Embedded Linux Primer: A Practical, Real-World Approach

Chapter 3. Processor Basics

In this chapter, we present some basic information to help you navigate the huge sea of embedded processor choices. We look at some of the processors on the market and the types of features they contain. Stand-alone processors are highlighted first. These tend to be the most powerful processors and require external chipsets to form complete systems. Next we present some of the many integrated processors that are supported under Linux. Finally, we look at some of the common hardware platforms in use today.

Literally dozens of embedded processors are available to choose from in a given embedded design. For the purposes of this chapter, we limit the available discussion to those that contain a hardware memory-management unit and, of course, to those that are supported under Linux. One of the fundamental architectural design aspects of Linux is that it is a virtual memory operating system.[16] Employing Linux on a processor that does not contain an MMU gives up one of the more valuable architectural features of the kernel and is beyond the scope of this book.

3.1. Stand-alone Processors

Stand-alone processors refer to processor chips that are dedicated solely to the processing function. As opposed to integrated processors, stand-alone processors require additional support circuitry for their basic operation. In many cases, this means a chipset or custom logic surrounding the processor to handle functions such as DRAM controller, system bus addressing configuration, and external peripheral devices such as keyboard controllers and serial ports. Stand-alone processors often offer the highest overall CPU performance.

Numerous processors exist in both 32-bit and 64-bit implementations [17] that have seen widespread use in embedded systems. These include the IBM PowerPC 970FX, the Intel Pentium M, and the Freescale MPC74xx Host Processors, among others.

Here we present a sample from each of the major manufactures of stand-alone processors. These processors are well supported under Linux and have been used in many embedded Linux designs.

3.1.1. IBM 970FX

The IBM 970FX processor core is a high-performance 64-bit capable stand-alone processor. The 970FX is a superscalar architecture. This means the core is capable of fetching, issuing, and obtaining results from more than one instruction at a time. This is done through a pipelining architecture, which provides the effect of multiple streams of instruction simultaneously. The IBM 970FX contains up to 25 stages of pipelining, depending on the instruction stream and operations contained therein.

Some of the key features of the 970FX are as follows:

• A 64-bit implementation of the popular PowerPC architecture

• Deeply pipelined design, for very-high-performance computing applications

• Static and dynamic power-management features

• Multiple sleep modes, to minimize power requirements and maximize battery life

• Dynamically adjustable clock rates, supporting lower-power modes

• Optimized for high-performance, low-latency storage management

The IBM 970FX has been incorporated into a number of high-end server blades and computing platforms, including IBM's own Blade Server platform.

3.1.2. Intel Pentium M

Certainly one of the most popular architectures, x86 in both 32- and 64-bit flavors (more properly called IA32 and IA64, respectively) has been employed for embedded devices in a variety of applications. In the most common form, these platforms are based on a variety of commercial off-the-shelf (COTS) hardware implementations. Numerous manufacturers supply x86 single-board computers and complete platforms in a variety of form factors. See Section 3.2, 'Integrated Processors: Systems on Chip,' later in this chapter for a discussion of the more common platforms in use today.

The Intel Pentium M has been used in a wide variety of laptop computers and has found a niche in embedded products. Like the IBM 970FX processor, the Pentium M is a superscalar architecture. These characteristics make it attractive in embedded applications:

• The Pentium M is based on the popular x86 architecture, and thus is widely supported by a large ecosystem of hardware and software vendors.

• It consumes less power than other x86 processors.

• Advanced power-management features enable low-power operating modes and multiple sleep modes.

• Dynamic clock speed capability enhances battery-powered operations such as standby.

• On chip thermal monitoring enables automatic transition to lower power modes, to reduce power consumption in overtemperature conditions.

• Multiple frequency and voltage operating points (dynamically selectable) are designed to maximize battery life in portable equipment.

Many of these features are especially useful for embedded applications. It is not uncommon for embedded products to require portable or battery-powered configurations. The Pentium M has enjoyed popularity in this application space because of its power- and thermal-management features.

3.1.3. Freescale MPC7448

The Freescale MPC7448 contains what is referred to as a fourth-generation PowerPC core, commonly called G4.[18] This high-performance 32-bit processor is commonly found in networking and telecommunications applications. Several companies manufacture blades that conform to an industry-standard platform specification, including this and other similar stand-alone Freescale processors. We examine these platforms in Section 3.3, 'Hardware Platforms,' later in this chapter.

The MPC7448 has enjoyed popularity in a wide variety of signal-processing and networking applications because of the advanced feature set highlighted here:

• Operating clock rates in excess of 1.5GHz

• 1MB onboard L2 cache

• Advanced power-management capabilities, including multiple sleep modes

• Advanced AltiVec vector-execution unit

• Voltage scaling for reduced-power configurations

The MPC7448 contains a Freescale technology called AltiVec to enable very fast algorithmic computations and other data-crunching applications. The AltiVec unit consists of a register file containing 32 very wide (128-bit) registers. Each value within one of these AltiVec registers can be considered a vector of multiple elements. AltiVec defines a set of instructions to manipulate this vector data effectively in parallel with core CPU instruction