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<h1>
<a name="1204473"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">1 Introduction</font>
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<p>
<a name="1204474"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This book describes each instruction in the ST20-C2 instruction set and explains the context within which that instruction is used. It is divided into two parts: a narrative that introduces each instruction within a logical group of instructions, and a reference section that gives a code-like specification of each instruction.</font>
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<a name="1204475"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This is a useful document to all ST20-C2 users, but it is aimed in particular at the following:</font>
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<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204476"> </a>high-level programmers who want to use low-level code inserts to enhance the performance of their code,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204477"> </a>compiler writers,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204478"> </a>operating system or run-time kernel writers,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204479"> </a>writers of high-level debugging tools,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204480"> </a>writers of run-time support libraries,</font></p>
<p> <font size=2 face="Verdana, Arial, Helvetica, sans-serif"><li ><a name="1204481"> </a>writers of bootstrap code.</font></p>
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<p>
<a name="1204848"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">High-level programmers in a language such as C might find that, either their particular compiler is limited in certain respects, or a particular piece of code is time critical, and they may need to write a low-level code sequence. Provided that the compiler/linker system they are using enables them to write instruction level sequences into their program, they can overcome such difficulties. For this, they need a good understanding of the capabilities and range of the entire instruction set.</font>
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<p>
<a name="1204483"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Compiler writers need to understand the exact action of each instruction in order to write the code generation part of a compiler.</font>
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<p>
<a name="1204484"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Writers of run-time kernels need to understand the ST20-C2's scheduling mechanism and the instructions which enable them to implement their own scheduling/interrupt scheme.</font>
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<p>
<a name="1204485"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Similarly writers of run time support libraries may need to write time critical code at assembly level.</font>
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<p>
<a name="1204486"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Writers of debugging tools need to be able to control low-level context switching (for example, for implementing breakpoints/single-step) and need to be able to access and manipulate certain registers and data structures.</font>
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<p>
<a name="1204487"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Bootstrap code must usually be very compact code and requires some machine specific instructions which are not available in high-level programming languages. For these reasons, a bootstrap program is written at assembly code level and the writer of this program needs to be familiar with the processor's instruction set.</font>
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<p>
<a name="1204856"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">In the narrative part of the book, issues in programming the ST20-C2 are considered subject by subject. Each subject introduces certain instructions. This provides the reader with the purpose for the instructions as they are introduced and describes any environmental issues, such as data structures and register/data-structure pre-conditions. Some of these subjects are of interest to all ST20-C2 users while some are of specific interest to certain categories of reader.</font>
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