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<a href='https://github.com/akkartik/mu/blob/main/115write-byte.subx'>https://github.com/akkartik/mu/blob/main/115write-byte.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr"> 1 </span><span class="subxComment"># Write a single byte to a stream.</span>
<span id="L2" class="LineNr"> 2 </span><span class="subxComment">#</span>
<span id="L3" class="LineNr"> 3 </span><span class="subxComment"># We need to do this in machine code because streams need to be opaque types,</span>
<span id="L4" class="LineNr"> 4 </span><span class="subxComment"># and we don't yet support opaque types in Mu.</span>
<span id="L5" class="LineNr"> 5 </span>
<span id="L6" class="LineNr"> 6 </span>== code
<span id="L7" class="LineNr"> 7 </span><span class="subxComment">#   instruction                     effective address                                                   register    displacement    immediate</span>
<span id="L8" class="LineNr"> 8 </span><span class="subxS1Comment"># . op          subop               mod             rm32          base        index         scale       r32</span>
<span id="L9" class="LineNr"> 9 </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="L10" class="LineNr">10 </span>
<span id="L11" class="LineNr">11 </span><span class="subxComment"># Write lower byte of 'n' to 'f'.</span>
<span id="L12" class="LineNr">12 </span><span class="subxFunction">append-byte</span>:  <span class="subxComment"># f: (addr stream byte), n: int</span>
<span id="L13" class="LineNr">13 </span>    <span class="subxS1Comment"># . prologue</span>
<span id="L14" class="LineNr">14 </span>    55/push-ebp
<span id="L15" class="LineNr">15 </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="L16" class="LineNr">16 </span>    <span class="subxS1Comment"># . save registers</span>
<span id="L17" class="LineNr">17 </span>    50/push-eax
<span id="L18" class="LineNr">18 </span>    51/push-ecx
<span id="L19" class="LineNr">19 </span>    57/push-edi
<span id="L20" class="LineNr">20 </span>    <span class="subxComment"># edi = f</span>
<span id="L21" class="LineNr">21 </span>    8b/copy                         1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          7/r32/edi   8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># copy *(ebp+8) to edi</span>
<span id="L22" class="LineNr">22 </span>    <span class="subxComment"># ecx = f-&gt;write</span>
<span id="L23" class="LineNr">23 </span>    8b/copy                         0/mod/indirect  7/rm32/edi   <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"># copy *edi to ecx</span>
<span id="L24" class="LineNr">24 </span>    <span class="subxComment"># if (f-&gt;write &gt;= f-&gt;size) abort</span>
<span id="L25" class="LineNr">25 </span>    3b/compare                      1/mod/*+disp8   7/rm32/edi   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          1/r32/ecx   8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># compare ecx with *(edi+8)</span>
<span id="L26" class="LineNr">26 </span>    7d/jump-if-&gt;=  $append-byte:<a href='317abort.subx.html#L5'>abort</a>/disp8
<span id="L27" class="LineNr">27 </span><span class="Constant">$append-byte:to-stream</span>:
<span id="L28" class="LineNr">28 </span>    <span class="subxComment"># write to stream</span>
<span id="L29" class="LineNr">29 </span>    <span class="subxComment"># f-&gt;data[f-&gt;write] = LSB(n)</span>
<span id="L30" class="LineNr">30 </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="L31" class="LineNr">31 </span>    8a/copy-byte                    1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          0/r32/AL    0xc/disp8      <span class="Normal"> . </span>                <span class="subxComment"># copy byte at *(ebp+12) to AL</span>
<span id="L32" class="LineNr">32 </span>    88/copy-byte                    1/mod/*+disp8   4/rm32/sib    7/base/edi  1/index/ecx  <span class="Normal"> . </span>          0/r32/AL    0xc/disp8      <span class="Normal"> . </span>                <span class="subxComment"># copy AL to *(edi+ecx+12)</span>
<span id="L33" class="LineNr">33 </span>    <span class="subxComment"># ++f-&gt;write</span>
<span id="L34" class="LineNr">34 </span>    ff          0/subop/increment   0/mod/indirect  7/rm32/edi   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>         <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># increment *edi</span>
<span id="L35" class="LineNr">35 </span><span class="Constant">$append-byte:end</span>:
<span id="L36" class="LineNr">36 </span>    <span class="subxS1Comment"># . restore registers</span>
<span id="L37" class="LineNr">37 </span>    5f/pop-to-edi
<span id="L38" class="LineNr">38 </span>    59/pop-to-ecx
<span id="L39" class="LineNr">39 </span>    58/pop-to-eax
<span id="L40" class="LineNr">40 </span>    <span class="subxS1Comment"># . epilogue</span>
<span id="L41" class="LineNr">41 </span>    89/copy                         3/mod/direct    4/rm32/esp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          5/r32/ebp  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy ebp to esp</span>
<span id="L42" class="LineNr">42 </span>    5d/pop-to-ebp
<span id="L43" class="LineNr">43 </span>    c3/return
<span id="L44" class="LineNr">44 </span>
<span id="L45" class="LineNr">45 </span><span class="Constant">$append-byte:<a href='317abort.subx.html#L5'>abort</a></span>:
<span id="L46" class="LineNr">46 </span>    (<a href='317abort.subx.html#L5'>abort</a> <span class="Constant">&quot;append-byte: out of space&quot;</span>)
<span id="L47" class="LineNr">47 </span>    <span class="subxComment"># never gets here</span>
<span id="L48" class="LineNr">48 </span>
<span id="L49" class="LineNr">49 </span><span class="subxTest">test-append-byte-single</span>:
<span id="L50" class="LineNr">50 </span>    <span class="subxH1Comment"># - check that append-byte writes to first byte of 'file'</span>
<span id="L51" class="LineNr">51 </span>    <span class="subxComment"># setup</span>
<span id="L52" class="LineNr">52 </span>    <span class="subxS1Comment"># . clear-stream(_test-stream)</span>
<span id="L53" class="LineNr">53 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L54" class="LineNr">54 </span>    68/push  <a href='108write.subx.html#L125'>_test-stream</a>/imm32
<span id="L55" class="LineNr">55 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L56" class="LineNr">56 </span>    e8/call  <a href='106stream.subx.html#L20'>clear-stream</a>/disp32
<span id="L57" class="LineNr">57 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L58" class="LineNr">58 </span>    81          0/subop/add         3/mod/direct    4/rm32/esp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>         <span class="Normal"> . </span>              4/imm32           <span class="subxComment"># add to esp</span>
<span id="L59" class="LineNr">59 </span>    <span class="subxComment"># append-byte(_test-stream, 'A')</span>
<span id="L60" class="LineNr">60 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L61" class="LineNr">61 </span>    68/push  0x41/imm32
<span id="L62" class="LineNr">62 </span>    68/push  <a href='108write.subx.html#L125'>_test-stream</a>/imm32
<span id="L63" class="LineNr">63 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L64" class="LineNr">64 </span>    e8/call  <a href='115write-byte.subx.html#L12'>append-byte</a>/disp32
<span id="L65" class="LineNr">65 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L66" class="LineNr">66 </span>    81          0/subop/add         3/mod/direct    4/rm32/esp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>         <span class="Normal"> . </span>              8/imm32           <span class="subxComment"># add to esp</span>
<span id="L67" class="LineNr">67 </span>    <span class="subxComment"># check-stream-equal(_test-stream, &quot;A&quot;, msg)</span>
<span id="L68" class="LineNr">68 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L69" class="LineNr">69 </span>    68/push  <span class="Constant">&quot;F - test-append-byte-single&quot;</span>/imm32
<span id="L70" class="LineNr">70 </span>    68/push  <span class="Constant">&quot;A&quot;</span>/imm32
<span id="L71" class="LineNr">71 </span>    68/push  <a href='108write.subx.html#L125'>_test-stream</a>/imm32
<span id="L72" class="LineNr">72 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L73" class="LineNr">73 </span>    e8/call  <a href='109stream-equal.subx.html#L194'>check-stream-equal</a>/disp32
<span id="L74" class="LineNr">74 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L75" class="LineNr">75 </span>    81          0/subop/add         3/mod/direct    4/rm32/esp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>         <span class="Normal"> . </span>              0xc/imm32         <span class="subxComment"># add to esp</span>
<span id="L76" class="LineNr">76 </span>    <span class="subxS1Comment"># . end</span>
<span id="L77" class="LineNr">77 </span>    c3/return
<span id="L78" class="LineNr">78 </span>
<span id="L79" class="LineNr">79 </span><span class="subxS2Comment"># . . vim&#0058;nowrap:textwidth=0</span>
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