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<h2>
<a name="1102960"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">3.4 Instruction encoding</font>
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<a name="1102961"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The ST20-C1 is a zero-address machine. Instruction operands are always implicit and no bits are needed in the instruction representation to carry address or operand location information. This results in very short instructions and exceptionally high code density.</font>
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<a name="1102962"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The instruction encoding is designed so that the most commonly executed instructions occupy the least number of bytes. This reduces the size of the code, which saves memory and reduces the memory bandwidth needed for instruction fetching. This section describes the encoding mechanism.</font>
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<a name="1102963"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A sequence of single byte instruction components is used to encode an instruction. The ST20 interprets this sequence at the instruction fetch stage of execution. Most programmers, working at the level of microprocessor assembly language or high-level language, need not be aware of the existence of instruction components and do not generally need to consider the encoding. </font>
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<a name="1102967"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">This section has been included to provide a background. <a href="comp_for.htm#1051524"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Appendix燙: Compiling for the ST20-C1</i></font></a> discusses consequential issues which need to be considered in order to implement a code generator.</font>
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<a name="1102969"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">3.4.1 An instruction component</font>
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<a name="1102970"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Each instruction component is one byte long, and is divided into two 4-bit parts. The four most significant bits of the byte form a function code, and the four least significant bits are used to build an instruction data value as shown in <a href="arch5.htm#1106084"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Figure⌨️ 快捷键说明
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