ARM.SoC.Architecture

1 introduction the arm programmers model was

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Unformatted text preview: ypes, and in particular there is a defined set of types to represent floating-point values. There is no explicit support for these types within the ARM core, however, and in the absence of a floating-point coprocessor these types are interpreted by software which uses the standard types listed above. Memory organization There are two ways to store words in a byte-addressed memory, depending on whether the least significant byte is stored at a lower or higher address than the next most significant byte. Since there is no good reason for choosing one approach over the other the argument as to which is better is more a matter of religion than reason. The two schemes are illustrated in Figure 5.1 on page 107, which shows how an assortment of data types would be stored under the two schemes. ('half-word!2' is found at address 12, and so on.) The 'little-endian' and 'big-endian' terminology which is used to denote the two approaches is derived from Swift's Gulliver s Travels. The inhabitants of Lilliput, who are well known for being rather small are, in addition, constrained by law to break their eggs only at the little end. When this law is imposed, those of their fellow citizens who prefer to break their eggs at the big end take exception to the new rule and civil war breaks out. The big-endians eventually take refuge on a nearby island, which is the kingdom of Blefuscu. The civil war results in many casualties. The application of the 'big-endian' and 'little-endian' terms to the two ways to organize computer memory comes from 'On Holy Wars and a Plea for Peace' by Danny Cohen in the October 1981 issue of Computer. Introduction 107 Figure 5.1 Little- and big-endian memory organizations. To my knowledge, no one has yet been mortally wounded in an argument over byte ordering. However the issue causes significant practical difficulties when datasets are transferred between machines of opposite orderings. Most ARM chips remain strictly neutral in the dispute and can be configured to work with either memory arrangement, though they default to little-endian. Throughout this bo...
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