ARM.SoC.Architecture

Where several data items of different types are

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Unformatted text preview: different alignment conventions, or to pack data tightly to minimize memory use even though this will reduce performance. For such purposes the ARM C compiler can produce code that works with packed data structures where all the padding is removed: ..packed struct example3; S3 {char c; int x; short s;} A packed struct gives precise control of the alignment of all the fields but incurs the overhead of the ARM's relatively inefficient access to non-aligned operands, and therefore should only be used when strictly necessary. The memory occupancy of the packed structure declared above is illustrated in Figure 6.17. Figure 6.17 An example of packed struct memory allocation. Run-time environment 185 6.10 Run-time environment A C program requires an environment in which to operate; this is usually provided through a library of functions that the C program can call. In a PC or workstation a C programmer can expect to find the full ANSI C library, giving access to a broad range of functions such as file management, input and output (print f ()), the realtime clock, and so on. Minimal run-time library In a small embedded system such as a mobile telephone, most of these functions are irrelevant. ARM Limited supplies a minimal stand-alone run-time library which, once ported to the target environment, allows basic C programs to run. This library therefore reflects the minimal requirements of a C program. It comprises: Division and remainder functions. Since the ARM instruction set does not include divide instructions, these are implemented as library functions. Stack-limit checking functions. A minimal embedded system is unlikely to have memory management hardware available for stack overflow detection; therefore these library functions are needed to ensure programs operate safely. Stack and heap management. All C programs use the stack for (many) function calls, and all but the most trivial create data structures on the heap. Program start up. Once the stack and heap are initial...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.

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