4959

It's always seemed ugly to explain the rules for segment names. Let's just
always require a fixed name for the code and data segments.

3 files changed, 142 insertions, 105 deletions

diff --git a/html/subx/034compute_segment_address.cc.html b/html/subx/034compute_segment_address.cc.html
index 8f579b69..ae9a2461 100644
--- a/html/subx/034compute_segment_address.cc.html
+++ b/html/subx/034compute_segment_address.cc.html
@@ -71,7 +71,7 @@ if ('onhashchange' in window) {
 <span id="L11" class="LineNr"> 11 </span><span class="traceContains">+load: 0x09000056 -&gt; 0b</span>
 <span id="L12" class="LineNr"> 12 </span><span class="traceContains">+load: 0x09000057 -&gt; 0c</span>
 <span id="L13" class="LineNr"> 13 </span><span class="traceContains">+load: 0x09000058 -&gt; 0d</span>
-<span id="L14" class="LineNr"> 14 </span><span class="traceContains">+run: add <a href='037heap.cc.html#L23'>imm32</a> 0x0d0c0b0a to <a href='010---vm.cc.html#L23'>reg</a> <a href='010---vm.cc.html#L13'>EAX</a></span>
+<span id="L14" class="LineNr"> 14 </span><span class="traceContains">+run: add imm32 0x0d0c0b0a to reg EAX</span>
 <span id="L15" class="LineNr"> 15 </span><span class="traceContains">+run: storing 0x0d0c0b0a</span>
 <span id="L16" class="LineNr"> 16 </span>
 <span id="L17" class="LineNr"> 17 </span><span class="Comment">//: Update the parser to handle non-numeric segment name.</span>
@@ -81,45 +81,45 @@ if ('onhashchange' in window) {
 <span id="L21" class="LineNr"> 21 </span><span class="Comment">//: the new data to existing data for the segment.</span>
 <span id="L22" class="LineNr"> 22 </span>
 <span id="L23" class="LineNr"> 23 </span><span class="Delimiter">:(before &quot;End Globals&quot;)</span>
-<span id="L24" class="LineNr"> 24 </span>map&lt;<span class="Comment">/*</span><span class="Comment">name</span><span class="Comment">*/</span>string<span class="Delimiter">,</span> <span class="Normal">int</span>&gt; <span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">;</span>
-<span id="L25" class="LineNr"> 25 </span><span class="Normal">bool</span> <span class="Special"><a href='034compute_segment_address.cc.html#L25'>Currently_parsing_named_segment</a></span> =<span class="Constant"> false</span><span class="Delimiter">;</span>  <span class="Comment">// global to permit cross-layer communication</span>
-<span id="L26" class="LineNr"> 26 </span><span class="Normal">int</span> <span class="Special"><a href='034compute_segment_address.cc.html#L26'>Currently_parsing_segment_index</a></span> = -<span class="Constant">1</span><span class="Delimiter">;</span>  <span class="Comment">// global to permit cross-layer communication</span>
+<span id="L24" class="LineNr"> 24 </span>map&lt;<span class="Comment">/*</span><span class="Comment">name</span><span class="Comment">*/</span>string<span class="Delimiter">,</span> <span class="Normal">int</span>&gt; <span class="Special">Segment_index</span><span class="Delimiter">;</span>
+<span id="L25" class="LineNr"> 25 </span><span class="Normal">bool</span> <span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> false</span><span class="Delimiter">;</span>  <span class="Comment">// global to permit cross-layer communication</span>
+<span id="L26" class="LineNr"> 26 </span><span class="Normal">int</span> <span class="Special">Currently_parsing_segment_index</span> = -<span class="Constant">1</span><span class="Delimiter">;</span>  <span class="Comment">// global to permit cross-layer communication</span>
 <span id="L27" class="LineNr"> 27 </span><span class="Delimiter">:(before &quot;End Reset&quot;)</span>
-<span id="L28" class="LineNr"> 28 </span><span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">.</span>clear<span class="Delimiter">();</span>
-<span id="L29" class="LineNr"> 29 </span><span class="Special"><a href='034compute_segment_address.cc.html#L25'>Currently_parsing_named_segment</a></span> =<span class="Constant"> false</span><span class="Delimiter">;</span>
-<span id="L30" class="LineNr"> 30 </span><span class="Special"><a href='034compute_segment_address.cc.html#L26'>Currently_parsing_segment_index</a></span> = -<span class="Constant">1</span><span class="Delimiter">;</span>
+<span id="L28" class="LineNr"> 28 </span><span class="Special">Segment_index</span><span class="Delimiter">.</span>clear<span class="Delimiter">();</span>
+<span id="L29" class="LineNr"> 29 </span><span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> false</span><span class="Delimiter">;</span>
+<span id="L30" class="LineNr"> 30 </span><span class="Special">Currently_parsing_segment_index</span> = -<span class="Constant">1</span><span class="Delimiter">;</span>
 <span id="L31" class="LineNr"> 31 </span>
 <span id="L32" class="LineNr"> 32 </span><span class="Delimiter">:(before &quot;End Segment Parsing Special-cases(segment_title)&quot;)</span>
 <span id="L33" class="LineNr"> 33 </span><span class="Normal">if</span> <span class="Delimiter">(</span>!starts_with<span class="Delimiter">(</span>segment_title<span class="Delimiter">,</span> <span class="Constant">&quot;0x&quot;</span><span class="Delimiter">))</span> <span class="Delimiter">{</span>
-<span id="L34" class="LineNr"> 34 </span>  <span class="Special"><a href='034compute_segment_address.cc.html#L25'>Currently_parsing_named_segment</a></span> =<span class="Constant"> true</span><span class="Delimiter">;</span>
-<span id="L35" class="LineNr"> 35 </span>  <span class="Normal">if</span> <span class="Delimiter">(</span>!contains_key<span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">,</span> segment_title<span class="Delimiter">))</span> <span class="Delimiter">{</span>
-<span id="L36" class="LineNr"> 36 </span>    <a href='003trace.cc.html#L183'>trace</a><span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;parse&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;new segment '&quot;</span> &lt;&lt; segment_title &lt;&lt; <span class="Constant">&quot;'&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
-<span id="L37" class="LineNr"> 37 </span>    <span class="Normal">if</span> <span class="Delimiter">(</span>segment_title == <span class="Constant">&quot;code&quot;</span><span class="Delimiter">)</span>
-<span id="L38" class="LineNr"> 38 </span>      <a href='001help.cc.html#L252'>put</a><span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">,</span> segment_title<span class="Delimiter">,</span> <span class="Constant">0</span><span class="Delimiter">);</span>
-<span id="L39" class="LineNr"> 39 </span>    <span class="Normal">else</span> <span class="Normal">if</span> <span class="Delimiter">(</span>segment_title == <span class="Constant">&quot;data&quot;</span><span class="Delimiter">)</span>
-<span id="L40" class="LineNr"> 40 </span>      <a href='001help.cc.html#L252'>put</a><span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">,</span> segment_title<span class="Delimiter">,</span> <span class="Constant">1</span><span class="Delimiter">);</span>
-<span id="L41" class="LineNr"> 41 </span>    <span class="Normal">else</span>
-<span id="L42" class="LineNr"> 42 </span>      <a href='001help.cc.html#L252'>put</a><span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">,</span> segment_title<span class="Delimiter">,</span> max<span class="Delimiter">(</span><span class="Constant">2</span><span class="Delimiter">,</span> <a href='001help.cc.html#L166'>SIZE</a><span class="Delimiter">(</span>out<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">)));</span>
-<span id="L43" class="LineNr"> 43 </span>    out<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>segment<span class="Delimiter">());</span>
-<span id="L44" class="LineNr"> 44 </span>  <span class="Delimiter">}</span>
-<span id="L45" class="LineNr"> 45 </span>  <span class="Normal">else</span> <span class="Delimiter">{</span>
-<span id="L46" class="LineNr"> 46 </span>    <a href='003trace.cc.html#L183'>trace</a><span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;parse&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;prepending to segment '&quot;</span> &lt;&lt; segment_title &lt;&lt; <span class="Constant">&quot;'&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L34" class="LineNr"> 34 </span>  <span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> true</span><span class="Delimiter">;</span>
+<span id="L35" class="LineNr"> 35 </span>  <span class="Normal">if</span> <span class="Delimiter">(</span>!contains_key<span class="Delimiter">(</span><span class="Special">Segment_index</span><span class="Delimiter">,</span> segment_title<span class="Delimiter">))</span> <span class="Delimiter">{</span>
+<span id="L36" class="LineNr"> 36 </span>    trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;parse&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;new segment '&quot;</span> &lt;&lt; segment_title &lt;&lt; <span class="Constant">&quot;'&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L37" class="LineNr"> 37 </span>    <span class="Normal">if</span> <span class="Delimiter">(</span>out<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>empty<span class="Delimiter">()</span> &amp;&amp; segment_title != <span class="Constant">&quot;code&quot;</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L38" class="LineNr"> 38 </span>      raise &lt;&lt; <span class="Constant">&quot;first segment must be 'code' but is '&quot;</span> &lt;&lt; segment_title &lt;&lt; <span class="Constant">&quot;'\n&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L39" class="LineNr"> 39 </span>      <span class="Identifier">return</span><span class="Delimiter">;</span>
+<span id="L40" class="LineNr"> 40 </span>    <span class="Delimiter">}</span>
+<span id="L41" class="LineNr"> 41 </span>    <span class="Normal">if</span> <span class="Delimiter">(</span>SIZE<span class="Delimiter">(</span>out<span class="Delimiter">.</span>segments<span class="Delimiter">)</span> == <span class="Constant">1</span> &amp;&amp; segment_title != <span class="Constant">&quot;data&quot;</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L42" class="LineNr"> 42 </span>      raise &lt;&lt; <span class="Constant">&quot;second segment must be 'data' but is '&quot;</span> &lt;&lt; segment_title &lt;&lt; <span class="Constant">&quot;'\n&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L43" class="LineNr"> 43 </span>      <span class="Identifier">return</span><span class="Delimiter">;</span>
+<span id="L44" class="LineNr"> 44 </span>    <span class="Delimiter">}</span>
+<span id="L45" class="LineNr"> 45 </span>    put<span class="Delimiter">(</span><span class="Special">Segment_index</span><span class="Delimiter">,</span> segment_title<span class="Delimiter">,</span> SIZE<span class="Delimiter">(</span>out<span class="Delimiter">.</span>segments<span class="Delimiter">));</span>
+<span id="L46" class="LineNr"> 46 </span>    out<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>segment<span class="Delimiter">());</span>
 <span id="L47" class="LineNr"> 47 </span>  <span class="Delimiter">}</span>
-<span id="L48" class="LineNr"> 48 </span>  <span class="Special"><a href='034compute_segment_address.cc.html#L26'>Currently_parsing_segment_index</a></span> = get<span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L24'>Segment_index</a></span><span class="Delimiter">,</span> segment_title<span class="Delimiter">);</span>
-<span id="L49" class="LineNr"> 49 </span><span class="Delimiter">}</span>
-<span id="L50" class="LineNr"> 50 </span>
-<span id="L51" class="LineNr"> 51 </span><span class="Delimiter">:(before &quot;End flush(p, lines) Special-cases&quot;)</span>
-<span id="L52" class="LineNr"> 52 </span><span class="Normal">if</span> <span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L25'>Currently_parsing_named_segment</a></span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L53" class="LineNr"> 53 </span>  <span class="Normal">if</span> <span class="Delimiter">(</span>p<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">.</span>empty<span class="Delimiter">()</span> || <span class="Special"><a href='034compute_segment_address.cc.html#L26'>Currently_parsing_segment_index</a></span> &lt; <span class="Constant">0</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L54" class="LineNr"> 54 </span>    <a href='003trace.cc.html#L190'>raise</a> &lt;&lt; <span class="Constant">&quot;input does not start with a '==' section header\n&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
-<span id="L55" class="LineNr"> 55 </span>    <span class="Identifier">return</span><span class="Delimiter">;</span>
-<span id="L56" class="LineNr"> 56 </span>  <span class="Delimiter">}</span>
-<span id="L57" class="LineNr"> 57 </span>  <a href='003trace.cc.html#L183'>trace</a><span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;parse&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;flushing to segment&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
-<span id="L58" class="LineNr"> 58 </span>  vector&lt;line&gt;&amp; curr_segment_data = p<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Special"><a href='034compute_segment_address.cc.html#L26'>Currently_parsing_segment_index</a></span><span class="Delimiter">).</span><a href='011run.cc.html#L109'>lines</a><span class="Delimiter">;</span>
-<span id="L59" class="LineNr"> 59 </span>  curr_segment_data<span class="Delimiter">.</span>insert<span class="Delimiter">(</span>curr_segment_data<span class="Delimiter">.</span>begin<span class="Delimiter">(),</span> <a href='011run.cc.html#L109'>lines</a><span class="Delimiter">.</span>begin<span class="Delimiter">(),</span> <a href='011run.cc.html#L109'>lines</a><span class="Delimiter">.</span>end<span class="Delimiter">());</span>
-<span id="L60" class="LineNr"> 60 </span>  <a href='011run.cc.html#L109'>lines</a><span class="Delimiter">.</span>clear<span class="Delimiter">();</span>
-<span id="L61" class="LineNr"> 61 </span>  <span class="Special"><a href='034compute_segment_address.cc.html#L25'>Currently_parsing_named_segment</a></span> =<span class="Constant"> false</span><span class="Delimiter">;</span>
-<span id="L62" class="LineNr"> 62 </span>  <span class="Special"><a href='034compute_segment_address.cc.html#L26'>Currently_parsing_segment_index</a></span> = -<span class="Constant">1</span><span class="Delimiter">;</span>
+<span id="L48" class="LineNr"> 48 </span>  <span class="Normal">else</span> <span class="Delimiter">{</span>
+<span id="L49" class="LineNr"> 49 </span>    trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;parse&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;prepending to segment '&quot;</span> &lt;&lt; segment_title &lt;&lt; <span class="Constant">&quot;'&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L50" class="LineNr"> 50 </span>  <span class="Delimiter">}</span>
+<span id="L51" class="LineNr"> 51 </span>  <span class="Special">Currently_parsing_segment_index</span> = get<span class="Delimiter">(</span><span class="Special">Segment_index</span><span class="Delimiter">,</span> segment_title<span class="Delimiter">);</span>
+<span id="L52" class="LineNr"> 52 </span><span class="Delimiter">}</span>
+<span id="L53" class="LineNr"> 53 </span>
+<span id="L54" class="LineNr"> 54 </span><span class="Delimiter">:(before &quot;End flush(p, lines) Special-cases&quot;)</span>
+<span id="L55" class="LineNr"> 55 </span><span class="Normal">if</span> <span class="Delimiter">(</span><span class="Special">Currently_parsing_named_segment</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L56" class="LineNr"> 56 </span>  assert<span class="Delimiter">(</span>!p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>empty<span class="Delimiter">());</span>
+<span id="L57" class="LineNr"> 57 </span>  trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;parse&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;flushing to segment&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L58" class="LineNr"> 58 </span>  vector&lt;line&gt;&amp; curr_segment_data = p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>at<span class="Delimiter">(</span><span class="Special">Currently_parsing_segment_index</span><span class="Delimiter">).</span>lines<span class="Delimiter">;</span>
+<span id="L59" class="LineNr"> 59 </span>  curr_segment_data<span class="Delimiter">.</span>insert<span class="Delimiter">(</span>curr_segment_data<span class="Delimiter">.</span>begin<span class="Delimiter">(),</span> lines<span class="Delimiter">.</span>begin<span class="Delimiter">(),</span> lines<span class="Delimiter">.</span>end<span class="Delimiter">());</span>
+<span id="L60" class="LineNr"> 60 </span>  lines<span class="Delimiter">.</span>clear<span class="Delimiter">();</span>
+<span id="L61" class="LineNr"> 61 </span>  <span class="Special">Currently_parsing_named_segment</span> =<span class="Constant"> false</span><span class="Delimiter">;</span>
+<span id="L62" class="LineNr"> 62 </span>  <span class="Special">Currently_parsing_segment_index</span> = -<span class="Constant">1</span><span class="Delimiter">;</span>
 <span id="L63" class="LineNr"> 63 </span>  <span class="Identifier">return</span><span class="Delimiter">;</span>
 <span id="L64" class="LineNr"> 64 </span><span class="Delimiter">}</span>
 <span id="L65" class="LineNr"> 65 </span>
@@ -141,65 +141,84 @@ if ('onhashchange' in window) {
 <span id="L81" class="LineNr"> 81 </span><span class="traceContains">+load: 0x0900005c -&gt; 0c</span>
 <span id="L82" class="LineNr"> 82 </span><span class="traceContains">+load: 0x0900005d -&gt; 0d</span>
 <span id="L83" class="LineNr"> 83 </span>
-<span id="L84" class="LineNr"> 84 </span><span class="Comment">//: compute segment address</span>
-<span id="L85" class="LineNr"> 85 </span>
-<span id="L86" class="LineNr"> 86 </span><span class="Delimiter">:(before &quot;End Level-2 Transforms&quot;)</span>
-<span id="L87" class="LineNr"> 87 </span><span class="Special"><a href='011run.cc.html#L229'>Transform</a></span><span class="Delimiter">.</span>push_back<span class="Delimiter">(</span><a href='034compute_segment_address.cc.html#L90'>compute_segment_starts</a><span class="Delimiter">);</span>
+<span id="L84" class="LineNr"> 84 </span><span class="Delimiter">:(scenario error_on_missing_segment_header)</span>
+<span id="L85" class="LineNr"> 85 </span><span class="Special">% Hide_errors = true;</span>
+<span id="L86" class="LineNr"> 86 </span><span class="PreProc">0</span><span class="Constant">5</span>/add-to-EAX <span class="Constant">0</span>/imm32
+<span id="L87" class="LineNr"> 87 </span><span class="traceContains">+error: input does not start with a '==' section header</span>
 <span id="L88" class="LineNr"> 88 </span>
-<span id="L89" class="LineNr"> 89 </span><span class="Delimiter">:(code)</span>
-<span id="L90" class="LineNr"> 90 </span><span class="Normal">void</span> <a href='034compute_segment_address.cc.html#L90'>compute_segment_starts</a><span class="Delimiter">(</span>program&amp; p<span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L91" class="LineNr"> 91 </span>  <a href='003trace.cc.html#L183'>trace</a><span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;transform&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;-- compute segment addresses&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
-<span id="L92" class="LineNr"> 92 </span>  <span class="Normal">uint32_t</span> p_offset = <span class="Comment">/*</span><span class="Comment">size of ehdr</span><span class="Comment">*/</span><span class="Constant">0x34</span> + <a href='001help.cc.html#L166'>SIZE</a><span class="Delimiter">(</span>p<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">)</span>*<span class="Constant">0x20</span><span class="Comment">/*</span><span class="Comment">size of each phdr</span><span class="Comment">*/</span><span class="Delimiter">;</span>
-<span id="L93" class="LineNr"> 93 </span>  <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">size_t</span> i = <span class="Constant">0</span><span class="Delimiter">;</span>  i &lt; p<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">.</span>size<span class="Delimiter">();</span>  ++i<span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L94" class="LineNr"> 94 </span>    segment&amp; curr = p<span class="Delimiter">.</span><a href='011run.cc.html#L100'>segments</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">);</span>
-<span id="L95" class="LineNr"> 95 </span>    <span class="Normal">if</span> <span class="Delimiter">(</span>curr<span class="Delimiter">.</span>start == <span class="Constant">0</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L96" class="LineNr"> 96 </span>      curr<span class="Delimiter">.</span>start = <a href='012elf.cc.html#L135'>CODE_SEGMENT</a> + i*SPACE_FOR_SEGMENT + p_offset<span class="Delimiter">;</span>
-<span id="L97" class="LineNr"> 97 </span>      <a href='003trace.cc.html#L183'>trace</a><span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;transform&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;segment &quot;</span> &lt;&lt; i &lt;&lt; <span class="Constant">&quot; begins at address 0x&quot;</span> &lt;&lt; <a href='010---vm.cc.html#L413'>HEXWORD</a> &lt;&lt; curr<span class="Delimiter">.</span>start &lt;&lt; end<span class="Delimiter">();</span>
-<span id="L98" class="LineNr"> 98 </span>    <span class="Delimiter">}</span>
-<span id="L99" class="LineNr"> 99 </span>    p_offset += size_of<span class="Delimiter">(</span>curr<span class="Delimiter">);</span>
-<span id="L100" class="LineNr">100 </span>    assert<span class="Delimiter">(</span>p_offset &lt; <a href='010---vm.cc.html#L115'>SEGMENT_ALIGNMENT</a><span class="Delimiter">);</span>  <span class="Comment">// for now we get less and less available space in each successive segment</span>
-<span id="L101" class="LineNr">101 </span>  <span class="Delimiter">}</span>
-<span id="L102" class="LineNr">102 </span><span class="Delimiter">}</span>
-<span id="L103" class="LineNr">103 </span>
-<span id="L104" class="LineNr">104 </span><span class="Normal">uint32_t</span> size_of<span class="Delimiter">(</span><span class="Normal">const</span> segment&amp; s<span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L105" class="LineNr">105 </span>  <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
-<span id="L106" class="LineNr">106 </span>  <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">int</span> i = <span class="Constant">0</span><span class="Delimiter">;</span>  i &lt; <a href='001help.cc.html#L166'>SIZE</a><span class="Delimiter">(</span>s<span class="Delimiter">.</span><a href='011run.cc.html#L109'>lines</a><span class="Delimiter">);</span>  ++i<span class="Delimiter">)</span>
-<span id="L107" class="LineNr">107 </span>    sum += <a href='034compute_segment_address.cc.html#L112'>num_bytes</a><span class="Delimiter">(</span>s<span class="Delimiter">.</span><a href='011run.cc.html#L109'>lines</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
-<span id="L108" class="LineNr">108 </span>  <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
-<span id="L109" class="LineNr">109 </span><span class="Delimiter">}</span>
-<span id="L110" class="LineNr">110 </span>
-<span id="L111" class="LineNr">111 </span><span class="Comment">// Assumes all bitfields are packed.</span>
-<span id="L112" class="LineNr">112 </span><span class="Normal">uint32_t</span> <a href='034compute_segment_address.cc.html#L112'>num_bytes</a><span class="Delimiter">(</span><span class="Normal">const</span> line&amp; inst<span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L113" class="LineNr">113 </span>  <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
-<span id="L114" class="LineNr">114 </span>  <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">int</span> i = <span class="Constant">0</span><span class="Delimiter">;</span>  i &lt; <a href='001help.cc.html#L166'>SIZE</a><span class="Delimiter">(</span>inst<span class="Delimiter">.</span><a href='011run.cc.html#L118'>words</a><span class="Delimiter">);</span>  ++i<span class="Delimiter">)</span>
-<span id="L115" class="LineNr">115 </span>    sum += size_of<span class="Delimiter">(</span>inst<span class="Delimiter">.</span><a href='011run.cc.html#L118'>words</a><span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
-<span id="L116" class="LineNr">116 </span>  <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
-<span id="L117" class="LineNr">117 </span><span class="Delimiter">}</span>
-<span id="L118" class="LineNr">118 </span>
-<span id="L119" class="LineNr">119 </span><span class="Normal">int</span> size_of<span class="Delimiter">(</span><span class="Normal">const</span> word&amp; w<span class="Delimiter">)</span> <span class="Delimiter">{</span>
-<span id="L120" class="LineNr">120 </span>  <span class="Normal">if</span> <span class="Delimiter">(</span>has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">&quot;disp32&quot;</span><span class="Delimiter">)</span> || has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">&quot;imm32&quot;</span><span class="Delimiter">))</span>
-<span id="L121" class="LineNr">121 </span>    <span class="Identifier">return</span> <span class="Constant">4</span><span class="Delimiter">;</span>
-<span id="L122" class="LineNr">122 </span>  <span class="Normal">else</span> <span class="Normal">if</span> <span class="Delimiter">(</span>has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">&quot;disp16&quot;</span><span class="Delimiter">))</span>
-<span id="L123" class="LineNr">123 </span>    <span class="Identifier">return</span> <span class="Constant">2</span><span class="Delimiter">;</span>
-<span id="L124" class="LineNr">124 </span>  <span class="Comment">// End size_of(word w) Special-cases</span>
-<span id="L125" class="LineNr">125 </span>  <span class="Normal">else</span>
-<span id="L126" class="LineNr">126 </span>    <span class="Identifier">return</span> <span class="Constant">1</span><span class="Delimiter">;</span>
-<span id="L127" class="LineNr">127 </span><span class="Delimiter">}</span>
-<span id="L128" class="LineNr">128 </span>
-<span id="L129" class="LineNr">129 </span><span class="Comment">//: Dependencies:</span>
-<span id="L130" class="LineNr">130 </span><span class="Comment">//: - We'd like to compute segment addresses before setting up global variables,</span>
-<span id="L131" class="LineNr">131 </span><span class="Comment">//:   because computing addresses for global variables requires knowing where</span>
-<span id="L132" class="LineNr">132 </span><span class="Comment">//:   the data segment starts.</span>
-<span id="L133" class="LineNr">133 </span><span class="Comment">//: - We'd like to finish expanding labels before computing segment addresses,</span>
-<span id="L134" class="LineNr">134 </span><span class="Comment">//:   because it would make computing the sizes of segments more self-contained</span>
-<span id="L135" class="LineNr">135 </span><span class="Comment">//:   (num_bytes).</span>
-<span id="L136" class="LineNr">136 </span><span class="Comment">//:</span>
-<span id="L137" class="LineNr">137 </span><span class="Comment">//: Decision: compute segment addresses before expanding labels, by being</span>
-<span id="L138" class="LineNr">138 </span><span class="Comment">//: aware in this layer of certain operand types that will eventually occupy</span>
-<span id="L139" class="LineNr">139 </span><span class="Comment">//: multiple bytes.</span>
-<span id="L140" class="LineNr">140 </span><span class="Comment">//:</span>
-<span id="L141" class="LineNr">141 </span><span class="Comment">//: The layer to expand labels later hooks into num_bytes() to teach this</span>
-<span id="L142" class="LineNr">142 </span><span class="Comment">//: layer that labels occupy zero space in the binary.</span>
+<span id="L89" class="LineNr"> 89 </span><span class="Delimiter">:(scenario error_on_first_segment_not_code)</span>
+<span id="L90" class="LineNr"> 90 </span><span class="Special">% Hide_errors = true;</span>
+<span id="L91" class="LineNr"> 91 </span>== data
+<span id="L92" class="LineNr"> 92 </span><span class="PreProc">0</span><span class="Constant">5</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span>
+<span id="L93" class="LineNr"> 93 </span><span class="traceContains">+error: first segment must be 'code' but is 'data'</span>
+<span id="L94" class="LineNr"> 94 </span>
+<span id="L95" class="LineNr"> 95 </span><span class="Delimiter">:(scenario error_on_second_segment_not_data)</span>
+<span id="L96" class="LineNr"> 96 </span><span class="Special">% Hide_errors = true;</span>
+<span id="L97" class="LineNr"> 97 </span>== <a href='031check_operands.cc.html#L11'>code</a>
+<span id="L98" class="LineNr"> 98 </span><span class="PreProc">0</span><span class="Constant">5</span>/add-to-EAX <span class="Constant">0</span>/imm32
+<span id="L99" class="LineNr"> 99 </span>== bss
+<span id="L100" class="LineNr">100 </span><span class="PreProc">0</span><span class="Constant">5</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span> <span class="PreProc">0</span><span class="Constant">0</span>
+<span id="L101" class="LineNr">101 </span><span class="traceContains">+error: second segment must be 'data' but is 'bss'</span>
+<span id="L102" class="LineNr">102 </span>
+<span id="L103" class="LineNr">103 </span><span class="Comment">//: compute segment address</span>
+<span id="L104" class="LineNr">104 </span>
+<span id="L105" class="LineNr">105 </span><span class="Delimiter">:(before &quot;End Level-2 Transforms&quot;)</span>
+<span id="L106" class="LineNr">106 </span><span class="Special">Transform</span><span class="Delimiter">.</span>push_back<span class="Delimiter">(</span>compute_segment_starts<span class="Delimiter">);</span>
+<span id="L107" class="LineNr">107 </span>
+<span id="L108" class="LineNr">108 </span><span class="Delimiter">:(code)</span>
+<span id="L109" class="LineNr">109 </span><span class="Normal">void</span> compute_segment_starts<span class="Delimiter">(</span>program&amp; p<span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L110" class="LineNr">110 </span>  trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;transform&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;-- compute segment addresses&quot;</span> &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L111" class="LineNr">111 </span>  <span class="Normal">uint32_t</span> p_offset = <span class="Comment">/*</span><span class="Comment">size of ehdr</span><span class="Comment">*/</span><span class="Constant">0x34</span> + SIZE<span class="Delimiter">(</span>p<span class="Delimiter">.</span>segments<span class="Delimiter">)</span>*<span class="Constant">0x20</span><span class="Comment">/*</span><span class="Comment">size of each phdr</span><span class="Comment">*/</span><span class="Delimiter">;</span>
+<span id="L112" class="LineNr">112 </span>  <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">size_t</span> i = <span class="Constant">0</span><span class="Delimiter">;</span>  i &lt; p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>size<span class="Delimiter">();</span>  ++i<span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L113" class="LineNr">113 </span>    segment&amp; curr = p<span class="Delimiter">.</span>segments<span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">);</span>
+<span id="L114" class="LineNr">114 </span>    <span class="Normal">if</span> <span class="Delimiter">(</span>curr<span class="Delimiter">.</span>start == <span class="Constant">0</span><span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L115" class="LineNr">115 </span>      curr<span class="Delimiter">.</span>start = CODE_SEGMENT + i*SPACE_FOR_SEGMENT + p_offset<span class="Delimiter">;</span>
+<span id="L116" class="LineNr">116 </span>      trace<span class="Delimiter">(</span><span class="Constant">99</span><span class="Delimiter">,</span> <span class="Constant">&quot;transform&quot;</span><span class="Delimiter">)</span> &lt;&lt; <span class="Constant">&quot;segment &quot;</span> &lt;&lt; i &lt;&lt; <span class="Constant">&quot; begins at address 0x&quot;</span> &lt;&lt; HEXWORD &lt;&lt; curr<span class="Delimiter">.</span>start &lt;&lt; end<span class="Delimiter">();</span>
+<span id="L117" class="LineNr">117 </span>    <span class="Delimiter">}</span>
+<span id="L118" class="LineNr">118 </span>    p_offset += size_of<span class="Delimiter">(</span>curr<span class="Delimiter">);</span>
+<span id="L119" class="LineNr">119 </span>    assert<span class="Delimiter">(</span>p_offset &lt; SEGMENT_ALIGNMENT<span class="Delimiter">);</span>  <span class="Comment">// for now we get less and less available space in each successive segment</span>
+<span id="L120" class="LineNr">120 </span>  <span class="Delimiter">}</span>
+<span id="L121" class="LineNr">121 </span><span class="Delimiter">}</span>
+<span id="L122" class="LineNr">122 </span>
+<span id="L123" class="LineNr">123 </span><span class="Normal">uint32_t</span> size_of<span class="Delimiter">(</span><span class="Normal">const</span> segment&amp; s<span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L124" class="LineNr">124 </span>  <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
+<span id="L125" class="LineNr">125 </span>  <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">int</span> i = <span class="Constant">0</span><span class="Delimiter">;</span>  i &lt; SIZE<span class="Delimiter">(</span>s<span class="Delimiter">.</span>lines<span class="Delimiter">);</span>  ++i<span class="Delimiter">)</span>
+<span id="L126" class="LineNr">126 </span>    sum += num_bytes<span class="Delimiter">(</span>s<span class="Delimiter">.</span>lines<span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
+<span id="L127" class="LineNr">127 </span>  <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
+<span id="L128" class="LineNr">128 </span><span class="Delimiter">}</span>
+<span id="L129" class="LineNr">129 </span>
+<span id="L130" class="LineNr">130 </span><span class="Comment">// Assumes all bitfields are packed.</span>
+<span id="L131" class="LineNr">131 </span><span class="Normal">uint32_t</span> num_bytes<span class="Delimiter">(</span><span class="Normal">const</span> line&amp; inst<span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L132" class="LineNr">132 </span>  <span class="Normal">uint32_t</span> sum = <span class="Constant">0</span><span class="Delimiter">;</span>
+<span id="L133" class="LineNr">133 </span>  <span class="Normal">for</span> <span class="Delimiter">(</span><span class="Normal">int</span> i = <span class="Constant">0</span><span class="Delimiter">;</span>  i &lt; SIZE<span class="Delimiter">(</span>inst<span class="Delimiter">.</span>words<span class="Delimiter">);</span>  ++i<span class="Delimiter">)</span>
+<span id="L134" class="LineNr">134 </span>    sum += size_of<span class="Delimiter">(</span>inst<span class="Delimiter">.</span>words<span class="Delimiter">.</span>at<span class="Delimiter">(</span>i<span class="Delimiter">));</span>
+<span id="L135" class="LineNr">135 </span>  <span class="Identifier">return</span> sum<span class="Delimiter">;</span>
+<span id="L136" class="LineNr">136 </span><span class="Delimiter">}</span>
+<span id="L137" class="LineNr">137 </span>
+<span id="L138" class="LineNr">138 </span><span class="Normal">int</span> size_of<span class="Delimiter">(</span><span class="Normal">const</span> word&amp; w<span class="Delimiter">)</span> <span class="Delimiter">{</span>
+<span id="L139" class="LineNr">139 </span>  <span class="Normal">if</span> <span class="Delimiter">(</span>has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">&quot;disp32&quot;</span><span class="Delimiter">)</span> || has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">&quot;imm32&quot;</span><span class="Delimiter">))</span>
+<span id="L140" class="LineNr">140 </span>    <span class="Identifier">return</span> <span class="Constant">4</span><span class="Delimiter">;</span>
+<span id="L141" class="LineNr">141 </span>  <span class="Normal">else</span> <span class="Normal">if</span> <span class="Delimiter">(</span>has_operand_metadata<span class="Delimiter">(</span>w<span class="Delimiter">,</span> <span class="Constant">&quot;disp16&quot;</span><span class="Delimiter">))</span>
+<span id="L142" class="LineNr">142 </span>    <span class="Identifier">return</span> <span class="Constant">2</span><span class="Delimiter">;</span>
+<span id="L143" class="LineNr">143 </span>  <span class="Comment">// End size_of(word w) Special-cases</span>
+<span id="L144" class="LineNr">144 </span>  <span class="Normal">else</span>
+<span id="L145" class="LineNr">145 </span>    <span class="Identifier">return</span> <span class="Constant">1</span><span class="Delimiter">;</span>
+<span id="L146" class="LineNr">146 </span><span class="Delimiter">}</span>
+<span id="L147" class="LineNr">147 </span>
+<span id="L148" class="LineNr">148 </span><span class="Comment">//: Dependencies:</span>
+<span id="L149" class="LineNr">149 </span><span class="Comment">//: - We'd like to compute segment addresses before setting up global variables,</span>
+<span id="L150" class="LineNr">150 </span><span class="Comment">//:   because computing addresses for global variables requires knowing where</span>
+<span id="L151" class="LineNr">151 </span><span class="Comment">//:   the data segment starts.</span>
+<span id="L152" class="LineNr">152 </span><span class="Comment">//: - We'd like to finish expanding labels before computing segment addresses,</span>
+<span id="L153" class="LineNr">153 </span><span class="Comment">//:   because it would make computing the sizes of segments more self-contained</span>
+<span id="L154" class="LineNr">154 </span><span class="Comment">//:   (num_bytes).</span>
+<span id="L155" class="LineNr">155 </span><span class="Comment">//:</span>
+<span id="L156" class="LineNr">156 </span><span class="Comment">//: Decision: compute segment addresses before expanding labels, by being</span>
+<span id="L157" class="LineNr">157 </span><span class="Comment">//: aware in this layer of certain operand types that will eventually occupy</span>
+<span id="L158" class="LineNr">158 </span><span class="Comment">//: multiple bytes.</span>
+<span id="L159" class="LineNr">159 </span><span class="Comment">//:</span>
+<span id="L160" class="LineNr">160 </span><span class="Comment">//: The layer to expand labels later hooks into num_bytes() to teach this</span>
+<span id="L161" class="LineNr">161 </span><span class="Comment">//: layer that labels occupy zero space in the binary.</span>
 </pre>
 </body>
 </html>
diff --git a/subx/034compute_segment_address.cc b/subx/034compute_segment_address.cc
index b6c191e4..d37060a4 100644
--- a/subx/034compute_segment_address.cc
+++ b/subx/034compute_segment_address.cc
@@ -34,12 +34,15 @@ if (!starts_with(segment_title, "0x")) {
   Currently_parsing_named_segment = true;
   if (!contains_key(Segment_index, segment_title)) {
     trace(99, "parse") << "new segment '" << segment_title << "'" << end();
-    if (segment_title == "code")
-      put(Segment_index, segment_title, 0);
-    else if (segment_title == "data")
-      put(Segment_index, segment_title, 1);
-    else
-      put(Segment_index, segment_title, max(2, SIZE(out.segments)));
+    if (out.segments.empty() && segment_title != "code") {
+      raise << "first segment must be 'code' but is '" << segment_title << "'\n" << end();
+      return;
+    }
+    if (SIZE(out.segments) == 1 && segment_title != "data") {
+      raise << "second segment must be 'data' but is '" << segment_title << "'\n" << end();
+      return;
+    }
+    put(Segment_index, segment_title, SIZE(out.segments));
     out.segments.push_back(segment());
   }
   else {
@@ -50,10 +53,7 @@ if (!starts_with(segment_title, "0x")) {
 
 :(before "End flush(p, lines) Special-cases")
 if (Currently_parsing_named_segment) {
-  if (p.segments.empty() || Currently_parsing_segment_index < 0) {
-    raise << "input does not start with a '==' section header\n" << end();
-    return;
-  }
+  assert(!p.segments.empty());
   trace(99, "parse") << "flushing to segment" << end();
   vector<line>& curr_segment_data = p.segments.at(Currently_parsing_segment_index).lines;
   curr_segment_data.insert(curr_segment_data.begin(), lines.begin(), lines.end());
@@ -81,6 +81,25 @@ if (Currently_parsing_named_segment) {
 +load: 0x0900005c -> 0c
 +load: 0x0900005d -> 0d
 
+:(scenario error_on_missing_segment_header)
+% Hide_errors = true;
+05/add-to-EAX 0/imm32
++error: input does not start with a '==' section header
+
+:(scenario error_on_first_segment_not_code)
+% Hide_errors = true;
+== data
+05 00 00 00 00
++error: first segment must be 'code' but is 'data'
+
+:(scenario error_on_second_segment_not_data)
+% Hide_errors = true;
+== code
+05/add-to-EAX 0/imm32
+== bss
+05 00 00 00 00
++error: second segment must be 'data' but is 'bss'
+
 //: compute segment address
 
 :(before "End Level-2 Transforms")
diff --git a/subx/Readme.md b/subx/Readme.md
index de7a0fa7..b03176f0 100644
--- a/subx/Readme.md
+++ b/subx/Readme.md
@@ -274,8 +274,7 @@ SubX programs map to the same ELF binaries that a conventional Linux system
 uses. Linux ELF binaries consist of a series of _segments_. In particular, they
 distinguish between code and data. Correspondingly, SubX programs consist of a
 series of segments, each starting with a header line: `==` followed by a name.
-The first segment is assumed to be for code, and the second for data. By
-convention, I name them `code` and `data`.
+The first segment must be named `code`; the second must be named `data`.
 
 Execution always begins at the start of the `code` segment.