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Unformatted text preview: nents are packed as closely as possible consistent with these alignment rules. (Packed structures violate the alignment rules; use of memory is discussed more extensively in Section 6.9 on page 180.) ARM architectural support for C data types We have seen above that the ARM integer core provides native support for signed and unsigned 32-bit integers and for unsigned bytes, covering the C integer (given the decision to implement these as 32-bit values), long integer and unsigned character types. Pointers are implemented as native ARM addresses and are therefore supported directly. The ARM addressing modes provide reasonable support for arrays and structures: base plus scaled index addressing allows an array of objects of a size which is 2" bytes to be scanned using a pointer to the start of the array and a loop variable as the index; base plus immediate offset addressing gives access to an object within a structure. However, additional address calculation instructions will be necessary for more complex accesses. Current versions of the ARM include signed byte and signed and unsigned 16-bit loads and stores, providing some native support for short integer and signed character types. Floating-point types are discussed in the next section, however here we can note that the basic ARM core offers little direct support for them. These types (and the instructions that manipulate them) are, in the absence of specific floating-point support hardware, handled by complex software emulation routines. 6.3 Floating-point data types
Floating-point numbers attempt to represent real numbers with uniform accuracy. A generic way to represent a real number is in the form:
R = a x bn Equation 13 where n is chosen so that a falls within a defined range of values; b is usually implicit in the data type and is often equal to 2. Floating-point data types 159 IEEE 754 There are many complex issues to resolve with the handling of floating-point numbers in computers to ensure that the results are consistent when the same program is run on different machines....
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- Spring '09