Instruction Sets

86 CHAPTER 2 Instruction Setsand to take advantage of parallelism wherever possible. However, if the compilercannot determine that a set of instructions are independent,it must assume that theyare dependent and generate more restrictive,slower code.The restrict keyword canbe used to tell the compiler that a given pointer is the only one in the scope that canpoint to a particular object. The -pm option allows the compiler to perform moreglobal analysis and find more independent sets of instructions.SUMMARYWhen viewed from high above, all CPUs are similar—they read and write memory,perform data operations, and make decisions. However, there are many ways todesign an instruction set,as illustrated by the differences between the ARM and theC55x. When designing complex systems, we generally view the programs in highlevellanguage form,which hides many of the details of the instruction set. However,differences in instruction sets can be reflected in nonfunctional characteristics,suchas program size and speed.What We Learned■Both the von Neumann and Harvard architectures are in common use today.■ The programming model is a description of the architecture relevant toinstruction operation.■ ARM is a load-store architecture. It provides a few relatively complex instructions,such as saving and restoring multiple registers.■ The C55x provides a number of architectural features to support the arithmeticloops that are common on digital signal processing code.FURTHER READINGBooks by Jaggar [Jag95] and Furber [Fur96] describe the ARM architecture. TheARM Web site, www.arm.com, contains a large number of documents describingvarious versions of ARM.QUESTIONSQ2-1 What is the difference between a big-endian and little-endian datarepresentation?Q2-2 What is the difference between the Harvard and von Neumannarchitectures?