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POx86 Assembly: How do Disassemblers know how to break up instructions?
 

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    <p>How does a x86 disassembler know where to break up the instructions?</p> <p>I am looking at the <a href="http://datasheets.chipdb.org/Intel/x86/808x/datashts/8088/" rel="nofollow noreferrer">8088</a> instruction set. For example the move instruction has 7 variations that range from 2 to 4 bytes. The instructions themselves seem to follow no particular order. Another reason for <a href="https://stackoverflow.com/questions/2679882/why-is-x86-ugly-aka-why-is-x86-considered-inferior-when-compared-to-others">Why is x86 ugly?</a>.</p> <p>For example:</p> <pre><code> 76543210 76543210 76543210 76543210 reg/mem to/from reg 100010dw ||regr/m imm to reg/mem 1100011w ||000r/m dat dat w=1 imm to reg 1011wreg data dat w=1 imm to accum 1010000w addr-low addrhigh accum to mem 1010001w addr-low addrhigh reg/mem to seg 10001100 ||0ssr/m seg to reg/mem 10001100 ||0ssr/m Legend: ||=mod {NO-DISP=0,DISP-LOW,DISP-HIGH,REG} ss=seg enum{es=0,cs,ss,ds} reg=enum{ax=0,bx,cd,dx,bx,sp,bp,si,di (if w=1)} enum{al,bl...} (if w=0) r/m=reg or mem (mod=3 then REG, else mem) </code></pre> <p>many instruction can overlap in the first byte:</p> <pre><code> 76543210 76543210 76543210 76543210 push 11111111 ||110r/m inc 1111111w ||000r/m </code></pre> <p>The bitmasks appears to have arbitrary assignment. How does a disassembler break apart the instructions?</p> <p>This question is a sub set of <a href="https://stackoverflow.com/questions/924303/how-to-write-a-disassembler">How to write a disassembler.</a></p>
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    1. COThis is nearly a duplicate of http://stackoverflow.com/questions/3917098/how-are-hex-sequence-translated-to-assembly-without-ambiguity
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