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<a href='https://github.com/akkartik/mu/blob/master/069allocate.subx'>https://github.com/akkartik/mu/blob/master/069allocate.subx</a>
<pre id='vimCodeElement'>
<span id="L1" class="LineNr">  1 </span><span class="subxComment"># Helper to dynamically allocate memory on the heap.</span>
<span id="L2" class="LineNr">  2 </span><span class="subxComment">#</span>
<span id="L3" class="LineNr">  3 </span><span class="subxComment"># We'd like to be able to write tests for functions that allocate memory,</span>
<span id="L4" class="LineNr">  4 </span><span class="subxComment"># making assertions on the precise addresses used. To achieve this we'll pass</span>
<span id="L5" class="LineNr">  5 </span><span class="subxComment"># in an *allocation descriptor* to allocate from.</span>
<span id="L6" class="LineNr">  6 </span><span class="subxComment">#</span>
<span id="L7" class="LineNr">  7 </span><span class="subxComment"># Allocation descriptors are also useful outside of tests. Assembly and machine</span>
<span id="L8" class="LineNr">  8 </span><span class="subxComment"># code are of necessity unsafe languages, and one of the most insidious kinds</span>
<span id="L9" class="LineNr">  9 </span><span class="subxComment"># of bugs unsafe languages expose us to are dangling pointers to memory that</span>
<span id="L10" class="LineNr"> 10 </span><span class="subxComment"># has been freed and potentially even reused for something totally different.</span>
<span id="L11" class="LineNr"> 11 </span><span class="subxComment"># To reduce the odds of such &quot;use after free&quot; errors, SubX programs tend to not</span>
<span id="L12" class="LineNr"> 12 </span><span class="subxComment"># reclaim and reuse dynamically allocated memory. (Running out of memory is far</span>
<span id="L13" class="LineNr"> 13 </span><span class="subxComment"># easier to debug.) Long-running programs that want to reuse memory are mostly</span>
<span id="L14" class="LineNr"> 14 </span><span class="subxComment"># on their own to be careful. However, they do get one bit of help: they can</span>
<span id="L15" class="LineNr"> 15 </span><span class="subxComment"># carve out chunks of memory and then allocate from them manually using this</span>
<span id="L16" class="LineNr"> 16 </span><span class="subxComment"># very same 'allocate' helper. They just need a new allocation descriptor for</span>
<span id="L17" class="LineNr"> 17 </span><span class="subxComment"># their book-keeping.</span>
<span id="L18" class="LineNr"> 18 </span>
<span id="L19" class="LineNr"> 19 </span>== code
<span id="L20" class="LineNr"> 20 </span><span class="subxComment">#   instruction                     effective address                                                   register    displacement    immediate</span>
<span id="L21" class="LineNr"> 21 </span><span class="subxS1Comment"># . op          subop               mod             rm32          base        index         scale       r32</span>
<span id="L22" class="LineNr"> 22 </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="L23" class="LineNr"> 23 </span>
<span id="L24" class="LineNr"> 24 </span><span class="subxComment"># Claim the next 'n' bytes of memory starting at ad-&gt;curr and update ad-&gt;curr.</span>
<span id="L25" class="LineNr"> 25 </span><span class="subxComment"># Abort if there isn't enough memory in 'ad'.</span>
<span id="L26" class="LineNr"> 26 </span><span class="subxFunction">allocate</span>:  <span class="subxComment"># ad : (address allocation-descriptor), n : int -&gt; address-or-null/eax</span>
<span id="L27" class="LineNr"> 27 </span>    <span class="subxS1Comment"># . prolog</span>
<span id="L28" class="LineNr"> 28 </span>    55/push-ebp
<span id="L29" class="LineNr"> 29 </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="L30" class="LineNr"> 30 </span>    <span class="subxS1Comment"># . save registers</span>
<span id="L31" class="LineNr"> 31 </span>    51/push-ecx
<span id="L32" class="LineNr"> 32 </span>    52/push-edx
<span id="L33" class="LineNr"> 33 </span>    <span class="subxComment"># ecx = ad</span>
<span id="L34" class="LineNr"> 34 </span>    8b/copy                         1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          1/r32/ecx   8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># copy *(ebp+8) to ecx</span>
<span id="L35" class="LineNr"> 35 </span>    <span class="subxComment"># save ad-&gt;curr</span>
<span id="L36" class="LineNr"> 36 </span>    8b/copy                         0/mod/indirect  1/rm32/ecx   <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"># copy *ecx to eax</span>
<span id="L37" class="LineNr"> 37 </span>    <span class="subxComment"># check if there's enough space</span>
<span id="L38" class="LineNr"> 38 </span>    <span class="subxS1Comment"># . edx = ad-&gt;curr + n</span>
<span id="L39" class="LineNr"> 39 </span>    89/copy                         3/mod/direct    2/rm32/edx   <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"># copy eax to edx</span>
<span id="L40" class="LineNr"> 40 </span>    03/add                          1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          2/r32/edx   0xc/disp8      <span class="Normal"> . </span>                <span class="subxComment"># add *(ebp+12) to edx</span>
<span id="L41" class="LineNr"> 41 </span>    3b/compare                      1/mod/*+disp8   1/rm32/ecx   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          2/r32/edx   4/disp8        <span class="Normal"> . </span>                <span class="subxComment"># compare edx with *(ecx+4)</span>
<span id="L42" class="LineNr"> 42 </span>    73/jump-if-greater-or-equal-signed  $allocate:abort/disp8
<span id="L43" class="LineNr"> 43 </span><span class="Constant">$allocate:commit</span>:
<span id="L44" class="LineNr"> 44 </span>    <span class="subxComment"># update ad-&gt;curr</span>
<span id="L45" class="LineNr"> 45 </span>    89/copy                         0/mod/indirect  1/rm32/ecx   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          2/r32/edx  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy edx to *ecx</span>
<span id="L46" class="LineNr"> 46 </span><span class="Constant">$allocate:end</span>:
<span id="L47" class="LineNr"> 47 </span>    <span class="subxS1Comment"># . restore registers</span>
<span id="L48" class="LineNr"> 48 </span>    5a/pop-to-edx
<span id="L49" class="LineNr"> 49 </span>    59/pop-to-ecx
<span id="L50" class="LineNr"> 50 </span>    <span class="subxS1Comment"># . epilog</span>
<span id="L51" class="LineNr"> 51 </span>    89/copy                         3/mod/direct    4/rm32/esp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          5/r32/ebp  <span class="Normal"> . </span>             <span class="Normal"> . </span>                <span class="subxComment"># copy ebp to esp</span>
<span id="L52" class="LineNr"> 52 </span>    5d/pop-to-ebp
<span id="L53" class="LineNr"> 53 </span>    c3/return
<span id="L54" class="LineNr"> 54 </span>
<span id="L55" class="LineNr"> 55 </span><span class="Constant">$allocate:abort</span>:
<span id="L56" class="LineNr"> 56 </span>    <span class="subxS1Comment"># . _write(2/stderr, error)</span>
<span id="L57" class="LineNr"> 57 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L58" class="LineNr"> 58 </span>    68/push  <span class="Constant">&quot;allocate: failed to allocate\n&quot;</span>/imm32
<span id="L59" class="LineNr"> 59 </span>    68/push  2/imm32/stderr
<span id="L60" class="LineNr"> 60 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L61" class="LineNr"> 61 </span>    e8/call  <a href='050_write.subx.html#L14'>_write</a>/disp32
<span id="L62" class="LineNr"> 62 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L63" class="LineNr"> 63 </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="L64" class="LineNr"> 64 </span>    <span class="subxS1Comment"># . syscall(exit, 1)</span>
<span id="L65" class="LineNr"> 65 </span>    bb/copy-to-ebx  1/imm32
<span id="L66" class="LineNr"> 66 </span>    b8/copy-to-eax  1/imm32/exit
<span id="L67" class="LineNr"> 67 </span>    cd/syscall  0x80/imm8
<span id="L68" class="LineNr"> 68 </span>    <span class="subxComment"># never gets here</span>
<span id="L69" class="LineNr"> 69 </span>
<span id="L70" class="LineNr"> 70 </span><span class="subxTest">test-allocate-success</span>:
<span id="L71" class="LineNr"> 71 </span>    <span class="subxS1Comment"># . prolog</span>
<span id="L72" class="LineNr"> 72 </span>    55/push-ebp
<span id="L73" class="LineNr"> 73 </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="L74" class="LineNr"> 74 </span>    <span class="subxComment"># var ad/ecx : (address allocation-descriptor) = {11, 15}</span>
<span id="L75" class="LineNr"> 75 </span>    68/push  0xf/imm32/limit
<span id="L76" class="LineNr"> 76 </span>    68/push  0xb/imm32/curr
<span id="L77" class="LineNr"> 77 </span>    89/copy                         3/mod/direct    1/rm32/ecx   <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 ecx</span>
<span id="L78" class="LineNr"> 78 </span>    <span class="subxComment"># eax = allocate(ad, 3)</span>
<span id="L79" class="LineNr"> 79 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L80" class="LineNr"> 80 </span>    68/push  3/imm32
<span id="L81" class="LineNr"> 81 </span>    51/push-ecx
<span id="L82" class="LineNr"> 82 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L83" class="LineNr"> 83 </span>    e8/call  <a href='069allocate.subx.html#L26'>allocate</a>/disp32
<span id="L84" class="LineNr"> 84 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L85" class="LineNr"> 85 </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="L86" class="LineNr"> 86 </span>    <span class="subxComment"># check-ints-equal(eax, 11, msg)</span>
<span id="L87" class="LineNr"> 87 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L88" class="LineNr"> 88 </span>    68/push  <span class="Constant">&quot;F - <a href='069allocate.subx.html#L70'>test-allocate-success</a>: returns current pointer of allocation descriptor&quot;</span>/imm32
<span id="L89" class="LineNr"> 89 </span>    68/push  0xb/imm32
<span id="L90" class="LineNr"> 90 </span>    50/push-eax
<span id="L91" class="LineNr"> 91 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L92" class="LineNr"> 92 </span>    e8/call  <a href='051test.subx.html#L24'>check-ints-equal</a>/disp32
<span id="L93" class="LineNr"> 93 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L94" class="LineNr"> 94 </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="L95" class="LineNr"> 95 </span>    <span class="subxComment"># check-ints-equal(ad-&gt;curr, 14, msg)</span>
<span id="L96" class="LineNr"> 96 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L97" class="LineNr"> 97 </span>    68/push  <span class="Constant">&quot;F - <a href='069allocate.subx.html#L70'>test-allocate-success</a>: updates allocation descriptor&quot;</span>/imm32
<span id="L98" class="LineNr"> 98 </span>    68/push  0xe/imm32
<span id="L99" class="LineNr"> 99 </span>    ff          6/subop/push        0/mod/indirect  1/rm32/ecx   <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"># push *ecx</span>
<span id="L100" class="LineNr">100 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L101" class="LineNr">101 </span>    e8/call  <a href='051test.subx.html#L24'>check-ints-equal</a>/disp32
<span id="L102" class="LineNr">102 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L103" class="LineNr">103 </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="L104" class="LineNr">104 </span>    <span class="subxS1Comment"># . epilog</span>
<span id="L105" class="LineNr">105 </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="L106" class="LineNr">106 </span>    5d/pop-to-ebp
<span id="L107" class="LineNr">107 </span>    c3/return
<span id="L108" class="LineNr">108 </span>
<span id="L109" class="LineNr">109 </span><span class="subxMinorFunction">_pending-test-allocate-failure</span>:
<span id="L110" class="LineNr">110 </span>    <span class="subxS1Comment"># . prolog</span>
<span id="L111" class="LineNr">111 </span>    55/push-ebp
<span id="L112" class="LineNr">112 </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="L113" class="LineNr">113 </span>    <span class="subxComment"># var ad/ecx : (address allocation-descriptor) = {11, 15}</span>
<span id="L114" class="LineNr">114 </span>    68/push  0xf/imm32/limit
<span id="L115" class="LineNr">115 </span>    68/push  0xb/imm32/curr
<span id="L116" class="LineNr">116 </span>    89/copy                         3/mod/direct    1/rm32/ecx   <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 ecx</span>
<span id="L117" class="LineNr">117 </span>    <span class="subxComment"># eax = allocate(ad, 6)</span>
<span id="L118" class="LineNr">118 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L119" class="LineNr">119 </span>    68/push  6/imm32
<span id="L120" class="LineNr">120 </span>    51/push-ecx
<span id="L121" class="LineNr">121 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L122" class="LineNr">122 </span>    e8/call  <a href='069allocate.subx.html#L26'>allocate</a>/disp32
<span id="L123" class="LineNr">123 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L124" class="LineNr">124 </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="L125" class="LineNr">125 </span>    <span class="subxComment"># check-ints-equal(eax, 0, msg)</span>
<span id="L126" class="LineNr">126 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L127" class="LineNr">127 </span>    68/push  <span class="Constant">&quot;F - test-allocate-failure: returns null&quot;</span>/imm32
<span id="L128" class="LineNr">128 </span>    68/push  0/imm32
<span id="L129" class="LineNr">129 </span>    50/push-eax
<span id="L130" class="LineNr">130 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L131" class="LineNr">131 </span>    e8/call  <a href='051test.subx.html#L24'>check-ints-equal</a>/disp32
<span id="L132" class="LineNr">132 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L133" class="LineNr">133 </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="L134" class="LineNr">134 </span>    <span class="subxComment"># no change to ad-&gt;curr</span>
<span id="L135" class="LineNr">135 </span>    <span class="subxS1Comment"># . check-ints-equal(ad-&gt;curr, 11)</span>
<span id="L136" class="LineNr">136 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L137" class="LineNr">137 </span>    68/push  <span class="Constant">&quot;F - test-allocate-failure: updates allocation descriptor&quot;</span>/imm32
<span id="L138" class="LineNr">138 </span>    68/push  0xb/imm32
<span id="L139" class="LineNr">139 </span>    ff          6/subop/push        0/mod/indirect  1/rm32/ecx   <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"># push *ecx</span>
<span id="L140" class="LineNr">140 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L141" class="LineNr">141 </span>    e8/call  <a href='051test.subx.html#L24'>check-ints-equal</a>/disp32
<span id="L142" class="LineNr">142 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L143" class="LineNr">143 </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="L144" class="LineNr">144 </span>    <span class="subxS1Comment"># . epilog</span>
<span id="L145" class="LineNr">145 </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="L146" class="LineNr">146 </span>    5d/pop-to-ebp
<span id="L147" class="LineNr">147 </span>    c3/return
<span id="L148" class="LineNr">148 </span>
<span id="L149" class="LineNr">149 </span><span class="subxComment"># helper: create a nested allocation descriptor (useful for tests)</span>
<span id="L150" class="LineNr">150 </span><span class="subxFunction">allocate-region</span>:  <span class="subxComment"># ad : (address allocation-descriptor), n : int -&gt; new-ad : (address allocation-descriptor)</span>
<span id="L151" class="LineNr">151 </span>    <span class="subxS1Comment"># . prolog</span>
<span id="L152" class="LineNr">152 </span>    55/push-ebp
<span id="L153" class="LineNr">153 </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="L154" class="LineNr">154 </span>    <span class="subxS1Comment"># . save registers</span>
<span id="L155" class="LineNr">155 </span>    51/push-ecx
<span id="L156" class="LineNr">156 </span>    <span class="subxComment"># eax = allocate(ad, n)</span>
<span id="L157" class="LineNr">157 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L158" class="LineNr">158 </span>    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>          0xc/disp8      <span class="Normal"> . </span>                <span class="subxComment"># push *(ebp+12)</span>
<span id="L159" class="LineNr">159 </span>    ff          6/subop/push        1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>         <span class="Normal"> . </span>          8/disp8        <span class="Normal"> . </span>                <span class="subxComment"># push *(ebp+8)</span>
<span id="L160" class="LineNr">160 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L161" class="LineNr">161 </span>    e8/call  <a href='069allocate.subx.html#L26'>allocate</a>/disp32
<span id="L162" class="LineNr">162 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L163" class="LineNr">163 </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="L164" class="LineNr">164 </span>    <span class="subxComment"># if (eax == 0) abort</span>
<span id="L165" class="LineNr">165 </span>    3d/compare-eax-and  0/imm32
<span id="L166" class="LineNr">166 </span>    74/jump-if-equal  $allocate-region:abort/disp8
<span id="L167" class="LineNr">167 </span>    <span class="subxComment"># earmark 8 bytes at the start for a new allocation descriptor</span>
<span id="L168" class="LineNr">168 </span>    <span class="subxS1Comment"># . *eax = eax + 8</span>
<span id="L169" class="LineNr">169 </span>    89/copy                         3/mod/direct    1/rm32/ecx   <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"># copy eax to ecx</span>
<span id="L170" class="LineNr">170 </span>    81          0/subop/add         3/mod/direct    1/rm32/ecx   <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 ecx</span>
<span id="L171" class="LineNr">171 </span>    89/copy                         0/mod/indirect  0/rm32/eax   <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 ecx to *eax</span>
<span id="L172" class="LineNr">172 </span>    <span class="subxS1Comment"># . *(eax+4) = eax + n</span>
<span id="L173" class="LineNr">173 </span>    89/copy                         3/mod/direct    1/rm32/ecx   <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"># copy eax to ecx</span>
<span id="L174" class="LineNr">174 </span>    03/add                          1/mod/*+disp8   5/rm32/ebp   <span class="Normal"> . </span>         <span class="Normal"> . </span>           <span class="Normal"> . </span>          1/r32/ecx   0xc/disp8      <span class="Normal"> . </span>                <span class="subxComment"># add *(ebp+12) to ecx</span>
<span id="L175" class="LineNr">175 </span>    89/copy                         1/mod/*+disp8   0/rm32/eax   <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 ecx to *(eax+4)</span>
<span id="L176" class="LineNr">176 </span>    <span class="subxS1Comment"># . restore registers</span>
<span id="L177" class="LineNr">177 </span>    59/pop-to-ecx
<span id="L178" class="LineNr">178 </span>    <span class="subxS1Comment"># . epilog</span>
<span id="L179" class="LineNr">179 </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="L180" class="LineNr">180 </span>    5d/pop-to-ebp
<span id="L181" class="LineNr">181 </span>    c3/return
<span id="L182" class="LineNr">182 </span>
<span id="L183" class="LineNr">183 </span><span class="subxComment"># We could create a more general '$abort' jump target, but then we'd need to do</span>
<span id="L184" class="LineNr">184 </span><span class="subxComment"># a conditional jump followed by loading the error message and an unconditional</span>
<span id="L185" class="LineNr">185 </span><span class="subxComment"># jump. Or we'd need to unconditionally load the error message before a</span>
<span id="L186" class="LineNr">186 </span><span class="subxComment"># conditional jump, even if it's unused the vast majority of the time. This way</span>
<span id="L187" class="LineNr">187 </span><span class="subxComment"># we bloat a potentially cold segment in RAM so we can abort with a single</span>
<span id="L188" class="LineNr">188 </span><span class="subxComment"># instruction.</span>
<span id="L189" class="LineNr">189 </span><span class="Constant">$allocate-region:abort</span>:
<span id="L190" class="LineNr">190 </span>    <span class="subxS1Comment"># . _write(2/stderr, error)</span>
<span id="L191" class="LineNr">191 </span>    <span class="subxS2Comment"># . . push args</span>
<span id="L192" class="LineNr">192 </span>    68/push  <span class="Constant">&quot;allocate-region: failed to allocate\n&quot;</span>/imm32
<span id="L193" class="LineNr">193 </span>    68/push  2/imm32/stderr
<span id="L194" class="LineNr">194 </span>    <span class="subxS2Comment"># . . call</span>
<span id="L195" class="LineNr">195 </span>    e8/call  <a href='050_write.subx.html#L14'>_write</a>/disp32
<span id="L196" class="LineNr">196 </span>    <span class="subxS2Comment"># . . discard args</span>
<span id="L197" class="LineNr">197 </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="L198" class="LineNr">198 </span>    <span class="subxS1Comment"># . syscall(exit, 1)</span>
<span id="L199" class="LineNr">199 </span>    bb/copy-to-ebx  1/imm32
<span id="L200" class="LineNr">200 </span>    b8/copy-to-eax  1/imm32/exit
<span id="L201" class="LineNr">201 </span>    cd/syscall  0x80/imm8
<span id="L202" class="LineNr">202 </span>    <span class="subxComment"># never gets here</span>
<span id="L203" class="LineNr">203 </span>
<span id="L204" class="LineNr">204 </span><span class="subxS2Comment"># . . vim&#0058;nowrap:textwidth=0</span>
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