1.0 slots. Power Mac G5 systems with dual-core processors use PCI Express.

3.1.3.1 PCI-X

PCI-X was developed to increase the bus speed and reduce the latency of PCI (see the sidebar "A Primer on Local Busses"). PCI-X 1.0 was based on the existing PCI architecture. In particular, it is also a shared bus. It solves many—but not all—of the problems with PCI. For example, its split-transaction protocol improves bus bandwidth utilization, resulting in far greater throughput rates than PCI. It is fully backward compatible in that PCI-X cards can be used in Conventional PCI slots, and conversely, Conventional PCI cards—both 33MHz and 66MHz—can be used in PCI-X slots. However, PCI-X is not electrically compatible with 5V-only cards or 5V-only slots.

PCI-X 1.0 uses 64-bit slots. It provides two speed grades: PCI-X 66 (66MHz signaling speed, up to 533MBps peak throughput) and PCI-X 133 (133MHz signaling speed, up to 1GBps peak throughput).

PCI-X 2.0 provides enhancements such as the following:

An error correction code (ECC) mechanism for providing automatic 1-bit error recovery and 2-bit error detection
New speed grades: PCI-X 266 (266MHz signaling speed, up to 2.13GBps peak throughput) and PCI-X 533 (533MHz signaling speed, up to 4.26GBps peak throughput)
A new 16-bit interface for embedded or portable applications

Note how the slots are connected to the PCI-X bridge in Figure 3–1: Whereas one of them is "individually" connected (a point-to-point load), the other two "share" a connection (a multidrop load). A PCI-X speed limitation is that its highest speed grades are supported only if the load is point-to-point. Specifically, two PCI-X 133 loads will each operate at a maximum of 100MHz.^[9] Correspondingly, two of this Power Mac's slots are 100MHz each, whereas the third is a 133MHz slot.

3.1.3.2 PCI Express

An alternative to using a shared bus is to use point-to-point links to connect devices. PCI Express^[10] uses a high-speed, point-to-point architecture. It provides PCI compatibility using established PCI driver programming models. Software-generated I/O requests are transported to I/O devices through a split-transaction, packet-based protocol. In other words, PCI Express essentially serializes and packetizes PCI. It supports multiple interconnect widths—a link's bandwidth can be linearly scaled by adding signal pairs to form lanes. There can be up to 32 separate lanes.

A Primer on Local Busses

As CPU speeds have increased greatly over the years, other computer subsystems have not managed to keep pace. Perhaps an exception is the main memory, which has fared better than I/O bandwidth. In 1991,^[11] Intel introduced the Peripheral Component Interconnect (PCI) local bus standard. In the simplest terms, a bus is a shared communications link. In a computer system, a bus is implemented as a set of wires that connect some of the computer's subsystems. Multiple busses are typically used as building blocks to construct complex computer systems. The "local" in local bus implies its proximity to the processor.^[12] The PCI bus has proven to be an extremely popular interconnect mechanism (also called simply an interconnect), particularly in the so-called North Bridge/South Bridge implementation. A North Bridge typically takes care of communication between the processor, main memory, AGP, and the South Bridge. Note, however, that modern system designs are moving the memory controller to the processor die, thus making AGP obsolete and rendering the traditional North Bridge unnecessary.

A typical South Bridge controls various busses and devices, including the PCI bus. It is common to have the PCI bus work both as a plug-in bus for peripherals and as an interconnect allowing devices connected directly or indirectly to it to communicate with memory.

The PCI bus uses a shared, parallel multidrop architecture in which address, data, and control signals are multiplexed on the bus. When one PCI bus master^[13] uses the bus, other connected devices must either wait for it to become free or use a contention protocol to request control of the bus. Several sideband signals^[14] are required to keep track of communication directions, types of bus transactions, indications of bus-mastering requests, and so on. Moreover, a shared bus runs at limited clock speeds, and since the PCI bus can support a wide variety of devices with greatly varying requirements (in terms of bandwidth, transfer sizes, latency ranges, and so on), bus arbitration can be rather complicated. PCI has several other limitations that are beyond the scope of this chapter.

PCI has evolved into multiple variants that differ in backward compatibility, forward planning, bandwidth supported, and so on.

Conventional PCI— The original PCI Local Bus Specification has evolved into what is now called Conventional PCI. The PCI Special Interest Group (PCI-SIG) introduced PCI 2.01 in 1993, followed by revisions 2.1 (1995), 2.2 (1998), and 2.3 (2002). Depending on the revision, PCI bus characteristics include the following: 5V or 3.3V signaling, 32-bit or 64-bit bus width, operation at 33MHz or 66MHz, and a peak throughput of 133MBps, 266MBps, or 533MBps. Conventional PCI 3.0—the current standard—finishes the migration of the PCI bus from being a 5.0V signaling bus to a 3.3V signaling bus.
MiniPCI— MiniPCI defines a smaller form factor PCI card based on PCI 2.2. It is meant for use in products where space is a premium—such as notebook computers, docking stations, and set-top boxes. Apple's AirPort Extreme wireless card is based on MiniPCI.
CardBus— CardBus is a member of the PC Card family that provides a 32-bit, 33MHz PCI-like interface that operates at 3.3V. The PC Card Standard is maintained by the PCMCIA.^[15]

PCI-X (Section 3.1.3.1) and PCI Express (Section 3.1.3.2) represent further advancements in I/O bus architecture.

3.1.4 HyperTransport

HyperTransport (HT) is a high-speed, point-to-point, chip interconnect technology. Formerly known as Lightning Data Transport (LDT), it was developed in the late 1990s at Advanced Micro Devices (AMD) in collaboration with industry partners. The technology was formally introduced in July 2001. Apple Computer was one of the founding members of the HyperTransport Technology Consortium. The HyperTransport architecture is open and nonproprietary.

HyperTransport aims to simplify complex chip-to-chip and board-to-board interconnections in a system by replacing multilevel busses. Each connection in the HyperTransport protocol is between two devices. Instead of using a single bidirectional bus, each connection consists of two unidirectional links. HyperTransport point-to-point interconnects (Figure 3–2 shows an example) can be extended to support a variety of devices, including tunnels, bridges, and end-point devices. HyperTransport connections are especially well suited for devices on the main logic board—that is, those devices that require the lowest latency and the highest performance. Chains of HyperTransport links can also be used as I/O channels, connecting I/O devices and bridges to a host system.

Figure 3–2 HyperTransport I/O link

Some important HyperTransport features include the following.

HyperTransport uses a packet-based data protocol in which narrow and fast unidirectional point-to-point links carry command, address, and data (CAD) information encoded as packets.
The electrical characteristics of the links help in cleaner signal transmission, higher clock rates, and lower power consumption. Consequently, considerably fewer sideband signals are required.
Widths of various links do not need to be equal. An 8-bit-wide link can easily connect to a 32-bit-wide link. Links can scale from 2 bits to 4, 8, 16, or 32 bits in width. As shown in Figure 3–1, the HyperTransport bus between the U3H and the PCI-X bridge is 16 bits wide, whereas the PCI-X bridge and the K2 are connected by an 8-bit-wide HyperTransport bus.
Clock speeds of various links do not need to be equal and can scale across a wide spectrum. Thus, it is possible to scale links in both width and speed to suit specific needs.
HyperTransport supports split transactions, eliminating the need for inefficient retries, disconnects by targets, and insertion of wait states.
HyperTransport combines many benefits of serial and parallel bus architectures.
HyperTransport has comprehensive legacy support for PCI.

Split Transactions

When split transactions are used, a request (which requires a response) and completion of that request—the response ^[16]—are separate transactions on the bus. From the standpoint of operations that are performed as split transactions, the link is free after the request is sent and before the response is received. Moreover, depending on a chipset's implementation, multiple transactions could be pending^[17] at the same time. It is also easier to route such transactions across larger fabrics.

HyperTransport was designed to work with the widely used PCI bus standard—it is software compatible with PCI, PCI-X, and PCI Express. In fact, it could be viewed as a superset of PCI, since it can offer complete PCI transparency by preserving PCI definitions and register formats. It can conform to PCI ordering and configuration specifications. It can also use Plug-and-Play so that compliant operating systems can recognize and configure HyperTransport-enabled devices. It is designed to support both CPU-to-CPU communications and CPU-to-I/O transfers, while emphasizing low latency.

A HyperTransport tunnel device can be used to provide connection to other busses such as PCI-X. A system can use additional HyperTransport busses by using an HT-to-HT bridge.

Apple uses HyperTransport in G5-based systems to connect PCI, PCI-X, USB, FireWire, Audio, and Video links. The U3H acts as a North Bridge in this scenario.

System Architecture and Platform

From the standpoint of Mac OS X, we can define a system's architecture to be primarily a combination of its processor type, the North Bridge (including the memory controller), and the I/O controller. For example, the AppleMacRISC4PE system architecture consists of one or more G5-based processors, a U3-based North Bridge, and a K2-based I/O controller. The combination of a G3- or G4-based processor, a UniNorth-based host bridge, and a KeyLargo-based I/O controller is referred to as the AppleMacRISC2PE system architecture.

A more model-specific concept is that of a platform, which usually depends on the specific motherboard and is likely to change more frequently than system architecture. An example of a platform is PowerMac11,2, which corresponds to a 2.5GHz quad-processor (dual dual-core) Power Mac G5.

3.1.5 Elastic I/O Interconnect

The PowerPC 970 was introduced along with Elastic I/O, a high-bandwidth and high-frequency processor-interconnect (PI) mechanism that requires no bus-level arbitration.^[18] Elastic I/O consists of two 32-bit logical busses, each a high-speed source-synchronous bus (SSB) that represents a unidirectional point-to-point connection. As shown in Figure 3–1, one travels from the processor to the U3H companion chip, and the other travels from the U3H to the processor. In a dual-processor system, each processor gets its own dual-SSB bus. Note that the SSBs also support cache-coherency "snooping" protocols for use in multiprocessor systems.

Whereas the logical width of each SSB is 32 bits, the physical width is greater. Each SSB consists of 50 signal lines that are used as follows:

2 signals for the differential bus clock lines
44 signals for data, to transmit 35 bits of address and data or control information (AD), along with 1 bit for transfer-handshake (TH) packets for acknowledging such command or data packets received on the bus
4 signals for the differential snoop response (SR) bus to carry snoop-coherency responses, allowing global snooping activities to maintain cache coherency

The overall processor interconnect is shown in Figure 3–1 as logically consisting of three inbound segments (ADI, THI, SRI) and three outbound segments (ADO, THO, SRO). The direction of transmission is from a driver side (D), or master, to a receive side (R), or slave. The unit of data transmission is a packet.

Each SSB runs at a frequency that is an integer fraction of the processor frequency. The 970FX design allows several such ratios. For example, Apple's dual-processor 2.7GHz system has an SSB frequency of 1.35GHz (a PI bus ratio of 2:1), whereas one of the single-processor 1.8GHz models has an SSB frequency of 600MHz (a PI bus ratio of 3:1).

The bidirectional nature of the channel between a 970FX processor and the U3H means there are dedicated data paths for reading and writing. Consequently, throughput will be highest in a workload containing an equal number of reads and writes. Conventional bus architectures that are shared and unidirectional-at-a-time will offer higher peak throughput for workloads that are mostly reads or mostly writes. In other words, Elastic I/O leads to higher bus utilization for balanced workloads.

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