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<a href='https://github.com/akkartik/mu/blob/main/baremetal/112read-byte.subx'>https://github.com/akkartik/mu/blob/main/baremetal/112read-byte.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="subxComment"># instruction effective address register displacement immediate</span>
<span id="L2" class="LineNr"> 2 </span><span class="subxS1Comment"># . op subop mod rm32 base index scale r32</span>
<span id="L3" class="LineNr"> 3 </span><span class="subxS1Comment"># . 1-3 bytes 3 bits 2 bits 3 bits 3 bits 3 bits 2 bits 2 bits 0/1/2/4 bytes 0/1/2/4 bytes</span>
<span id="L4" class="LineNr"> 4 </span>
<span id="L5" class="LineNr"> 5 </span><span class="subxComment"># TODO: read-byte-buffered</span>
<span id="L6" class="LineNr"> 6 </span>
<span id="L7" class="LineNr"> 7 </span><span class="subxComment"># Return next byte value in eax, with top 3 bytes cleared.</span>
<span id="L8" class="LineNr"> 8 </span><span class="subxComment"># Abort on reaching end of stream.</span>
<span id="L9" class="LineNr"> 9 </span><span class="subxFunction">read-byte</span>: <span class="subxComment"># s: (addr stream byte) -> result/eax: byte</span>
<span id="L10" class="LineNr">10 </span> <span class="subxS1Comment"># . prologue</span>
<span id="L11" class="LineNr">11 </span> 55/push-ebp
<span id="L12" class="LineNr">12 </span> 89/copy 3/mod/direct 5/rm32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 4/r32/esp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="subxComment"># copy esp to ebp</span>
<span id="L13" class="LineNr">13 </span> <span class="subxS1Comment"># . save registers</span>
<span id="L14" class="LineNr">14 </span> 51/push-ecx
<span id="L15" class="LineNr">15 </span> 56/push-esi
<span id="L16" class="LineNr">16 </span> <span class="subxComment"># esi = s</span>
<span id="L17" class="LineNr">17 </span> 8b/copy 1/mod/*+disp8 5/rm32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 6/r32/esi 8/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy *(ebp+8) to esi</span>
<span id="L18" class="LineNr">18 </span> <span class="subxComment"># ecx = s->read</span>
<span id="L19" class="LineNr">19 </span> 8b/copy 1/mod/*+disp8 6/rm32/esi <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 1/r32/ecx 4/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy *(esi+4) to ecx</span>
<span id="L20" class="LineNr">20 </span> <span class="subxComment"># if (f->read >= f->write) abort</span>
<span id="L21" class="LineNr">21 </span> 3b/compare 0/mod/indirect 6/rm32/esi <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 1/r32/ecx <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="subxComment"># compare ecx with *esi</span>
<span id="L22" class="LineNr">22 </span> 0f 8d/jump-if->= $read-byte:end/disp32 <span class="subxComment"># TODO: abort</span>
<span id="L23" class="LineNr">23 </span> <span class="subxComment"># result = f->data[f->read]</span>
<span id="L24" class="LineNr">24 </span> 31/xor 3/mod/direct 0/rm32/eax <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 0/r32/eax <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="subxComment"># clear eax</span>
<span id="L25" class="LineNr">25 </span> 8a/copy-byte 1/mod/*+disp8 4/rm32/sib 6/base/esi 1/index/ecx <span class="Normal"> . </span> 0/r32/AL 0xc/disp8 <span class="Normal"> . </span> <span class="subxComment"># copy byte at *(esi+ecx+12) to AL</span>
<span id="L26" class="LineNr">26 </span> <span class="subxComment"># ++f->read</span>
<span id="L27" class="LineNr">27 </span> ff 0/subop/increment 1/mod/*+disp8 6/rm32/esi <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 4/disp8 <span class="Normal"> . </span> <span class="subxComment"># increment *(esi+4)</span>
<span id="L28" class="LineNr">28 </span><span class="Constant">$read-byte:end</span>:
<span id="L29" class="LineNr">29 </span> <span class="subxS1Comment"># . restore registers</span>
<span id="L30" class="LineNr">30 </span> 5e/pop-to-esi
<span id="L31" class="LineNr">31 </span> 59/pop-to-ecx
<span id="L32" class="LineNr">32 </span> <span class="subxS1Comment"># . epilogue</span>
<span id="L33" class="LineNr">33 </span> 89/copy 3/mod/direct 4/rm32/esp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="Normal"> . </span> 5/r32/ebp <span class="Normal"> . </span> <span class="Normal"> . </span> <span class="subxComment"># copy ebp to esp</span>
<span id="L34" class="LineNr">34 </span> 5d/pop-to-ebp
<span id="L35" class="LineNr">35 </span> c3/return
<span id="L36" class="LineNr">36 </span>
<span id="L37" class="LineNr">37 </span><span class="subxS2Comment"># . . vim:nowrap:textwidth=0</span>
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