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<a name="1204410"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">3.1 Word address and byte selector</font>
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<a name="1204821"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A machine address is a single word of data which identifies a byte in memory, that is, a byte address. It comprises two parts, a word address and a byte selector; the byte selector occupies the two least significant bits of the word, while the word address occupies the thirty most significant bits. An address is treated as a signed value, the range of which starts at the most negative integer and continues, through zero, to the most positive integer. This enables the standard comparison functions to be used on pointer (address) values in the same way that they are used on numerical values.</font>
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<a name="1204413"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">Certain values can never be used as pointers to program data because they represent reserved addresses at the bottom of memory space. These addresses are reserved for use by the processor and initialization, therefore, these values can be used in the context of pointers to data to indicate special conditions as they cannot be validly used as pointers to `real' data. For example, in the context of a pointer to a process then the special value <em>NotProcess</em> (which is the pointer to the reserved address <font size=2 face=Courier><strong>#80000000</strong></font>) is used to indicate the absence of a process, for example, to indicate that there is no waiting process. <em>NotProcess</em> points to an address in the reserved section at the base of memory, therefore it is not possible for it to be pointing to any `genuine' process so it can be used to indicate no process without ambiguity. In this text, names are used to represent these and other special values (for example, <em>NotProcess</em> and <em>Enabling</em>). A full list of names and values of constants used in this book is given in <a href="inst_s11.htm#1218402"><font color="#0000ff" face="Verdana, Arial, Helvetica, sans-serif"><i>Section⌨️ 快捷键说明
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